Courseware Sample - FestoCharacterizing a level process by using the open-loop step response method....

Instrumentation and Process Control

Courseware Sample

32621-F0

A

INSTRUMENTATION AND PROCESS CONTROL

COURSEWARE SAMPLE

bythe Staff

ofLab-Volt Ltd.

Copyright © 2001 Lab-Volt Ltd.

All rights reserved. No part of this publication may be reproduced,in any form or by any means, without the prior written permissionof Lab-Volt Ltd.

Printed in CanadaAugust 2007

III

Table of Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V

Courseware Outline

Pressure, Flow, and Level Process Control . . . . . . . . . . . . . . . . . . . . . . . . VII

Temperature Process Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XI

pH Process Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XIII

Sample Exercise Extracted from Pressure, Flow, and Level Process Control

Ex. 4-2 Differential-Pressure Level Meters . . . . . . . . . . . . . . . . . . . . . . . . . 3

Principle of operation of differential-pressure level meters. Measuringthe level of liquid in an open column with a differential-pressuretransmitter.

Sample Exercise Extracted from Temperature Process Control

Ex. 2-1 Resistance Temperature Detectors (RTDs) . . . . . . . . . . . . . . . . . 17

Construction and operation of RTDs. Comparison of the resistance-versus-temperature relationships of the most common types of RTDs.Nominal resistance, temperature coefficient, and sensitivity.Measurement of the voltage produced by an RTD with a Wheatstonebridge.

Sample Exercise Extracted from pH Process Control

Ex. 2-1 pH Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

An overview of the pH probe and of the pH Transmitter from theProcess Control Training System. Operation of the pH Transmitter inboth the fixed and the variable calibration modes. Measuring the pHof different solutions using the pH Probe.

Other Sample Extracted from Pressure, Flow, and Level Process Control

Unit Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Instructor Guide Sample Extracted from Pressure, Flow, and Level ProcessControl

Ex. 5-1 Pressure Process Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Bibliography

V

IntroductionThe Lab-Volt Process Control Training System, Model 6090, familiarizes studentswith the fundamentals of instrumentation and process control. It demonstrates thecontrol of pressure, flow, level, temperature, and pH processes. It can alsodemonstrate advanced process control techniques, such as feed-forward control,second-order control, and cascade control when used with a controller featuringthese functions.

The basic trainer demonstrates PID (proportional, integral, derivative) control of flow,pressure, and level processes. It comes with a variable-speed pump, a tank, acolumn, two-way valves, pressure gauges, flexible hoses, a venturi tube, an orificeplate, a rotameter, a paddle wheel flow transmitter, and a differential pressuretransmitter. A work surface provides a large area on which components can bemounted. Additional work area can be added with the provided expanding worksurface.

The trainer processes can be controlled by a computer-based PID Controllerimplemented with a personal computer (Pentium type), the included Process Controland Simulation Software (LVPROSIM), Model 3674, and the I/O Interface, Model9065. The trainer processes can also be controlled using any conventional PIDcontroller compatible with standard 4-20 mA signals or 0-5 V signals.

To demonstrate PID control of temperature and pH processes, additionalcomponents can be added to the basic trainer. The additional components requiredfor temperature process control include a heating unit, a cooling unit, andtemperature transmitters. Those required for pH process control include chemicaltanks, metering pumps, and a pH transmitter. Cascade and second-order processcontrol can also be studied with the basic trainer by having the students from twoworkstations work together at a single workstation, or by adding the followingcomponents to the basic trainer: a pressure transmitter, a column, and a PIDcontroller.

PRESSURE, FLOW, AND LEVEL PROCESS CONTROL

Courseware Outline

VII

Unit 1 Introduction to Process Control

Objective of process control. Open- and closed-loop process control.Process Instrumentation. I.S.A. instrumentation symbols.

Ex. 1-1 Familiarization with the Training System

Introduction to the Process Control Training System.Connection and operation of a basic flow circuit.

Unit 2 Pressure Processes

Nature of fluids. Definition and measurement of pressure. Pressure in awater system.

Ex. 2-1 Pressure Measurement

Types of pressure measurement devices. Construction andoperation of liquid manometers, Bourdon-tube pressuregauges, and strain-gauge pressure transmitters. Measuringpressure with a pressure gauge, a pressure transmitter, and aliquid manometer.

Ex. 2-2 Pressure Losses

Characteristics of fluids. Types of fluid flow. Reynolds number.Definition of pressure loss. Factors affecting pressure loss.Measuring the pressure losses in a water system.

Ex. 2-3 Centrifugal Pumps

Basic operation of a liquid pump. Types of liquid pumps.Construction and operation of a centrifugal pump. Performancechart, cavitation, NPSHR and NPSHA. Measuring the pressure-versus-flow curve of a centrifugal pump at different rotationspeeds.

Ex. 2-4 Characterization of a Pressure Process

Dynamic characteristics of a process. Capacitance andresistance of a process. Classification of processes.Characterizing a pressure process by using the open-loop stepresponse method.

Unit 3 Flow Processes

Definition and measurement of flow rate. Types of devices used tomeasure the flow rate of liquids.

PRESSURE, FLOW, AND LEVEL PROCESS CONTROL (cont'd)

Courseware Outline

VIII

Ex. 3-1 Rotameters and Paddle Wheel Flow Meters

Construction and operation of rotameters and paddle wheelflow meters. Plotting the voltage-versus-flow curve of a paddlewheel flow transmitter.

Ex. 3-2 Orifice Plates

Bernouilli's principle. Introduction to differential-pressure flowmeters. Construction and operation of orifice plates. Beta ratio,vena contracta, and permanent pressure loss. Plotting andlinearizing the pressure drop-versus-flow curve of an orificeplate.

Ex. 3-3 Venturi Tubes

Construction and operation of venturi tubes. Throat, angle ofconvergence, and angle of divergence. Plotting and linearizingthe pressure drop-versus-flow curve of a venturi tube.

Ex. 3-4 Permanent Pressure Losses Compared

Definition and measurement of power. Power conversion in awater system. Power dissipated by a differential-pressure flowmeter. Calculation and comparison of the yearly electricity costsof an orifice place and a venturi tube of equivalent size.

Ex. 3-5 Characterization of a Flow Process

Characterizing a flow process by using the open-loop stepresponse method.

Unit 4 Level Processes

Measurement of level. Point level and continuous level measurementdevices.

Ex. 4-1 Float Switches

Construction and operation of float switches. Float switchapplications. Using a float switch to limit the amount of liquidpumped into a column.

Ex. 4-2 Differential-Pressure Level Meters

Principle of operation of differential-pressure level meters.Measuring the level of liquid in an open column with adifferential-pressure transmitter.


Courseware Outline

IX

Ex. 4-3 Zero Suppression and Zero Elevation

Calibrating a differential-pressure level meter so as tocompensate for the depression or elevation of its sensingelement relative to the reference level.

Ex. 4-4 Wet Reference Legs

Measuring the level of liquid in a pressurized column with adifferential-pressure level meter. Dry and wet reference legs.Compensation of the hydrostatic pressure caused by a wetreference leg.

Ex. 4-5 Characterization of a Level Process

Characterizing a level process by using the open-loop stepresponse method.

Ex. 4-6 Bubblers (Optional Exercise)

Principle of operation of bubblers. Measuring the level of liquidin an open column by using a bubbler.

Unit 5 PID Process Control

Closed-loop control of a process. The PID controller. Types ofPID controller configurations. Tuning of a PID controller.

Ex. 5-1 Pressure Process Control

The trial and error method of controller tuning. PerformingPID control of a pressure process.

Ex. 5-2 Flow Process Control

The ultimate-cycle method of controller tuning. PerformingPID control of a flow process.

Ex. 5-3 Level Process Control

The open-loop step response method of controller tuning.Performing PID control of a level process.

Ex. 5-4 Cascade Process Control

Comparison of the single-loop and cascade control modes.Tuning of a cascade control system. Performing cascadecontrol of a level process.


Courseware Outline

X

Ex. 5-5 Second-Order Process Control

Transfer function of a process. First- and second-orderprocesses. Characterization of a second-order process usingthe open-loop step response method. Performing PID control ofa second-order level process.

Appendixes A Equipment Utilization ChartB Connection DiagramsC Detail of Pump ComponentsD I.S.A. Instrumentation Symbols (Meaning of

the letter codes used in tag numbers)E Selection Guide for Primary Sensing ElementsF Conversion FactorsG Using the Process Control and

Simulation Software (LVPROSIM)

Bibliography

We Value Your Opinion!

TEMPERATURE PROCESS CONTROL

Courseware Outline

XI

Unit 1 Introduction to Temperature Process Control

Definition of thermal energy and temperature. Basic principles of thermalenergy (heat) transfer. Mechanisms of thermal energy transfer: conduction,convection, and radiation.


The heating and cooling modes of control of temperatureprocesses. Familiarization with the components of the ProcessControl Training System that are used for the measurement andcontrol of temperature.

Unit 2 Temperature Measurement

Temperature scales. Conversion between Celsius and Fahrenheit. Typesof temperature measurement devices commonly used in industrialprocesses. Static and dynamic characteristics of temperature measurementdevices.

Ex. 2-1 Resistance Temperature Detectors (RTDs)

Construction and operation of RTDs. Comparison of the resistance-versus-temperature relationships of the most common types ofRTDs. Nominal resistance, temperature coefficient, and sensitivity.Measurement of the voltage produced by an RTD with aWheatstone bridge.

Ex. 2-2 Thermocouples

The Seebeck effect. Construction and operation of thermocouples.Comparison of the voltage-versus-temperature relationships of themost common types of thermocouples. Cold junction compensation.Thermocouple sensitivity to noise.

Ex. 2-3 Thermal Energy Transfer in Temperature Processes

Measurement of thermal energy. Specific heat capacity. Conversionof energy in temperature processes. Thermal energy balance.Determining the rate at which thermal energy is gained or lost bywater in a temperature process at thermal equilibrium.

Unit 3 Characterization of Temperature Processes

The Ziegler-Nichols and 28.3%-63.2% methods of process characterization.

TEMPERATURE PROCESS CONTROL (cont'd)

Courseware Outline

XII

Ex. 3-1 Characterization of a Temperature Processin the Heating Mode

Characterizing a temperature process in the heating mode by usingthe Ziegler-Nichols method of process characterization.

Ex. 3-2 Characterization of a Temperature Processin the Cooling Mode

Characterizing a temperature process in the cooling mode by usingthe 28.3%-63.2% method of process characterization.

Unit 4 PI Control of Temperature Processes

The control of temperature in the heating and cooling modes in a chemicalconversion process.

Ex. 4-1 PI Control of a Temperature Process in theHeating Mode

Review of the principles of the ultimate-cycle method of PI controllertuning. Performing PI control of a temperature process in theheating mode.

Ex. 4-2 PI Control of a Temperature Process in theCooling Mode

Performing PI control of a temperature process in the cooling mode.Calculation and tuning of the controller P and I constants, based onthe process characteristics measured in Exercise 3-2 with the28.3%-63.2% method of process characterization.

Appendices A Equipment Utilization ChartB Connection DiagramsC I.S.A. Instrumentation Symbols (Meaning of

the letter codes used in tag numbers)D Selection Guide for Temperature Sensing ElementsE Conversion FactorsF Using the Process Control and Simulation

Software (LVPROSIM)

Bibliography


pH PROCESS CONTROL

Courseware Outline

XIII

Unit 1 Introduction to pH Process Control

Concentration units. Definition of pH, acid solution, alkaline solution, andbuffer solution. Properties of acids and bases. Dissociation of acids andbases. Analysis of titration curves.


Familiarization with the components of the Process ControlTraining System that are used for the measurement and controlof pH. Introduction to the laboratory safety rules.

Unit 2 pH Measurement

Acids and bases characteristics. pH scale. Temperature effect on pH. Staticand dynamic characteristics of pH measurement devices. Mixing in pHprocess reactors.

Ex. 2-1 pH Electrodes

An overview of the pH probe and of the pH Transmitter from theProcess Control Training System. Operation of the pHTransmitter in both the fixed and the variable calibration modes.Measuring the pH of different solutions using the pH Probe.

Ex. 2-2 Titration of a Strong Acid

Strong acids and strong bases. Calculation of the pH of a strongacid solution and of a strong base solution . Titration of a strongacid solution with a strong base solution. The effect of badmixing inside a reactor.

Ex. 2-3 Titration of Weak Acids

The 5% rule. Weak acids and weak bases. Calculation of the pHof a weak acid solution and of a weak base solution. Titration ofweak acid solutions with a strong base.

Ex. 2-4 Titration of a Buffer Solution

Definition of buffer solution and buffer capacity. Using a sodiumbicarbonate solution as a buffer. Titration of a buffer solution witha weak acid.

Unit 3 Characterization of pH Processes

Determining the dynamic characteristics of a process. The Ziegler-Nicholls,28.3%-63.2%, and approximative method of process characterization.

pH PROCESS CONTROL (cont'd)

Courseware Outline

XIV

Ex. 3-1 Characterization of a Strong-Acid pH Process

Characterizing a strong-acid pH process using theZeigler-Nichols method of process characterization.

Ex. 3-2 Characterization of a Weak-Acid pH Process

Characterizing a weak-acid pH process using the Zeigler-Nicholsmethod of process characterization.

Unit 4 pH Process Control

pH processes. Batch and continuous processes. Batch and continuousprocesses examples.

Ex. 4-1 PID and On-Off Control of a Batch pH Process

On-off controller. On-off controller with a dead band. PerformingPID and on-off control of a batch pH process.

Ex. 4-2 PID Control of a Continuous pH Process

The open-loop step response method of controller tuning.Performing PID control of a continuous pH process.

Appendices A Equipment Utilization ChartB Connection DiagramsC Physical ConstantsD Periodic Table of the ElementsE Units Conversion TableF Calibrating the pH TransmitterG Useful Mathematical FormulasH Using the Process Control and Simulation Software

(LVPROSIM)I Neutralizing the systemJ Rinsing the systemK Storage of the pH probeL Material Safety Data SheetsM Glossary

Bibliography


Sample Exercise

Extracted from

Pressure, Flow, and

Level Process Control

3

Exercise 4-2

Differential-Pressure Level Meters

EXERCISE OBJECTIVES

C To describe how differential-pressure level meters operate;C To describe the relationship between the hydrostatic pressure, density, and level

of liquid in a vessel;C To measure level in a column open to atmosphere by using a pressure

transmitter.

DISCUSSION

Measuring hydrostatic pressure in order to infer level

Unlike float switches, which allow detection of a single discrete level, differential-pressure level meters provide continuous measurement of the level of liquid in avessel.

Differential-pressure level meters do not measure the level directly. Instead theymeasure a parameter that varies directly with the level of liquid in the vessel: thehydrostatic pressure of the liquid. The hydrostatic pressure, also called hydraulichead, is caused by the weight of the liquid in the vessel. The hydrostatic pressurecorresponds to the vertical height of the liquid column which would occur if the samepressure was applied to the liquid.

The equation that relates the level of liquid in a vessel, h, to the hydrostatic pressureof the liquid, Pg, is as follows:

S.I. system of units:

where h = level of the liquid (m);Pg = hydrostatic pressure of the liquid (kPa, gauge);D = mass density of the liquid (kg/m3);g = gravitational acceleration (m/s2);

SG = specific gravity of the liquid (dimensionless).


4

U.S. system of units:

where h = level of the liquid (ft);Pg = hydrostatic pressure of the liquid (psig);gc = dimensional constant (lbmAft/lbfAs2);D = mass density of the liquid (lbm/ft3);g = gravitational acceleration (ft/s2);

SG = specific gravity of the liquid (dimensionless).

* As previously mentioned, a pressure of 1 kPa corresponds to a column (head) of water of 0.102 mat 15.5°C. Similarly, a pressure of 1 psi corresponds to a column of water of 2.31 ft at 60°F.

The equation shows that the level of the liquid varies in direct proportion to thehydrostatic pressure of the liquid. This direct relationship is true provided that thetemperature and the density of the liquid remain constant in the vessel.

Note that the volume of the liquid also varies in direct proportion to the hydrostaticpressure of the liquid, provided that the vessel is vertical and cylindrical in shape,and that the temperature and the density of the liquid remain constant.

By measuring the hydrostatic pressure of a liquid with a pressure transmitter, we canobtain a voltage or current proportional to the level of the liquid. This exercise will becentered on the measurement of level in vessels that are open to atmosphere.

Measurement of the level in open vessels

In vessels that are open to atmosphere, the level is inferred by measuring thehydrostatic gauge pressure of the liquid. To do this, either a gauge-pressuretransmitter or a differential-pressure transmitter can be used, as Figure 4-7 (a)shows.

Note: The pressure transmitters of Figure 4-7 are represented bythe letters "LT" (standing for level transmitter) rather than by theletters "PT" or "PDT". This occurs because flow diagrams representcomponents according to their function, not their construction.

• When a gauge-pressure transmitter (LT1) is used, the transmitter is connectedat the bottom of the vessel. The transmitter sensing line can contain the processliquid, or it can contain a filling fluid that is removed from direct contact with theprocess liquid by a seal.

• When a differential-pressure transmitter (LT2) is used, the high-pressure side ofthe transmitter is connected at the bottom of the vessel, while the low-pressureside is left open to atmosphere.


5

Figure 4-7. Measurement of the level in a vessel that is open to atmosphere.

In either case, the voltage or current generated by the pressure transmitter will varyin direct proportion to the level of the liquid in the vessel, as Figure 4-7 (b) shows.This relationship is true provided that the temperature and the density of the liquidremain constant in the vessel, which is normally the case.

The minimum measurable level is determined by where the primary sensing elementof the pressure transmitter is connected with respect to the bottom of the vessel. Theminimum level to be measured, or reference level, is adjusted by using the zero knobof the transmitter, as Figure 4-7 (b) shows. The reference level can be set at theminimum measurable level or above it.

The maximum measurable level is determined by the maximum height the liquid canreach above the primary sensing element of the pressure transmitter. The maximumlevel to be measured is adjusted by using the span knob of the transmitter, as


6

Figure 4-7 (b) shows. The maximum level to be measured can be set at themaximum measurable level or below it.

Advantages and limitations

The measurement of liquid level through the measurement of hydrostatic pressureis a method that is simple to apply, that requires minimum maintenance, and thatprovides a good accuracy, generally in the order of ±1% of the actual span.

However, this method cannot be used with liquids that crystallize as theirconcentration increases.

Moreover, a change in the temperature or density of the liquid dictates recalibrationof the transmitter. For example, heating the liquid will decrease the density of theliquid and, therefore, will increase its volume. Consequently, the level of the liquidwill increase but the weight (and therefore the hydrostatic pressure) of the liquid willremain the same, causing the pressure transmitter to indicate a level lower than theactual level. Note, however, that some manufacturers offer "smart" transmitters thatcan be programmed to compensate for the variation in liquid density.

Similarly, a change in the type of metered liquid dictates recalibration of the pressuretransmitter to account for the change in specific gravity (and therefore in density) ofthe liquid. Figure 4-8, for example, shows the transmitter output-versus-level curvesobtained for three liquids of differing specific gravities when the pressure transmitteris calibrated to accurately measure level at the specific gravity of water (1.00 at15.5°C/60°F and normal atmospheric pressure). The transmitter indicates a levelhigher than the actual level for the sulfuric acid and the chloroform. This occursbecause these denser (and therefore heavier) liquids produce a higher hydrostaticpressure for any given level.

Figure 4-8. Pressure transmitter-versus-level curves obtained for three liquids of differing specificgravities, when the transmitter is calibrated to accurately measure level at the specific gravity ofwater (1.00).


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Finally, in many installations, it is not possible to position the primary sensingelement of the pressure transmitter at exactly the same height as the desiredreference level. Thus, the sensing element may be located above or below thereference level.

In those applications, the zero adjustment of the pressure transmitter must becompensated to account for the elevation or depression of its sensing element withrespect to the desired reference level. You will learn more about this in the exercisesto follow.

Procedure summary

In this exercise, you will use a pressure transmitter to measure the level of the waterin a column that is open to atmosphere. You will plot the relationship between thetransmitter voltage and the level, demonstrating that the voltage varies linearly withthe level.

EQUIPMENT REQUIRED

Refer to the Equipment Utilization Chart in Appendix A of the manual to obtain thelist of equipment required to perform this exercise.

PROCEDURE

System setup

G 1. Get the Pressure Transmitter and Column from your storage location.

Mount these components on the Expanding Work Surface so that thepressure ports of the Pressure Transmitter are approximately one row ofperforations above the bottom of the Column.

G 2. Set up the system shown in Figure 4-9. Make sure to connect the Rotameteroutlet to the port of the Column which is attached to a pipe that extendsdown into column, which will reduce agitation of the water in the Column. Onthe Column, make sure the cap of the insertion opening of the Float Switchis tightened firmly.

Note: Level transmitter LT1 in Figure 4-9 is actually a pressuretransmitter. This occurs because flow diagrams representcomponents according to their function, not their construction.

Note: The Column will first be operated in the pressurized modein order to purge air from the hose connecting the bottom of theColumn to the Pumping Unit. Failure to purge air from this hosecan prevent the water in the Column from decreasing below acertain level when the pump speed is decreased or the pump isstopped.


8

Figure 4-9. Measuring the water level in an open column by using a Pressure Transmitter.

G 3. Power up the Pressure Transmitter.

G 4. Make the following settings on the Pressure Transmitter:

ZERO adjustment knob . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MAX.SPAN adjustment knob . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MAX.LOW PASS FILTER switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . I (ON)

G 5. Make sure the reservoir of the Pumping Unit is filled with about 12 liters(3.2 gallons US) of water. Make sure the baffle plate is properly installed atthe bottom of the reservoir.

G 6. On the Pumping Unit, adjust valves HV1 to HV3 as follows:

– Open HV1 completely;– Close HV2 completely;– Set HV3 for directing the full reservoir flow to the pump inlet.


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G 7. Turn on the Pumping Unit.

G 8. With the controller in the manual (open-loop) mode, set the controller outputat 100%, which will cause the water level to rise in the Column.

G 9. Close valve HV1 of the Pumping Unit completely, which will cause the waterlevel to rise further in the Column.

G 10. Set the controller output at 0% to stop the Pumping Unit. This will cause partof the water in the Column to siphon back out the tube in the Column.

G 11. Remove the plug connected to the hose port at the top of the Column.Connect this port to either of the auxiliary return ports of the Pumping Unit,using an extra-long hose.

This hose will serve as an overflow if the Column gets full. Moreover, it willcause the Column to be open to atmosphere through the reservoir of thePumping Unit.

G 12. Set the controller output at 50% to make the water level rise into theColumn. Then open valve HV1 of the Pumping Unit completely.

Transmitter calibration

Note: In steps 13 through 17, you will be adjusting the ZERO andSPAN knobs of the Pressure Transmitter so that its output voltagevaries between 0.00 and 5.0 V DC when the level of the water inthe Column is varied between 5 and 56 cm (2 and 22 in).

G 13. Connect a DC voltmeter to the 0-5 V OUTPUT of the Pressure Transmitter.

G 14. Adjust the controller output until the water level is stable at 5 cm (2 in) in theColumn. This will be the reference level.

Note: From now on, it is important that the water level in theColumn not be allowed to fall below 4 cm (1.5 in), as this wouldcause air to enter the line between the bottom of the Column andthe Pumping Unit, which in turn would prevent the water in theColumn from dropping below a certain level when the pumpspeed is reduced or the pump is stopped. Should this situationoccur, you will have to purge air from this line by placing theColumn in the pressurized mode as shown in Figure 4-9 and thenrepeating procedure steps 4 through 12.


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G 15. While observing the DC voltmeter reading, turn the ZERO adjustment knobof the Pressure Transmitter counterclockwise to decrease the voltage andstop turning it as soon as the voltmeter reads 0.00 V.

G 16. Readjust the controller output to raise and stabilize the water level to 56 cm(22 in) in the Column.

G 17. Adjust the SPAN knob of the Pressure Transmitter until the DC voltmeterreads 5.0 V.

Note: Due to interaction between the ZERO and SPANadjustments of the Pressure Transmitter, it will be necessary torepeat steps 14 through 17 until the Pressure Transmitter outputvoltage actually varies between 0.00 and 5.0 V DC when the levelof the water is varied between 5 and 56 cm (2 and 22 in).

Determining the relationship between the level and the pressure transmittervoltage

G 18. By varying the controller output, raise the water level in the Column from 5to 55 cm by steps of 5 cm (or from 2 to 22 in by steps of 2 in).

After each new level setting, measure the voltage generated by thePressure Transmitter and record it in Table 4-1.

LEVEL [cm (in)] PRESSURE TRANSMITTERVOLTAGE (V)

5 cm (2 in) 0.00 V

Table 4-1. Voltage generated by the Pressure Transmitter as a function of the water level in theopen Column.


11

G 19. Stop the variable-speed drive of the Pumping Unit by setting the controlleroutput at 0%.

G 20. From the data recorded in Table 4-1, plot in Figure 4-10 the relationshipbetween the water level and the Pressure Transmitter voltage.

G 21. According to the curve obtained in Figure 4-10, does the voltage generatedby the Pressure Transmitter increase linearly as the level is increased?Why?

G 22. If the water in the Column was replaced by mercury without recalibrating thePressure Transmitter, would the transmitter indicate higher or lower than theactual liquid level? Explain.


12

Figure 4-10. Relationship between the water level and the Pressure Transmitter voltage.

G 23. Turn off the Pumping Unit by setting its POWER switch at O.

G 24. Disconnect the circuit. Return the components and hoses to their storagelocation.


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G 25. Wipe off any water from the floor and the Process Control Training System.

CONCLUSION

In this exercise, you measured the voltage produced by a pressure transmitter as afunction of the water level in an open column. This allowed you to see that thehydrostatic gauge pressure of the water in the column varies in direct proportion tothe water level in the column. This direct relationship is true, provided that thedensity and the temperature of the water remain constant.

REVIEW QUESTIONS

1. How does the level of the liquid in an open vessel vary with the hydrostaticgauge pressure of the liquid in the vessel, for any given density andtemperature?

2. What effect does increasing the temperature of the liquid in a vessel have on thedensity and level of the liquid?

3. What is the level of the water in an open vessel if the hydrostatic pressure of thewater is 100 kPa, gauge (14.5 psig) at 15.5°C (60°F)?

4. If the liquid in a vessel was changed for a liquid of lower specific gravity than thatat which the pressure transmitter was calibrated, would the transmitter readhigher or lower than the actual liquid level? Explain.


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5. How could a pressure transmitter calibrated to accurately read level at a specificgravity of 1.00 be used to measure the level of a liquid with a specific gravity of2.00?

Sample Exercise

Extracted from

Temperature Process Control

17

Exercise 2-1

Resistance Temperature Detectors (RTDs)

EXERCISE OBJECTIVES

• To explain how resistance temperature detectors (RTDs) operate;• To describe the relationship between the temperature and the electrical

resistance of the most common types of RTDs;• To define the following terms: nominal resistance, temperature coefficient, and

sensitivity.• To explain how a Wheastone bridge can be used to measure the voltage

produced across an RTD.

DISCUSSION

Electrical resistance

An important characteristic of all metals is their electrical resistance. Electricalresistance is the opposition of the metal to the flow of electrical current. Electricalresistance is measured in ohms (S) in both the S.I. and U.S. systems of units.

The electrical resistance of a metal is dependent upon the temperature at which themetal is. Figure 2-4, for example, shows what happens to the relative resistance ofdifferent metals as their temperature increases. The relative resistance is the ratiobetween the resistance at the applied temperature to the resistance at a referencetemperature of 0°C (32°F).

As the figure shows, the relative resistance of the metals increases as theirtemperature gets higher. Moreover, the relative resistance increases almost linearlywith temperature, at least over a substantial range of temperatures. Besides, therelative resistance of nickel increases more sharply with temperature than that ofcopper or platinum.


18

Figure 2-4. Relative resistance-versus-temperature relationship of different metals.

Temperature coefficient

All metals have a specific temperature coefficient that indicates their averagechange in relative resistance per unit of temperature between 0 and 100°C (between32 and 212°F).

The temperature coefficient is symbolized by the Greek letter alpha ("). It is usuallymeasured in ohms per ohm degree Celsius (°C-1) or in ohms per ohm degree Fahr-enheit (°F-1).

Figure 2-4, for example, indicates that the temperature coefficient of platinum is0.00392°C-1 (0.00218°F-1). Consequently, the relative resistance of platinum variesby 0.392 between 0 and 100°C (32 and 212°F). Beyond 100°C (212°F), the shapeof the platinum curve indicates that the temperature coefficient decreases slightly asthe temperature gets higher.


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Resistance temperature detectors

A resistance temperature detector (RTD) is a primary element that is used to sensetemperature. The RTD works on the principle that the electrical resistance of metalschanges with temperature.

The RTD consists of a metallic conductor usually wound into a coil. The RTD is tobe connected to an electrical circuit in order to make a constant excitation currentflow through it. As the temperature increases, the electrical resistance of the metallicconductor increases and, therefore, the voltage across the RTD increases.

Consequently, by measuring the voltage across the RTD, a signal proportional to thetemperature of the RTD can be obtained. This signal can be conditioned into acurrent, voltage, or pressure of normalized range that is suitable for instrumentationand control, the combination of the RTD and the conditioning circuit thus forming atemperature transmitter.

RTD metals

The selection of a metal for use as an RTD depends on several factors. Amongthese, the most important are the capability to follow rapidly changing temperatures,a good linearity, a good reproducibility, and a relatively high change of resistance fora given change in temperature (i.e. a high temperature coefficient).

The metals most commonly used for RTDs are platinum, nickel, and copper (referto Figure 2-4):

• Platinum is the preferred metal for RTDs. It has been chosen as theinternational standard metal for RTD temperature measurement. Platinum hasa nearly linear resistance-versus-temperature relationship over a widetemperature range. Platinum offers good stability and reproducibility. It is well-suited for the measurement of high temperatures up to 650°C (1200°F).

• Nickel is the second mostly used metal for RTDs. It is less expensive thanplatinum and it is more sensitive because of its higher temperature coefficient.However, nickel has a narrower sensing range than platinum and is limited to themeasurement of temperatures below 300°C (570°F).

• Copper is the least expensive of the three metals and it has the most linearrelationship. Similar to platinum, copper is well suited for the measurement ofhigh temperatures. However, copper is subject to oxidation, and it has poorerstability and reproducibility than platinum.

RTD characteristics

Two important characteristics of RTDs are their nominal resistance and theirtemperature coefficient:

• The nominal resistance is the resistance of the RTD at a given referencetemperature, as specified by the manufacturer. Platinum RTDs, for example, areusually designed so that their nominal resistance is 100 S at the ice referencepoint of 0°C (32°F).


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• The temperature coefficient is the mean change in relative resistance of themetal per unit of temperature between 0 and 100°C (32 and 212°F), aspreviously explained.

The nominal resistance and the temperature coefficient of an RTD determine thesensitivity of the RTD within the 0-100°C (32-212°F) temperature range. Thesensitivity is the amount by which the resistance of the RTD will change per unit oftemperature, in S/°C (or S/°F).

For example, a platinum RTD having a nominal resistance of 100 S at 0°C (32°F)and a temperature coefficient of 0.00392°C-1 (0.00218°F-1) will have a sensitivity of0.392 S/°C (0.218 S/°F) within the 0-100°C (32-212°F) temperature range.

Measurement of the voltage across an RTD

As previously mentioned, the voltage produced across an RTD, which is directlyproportional to temperature, can be used for process instrumentation and control.

The traditional method of measuring the voltage across an RTD is to use aWheatstone bridge, as Figure 2-5 (a) shows.

• The RTD and its two lead wires constitute one leg of the bridge. Resistors R1 andR2 are of equal resistance, while resistor R3 is adjustable and is used as areference.

• A DC voltage source supplies an excitation current to the RTD.

• A differential amplifier produces a voltage VO proportional to the bridge outputvoltage (measured between points a and b).

With the RTD placed in an ice bath at 0°C (32°F), resistor R3 is initially adjusted inorder to obtain a null voltage (0 V) at the output of the differential amplifier. In thiscondition, the bridge is said to be null balanced.

Once the bridge has been null balanced, the amplifier output voltage will vary indirect proportion to the temperature of the RTD.


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Figure 2-5. Measurement of the voltage across an RTD.

If the two leads that connect the RTD to the bridge are more than a few centimeters(inches) long, they will introduce an error in the temperature measurement. Thisoccurs because the resistance of the leads will contribute to the voltage producedat the output of the bridge, causing the measured temperature to be higher than thatactually measured.

To minimize this error, RTDs are available in a three-wire version. The extra wire isused to cancel the resistances of lead wires 1 and 2 by balancing the bridge, asFigure 2-5 (b) shows. This has the effect of removing the error produced by leadwires 1 and 2 as long as these wires are of equal resistance (i.e. of equal length andtemperature).

Advantages and limitations of RTDs

RTDs have the following advantages: they provide a good sensitivity, a goodreproducibility, and a good stability. They also provide a high accuracy, someplatinum RTDs being able to measure a few thousandths of a degree.

However, RTDs are relatively expensive, and they have a slower response time thanthermocouples. Moreover, the measurement accuracy of RTDs is dependent uponthe thermal stability of the resistors and power supply used in the Wheatstonebridge.

The RTD probe and the RTD Temperature Transmitter of the Process ControlTraining System

The Process Control Training System comes with a three-wire RTD probe that usesa platinum RTD of 100 S at 0°C (32°F). The RTD probe is intended to be used withthe RTD Temperature Transmitter to measure the temperature of the water in thetrainer Column, as Figure 2-6 shows.


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The tip of the RTD probe, which contains the RTD, is to be inserted into the Columnthrough the opening of the Float Switch. The other end of the RTD probe, which hasthree leads, is to be connected to the "100-S RTD" terminals of the RTDTemperature Transmitter.

The RTD Temperature Transmitter produces an excitation current through the RTDand it measures the resulting voltage produced across the RTD. This voltage, whichis proportional to the temperature of the RTD, is conditioned into normalized voltagesand current that are available at the transmitter OUTPUTS.

The transmitter also contains a calibration source that can be used to simulate thevoltage produced across the RTD for any RTD temperature comprised between 0and 100°C (32 and 212°F). The source eliminates the need to set the RTD at a well-known temperature when performing calibration of the transmitter OUTPUTS.

The following is a detailed description of the RTD Temperature Transmitter terminalsand adjustments (refer to Figure 2-6):

Î POWER INPUT terminals: used to power the transmitter with a DC voltageof 24 V.

Ï CALIBRATION SOURCE adjustment knob: sets the probe temperature tobe simulated by the calibration source signal. This temperature can beadjusted between 0 and 100°C (32 and 212°F).

Ð INPUT SELECTOR: selects between the actual probe signal or thesimulated probe signal produced by the calibration source.

Ñ CALIBRATION SELECTOR switch: places the 0-5 V and 4-20 mAOUTPUTS in either fixed or variable calibration mode.

Ò ZERO and SPAN adjustment knobs: used in the variable calibration mode(CALIBRATION SELECTOR switch at VARIABLE) to set the temperaturerange for which the 0-5 V and 4-20 mA OUTPUTS will pass from minimumto maximum:

– The ZERO knob sets the temperature for which the outputs will beminimum (0 V and 4 mA), i.e. the minimum temperature to bedetected. The minimum temperature can be adjusted between 0 and50°C (32 and 122°F).

– The SPAN knob sets the temperature for which the outputs will bemaximum (5 V and 20 mA), i.e. the maximum temperature to bedetected. The maximum temperature can be adjusted between 15and 30°C (27 and 54°F) above the minimum temperature set by theZERO knob.


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Figure 2-6. The RTD probe and RTD Temperature Transmitter of the Training System.

Ó CALibrated OUTPUT: provides a voltage proportional to the temperaturesensed by the probe or to the simulated probe signal produced by thecalibration source, depending on the position of the INPUT SELECTORswitch.


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This output has a fixed calibration of 100 mV per sensed °C above 0°C(or 56 mV per sensed °F above 32°F). It will pass from 0 to 10 V whenthe actual or simulated temperature changes from 0 to 100°C (32 to212°F).

Ô 0-5 V and 4-20 mA OUTPUTS terminals: provide a voltage and a currentproportional to the temperature sensed by the probe or to the probetemperature signal simulated by the calibration source, depending on theposition of the INPUT SELECTOR switch.

The calibration of the 0-5 V and 4-20 mA OUTPUTS can be either fixedor variable, depending on the position of the CALIBRATIONSELECTOR switch:

– In the fixed calibration mode (CALIBRATION SELECTOR switch atFIXED), the temperature range for which the outputs will pass fromminimum to maximum is fixed and is 0-100°C (32-212°F).

– In the variable calibration mode (CALIBRATION SELECTOR switchat VARIABLE), the temperature range for which the outputs will passfrom minimum to maximum can be adjusted by means of the ZEROand SPAN adjustment knobs.

Õ 100-S RTD input terminals: used to connect the RTD probe to thetransmitter.

Procedure summary

In the first part of the exercise, you will familiarize yourself with the operation of anRTD Temperature Transmitter in the fixed calibration mode.

In the first part of the exercise, you will familiarize yourself with the operation of anRTD Temperature Transmitter in the variable calibration mode.

In the third part of the exercise, you will set up and operate a temperature process.You will use an RTD Temperature Transmitter to measure the temperature of thewater in a column.

EQUIPMENT REQUIRED



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PROCEDURE

Operation of the RTD Temperature Transmitter in the fixed calibration mode

G 1. Get the RTD Temperature Transmitter and 24-V DC Power Supply fromyour storage area. Mount these components on the Main Work Surface.

G 2. Power up the RTD Temperature Transmitter.

G 3. Get the RTD probe from your storage location and connect it to the100-S RTD input of the RTD Temperature Transmitter.

Let the probe tip lie on the Work Surface.

G 4. Make the following settings on the RTD Temperature Transmitter:

INPUT SELECTOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RTDCALIBRATION SELECTOR . . . . . . . . . . . . . . . . . . . . . . . . . . FIXED

This selects the RTD probe signal as the transmitter input signal and placesthe transmitter OUTPUTS in the fixed calibration mode.

G 5. Connect a DC voltmeter to the 0-5 V OUTPUT of the RTD TemperatureTransmitter.

Since this output is in the fixed calibration mode, it generates a fixed voltageof 50 mV per sensed °C above 0°C (or 28 mV per sensed °F above 32°F).

According to the voltmeter reading, what is the ambient temperature?

G 6. Further experiment with the operation of the transmitter in the fixedcalibration mode:

– Fill a suitable container with ice water (a mixture of ice cubes andwater).

– Immerse the tip of the RTD probe into the ice water. The 0-5 VOUTPUT voltage should decrease and stabilize at about 0.0 V, which,in the fixed calibration mode, corresponds to an RTD temperature of0°C (32°F).

– Fill a suitable container with boiling water heated by an electric kettle ora microwave oven.

– Immerse the tip of the RTD probe into the boiling water. The 0-5 VOUTPUT voltage should increase and stabilize at about 5.0 V, which,


26

in the fixed calibration mode, corresponds to an RTD temperature of100°C (212°F).

Note: The 0-5 V OUTPUT of the RTD Temperature Transmitterwill stabilize at a voltage lower than 5.0 V if the atmosphericpressure is lower than 101.3 kPa, absolute (14.7 psia).

Record below your observations.

Operation of the RTD Temperature Transmitter in the variable calibration mode

Note: In the following steps, you will use the calibration sourceof the RTD Temperature Transmitter to calibrate its 0-5 VOUTPUT so that the voltage at this output passes from 0.0 to5.00 V when the probe temperature simulated by the calibrationsource passes from 25 to 55°C (77 to 131°F), respectively.

G 7. Make the following settings on the RTD Temperature Transmitter:

INPUT SELECTOR . . . . . . . . . . . . . . . . . . . . . . . . . . CAL. SOURCECALIBRATION SELECTOR . . . . . . . . . . . . . . . . . . . . . . . VARIABLEZERO adjustment knob . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MAX.SPAN adjustment knob . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MAX.

This selects the calibration source signal as the transmitter input signal andplaces the transmitter OUTPUTS in the variable calibration mode.

G 8. Set the probe temperature to be simulated by the calibration source of thetransmitter at 25°C (77°F).

To do so, adjust the CALIBRATION SOURCE knob of the transmitter untilyou obtain a voltage of 2.5 V at the CAL. OUTPUT of the transmitter.

G 9. While monitoring the voltage at the 0-5 V OUTPUT of the transmitter, turnthe ZERO adjustment knob counterclockwise and stop turning it as soon asthe voltage ceases to decrease, which should occur around 0.01 V. Thenvery slowly turn the knob in the clockwise direction and stop turning it assoon as the voltage starts to increase.

This sets the minimum temperature to be detected at 25°C (77°F)approximately.

G 10. Now set the probe temperature to be simulated by the calibration source ofthe transmitter at 55°C (131°F).

To do so, adjust the CALIBRATION SOURCE knob of the transmitter untilyou obtain a voltage of 5.5 V at the CAL. OUTPUT of the transmitter.


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G 11. Adjust the SPAN knob in order to obtain a voltage of 5.00 V at thetransmitter 0-5 V OUTPUT.

This sets the maximum temperature to be detected at 55°C (131°F)approximately.

G 12. Now that the RTD Temperature Transmitter is calibrated, proceed to thenext part of the exercise.

Measuring temperature with an RTD

Preliminary setup

G 13. Get the Expanding Work Surface from your storage location and mount itvertically (at an angle of 90°) to the Main Work Surface, if this has notalready been done.

G 14. Connect the system shown in Figure 2-7, being careful not to modify thecalibration settings just made on the RTD Temperature Transmitter.Figure 2-8 shows the suggested setup.

The speed of the variable-speed drive of the Pumping Unit will be controlledwith a controller, FIC1, placed in the manual (open-loop) mode. The Heatingand Cooling Units will be controlled manually. (This is the reason why thereis no temperature controller, or "TC" instrumentation symbol illustrated nextto these units in the flow diagram of Figure 2-7.)

The Column will first be operated in the pressurized mode in order to purgeair from the components downstream of the Column. Consequently, let thetip of the RTD probe lie on the Work Surface for now.

Note: Make sure to mount the Heating Unit at the highestpossible location on the Expanding Work Surface, in order for thisunit to be above the other process components, as Figure 2-8shows. Failure to do so may result in water entering the HeatingUnit upon disconnection of the hoses, which in turn might causedamage to the Heating Unit.

Moreover, mount the 24-V DC Power Supply and the RTDTemperature Transmitter in such a manner that water cannotenter these components and their electrical terminals when hosesare disconnected.

The Heating Unit must be connected for the direction of flowindicated by the arrow heads in the symbol on its front panel.


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On the other hand, the Cooling Unit will operate regardless of thedirection of water flow through it. However, to minimize the risk ofcavitation caused by air suction within the pump when the waterbecomes hot, connect the Cooling Unit as indicated in Figure 2-8,that is, with the upper unit port used as the hot water inlet and thelower unit port used as the cooled water outlet. For the samereason, mount the Column at the highest possible location on theExpanding Work Surface in order to create a substantial head ofwater upstream of the Cooling Unit.

Note: If the controller you are using as flow controller FIC1 is theLab-Volt Process Control and Simulation Software (LVPROSIM),model 3674, you can refer to Figure B-1 of Appendix B for detailsof how to connect the LVPROSIM computer to the variable-speeddrive (SC1) of the Pumping Unit.

Figure 2-7. Measuring temperature with an RTD temperature transmitter.


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Figure 2-8. Suggested setup for the diagram of Figure 2-7 (see table next page for the detail of thecomponents).


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Î : Column Ò : Cooling Unit

Ï : Heating Unit Ó : RTD probe

Ð : Paddle Wheel Flow Transmitter Ô : RTD Temperature Transmitter

Ñ : Pumping Unit Õ : DC Power Supply

G 15. Make the following settings:

On the Heating Unit:

S1 switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Manual control knob . . . . . . . . . . . . turned fully counterclockwise

On the Cooling Unit:

S1 switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Manual control knob . . . . . . . . . . . . turned fully counterclockwiseS2 switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

On the RTD Temperature Transmitter:

SELECTOR switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RTD

Note: The 0-5 V OUTPUT of the RTD Temperature Transmittershould still be calibrated for a temperature measurement range of25-55°C (77-131°F) from the first part of the exercise.

G 16. Power up the Heating Unit:

– Connect the AC line cord of this unit to a wall outlet.– Set the POWER switch at I.

G 17. Power up the Cooling Unit and the Paddle Wheel Flow Transmitter byconnecting their POWER INPUT terminals to the 24-V DC Power Supply.

Purging air from the components downstream of the Column

G 18. Make sure flow controller FIC1 is in the manual (open-loop) mode. Set theoutput of this controller at 0% (0 V).

G 19. On the Column, make sure the cap of the insertion opening of the FloatSwitch is tightened firmly.


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G 20. Make sure the reservoir of the Pumping Unit is filled with about 12 liters(3.2 gallons US) of water. Make sure the baffle plate is properly installed atthe bottom of the reservoir.

G 21. Turn on the Pumping Unit by setting its POWER switch at I.

G 22. On the Pumping Unit, adjust valves HV1 through HV3 as follows:

– Open HV1 completely;– Close HV2 completely;– Set HV3 for directing the full reservoir flow to the pump inlet (turn handle

fully clockwise).

G 23. Make the variable-speed drive of the Pumping Unit rotate at the maximumspeed: with controller FIC1 in the manual (open-loop) mode, set thecontroller output at 100% (5 V).

G 24. Allow the level of the water to rise in the pressurized Column until itstabilizes at some intermediate level. This will force air out of thecomponents downstream of the Column.

Note: If the cap of the insertion opening of the Float Switch onthe Column has not been tightened firmly, air will be allowed toescape from the Column and the water level will not stabilize inthe Column. Should this case occur, stop the variable-speed driveof the Pumping Unit. Open valves HV1 and HV2 of the PumpingUnit in order to drain the Column to the reservoir. When theColumn is empty, tighten the cap of the insertion opening of theFloat Switch on the Column with more force. Then resume theprocedure from step 22.

Placing the system in the water recirculating mode

Note: In the following steps, you will place the system in thewater recirculating mode by setting the Pumping Unit valves so asto direct the return flow directly to the pump inlet, not to thereservoir. This will reduce the time required to raise or decreasethe temperature of the process water. For the same reason, thewater level in the Column will be set at a low, minimum level of7.5 cm (3 in).

G 25. On the Pumping Unit, close valve HV1, which will cause the water level torise further in the Column. Then set valve HV3 for directing the full returnflow directly to the pump inlet (turn handle fully counterclockwise).

G 26. On the Column, remove the cap of the insertion opening of the Float Switchto depressurize the Column. (The water level in the Column will remainstable).


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G 27. On the Pumping Unit, open valve HV2 in order to decrease the water levelin the Column to 7.5 cm (3 in), then close this valve.

G 28. Readjust the output of controller FIC1 until you read 4.0 V approximately atthe "F (cal.)" output of the Paddle Wheel Flow Transmitter. This will set theflow rate at about 4 l/min (1.1 gal US/min).

Note: Small, continuous variations of a few tenths of volts aroundthe adjusted mean value of 4.0 V are normal at the flowtransmitter output.

However, large variations of one volt or more are abnormal, andindicate that air has entered the system through an untightconnector or component on the suction side of the pump.

Should that case occur, stop the variable-speed drive of thePumping Unit in order to drain the column to the reservoir. Whenthe Column is empty, check the inside of the connector on thePumping Unit return line hose for any dirt or particles. Also, checkthe o-rings on the two hose connectors of the Cooling Unit for anyfissure or crack. Once you have located and eliminated the causeof the leak, reconnect the system as in Figure 2-7 and resume theprocedure from step 19.

Measuring temperature with the RTD

G 29. Insert the RTD probe all the way into the Column in order for its tip to besubmerged in the water.

G 30. Have the signal at the 0-5 V OUTPUT of the RTD Temperature Transmitterplotted on the trend recorder of controller FIC1.

Adjust the update rate of the trend recorder (sampling interval) in order tobe able to monitor the transmitter signal over a period of 10 minutesapproximately.

Note: If the controller you are using as controller FIC1 is the Lab-Volt Process Control and Simulation Software (LVPROSIM),model 3674, refer to Figure B-5 of Appendix B for details of howto connect the LVPROSIM computer to the RTD TemperatureTransmitter. On the I/O Interface, make sure the RANGE switchof ANALOG INPUT 1 is set at 5 V.

In LVPROSIM, select Analog Input 1 from the Trend Recorderselection list to have the RTD Temperature Transmitter signalplotted on the trend recorder. Set the LVPROSIM samplinginterval at 1500 ms. Access the Configure Analog Inputs windowand set the minimum and maximum range values of AnalogInput 1 at 25 and 55°C (77 and 131°F), respectively, whichcorresponds to the current measurement range of the RTDTemperature Transmitter. Set the filter time constant of this inputat 0.5 second. Make sure the square root extracting function isunselected. Accept setup and return to main screen.


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G 31. On the trend recorder, observe the RTD Temperature Transmitter outputsignal.

Since no electrical power is applied to the heating element of the HeatingUnit, theoretically, the water in the Column should be at ambienttemperature.

Assuming that the ambient temperature is below 25°C (77°F), the level ofthe RTD Temperature Transmitter signal should be at 0% of span on thetrend recorder, since the minimum temperature the transmitter can detecthas been adjusted to 25°C (77°F).

Yet, you may observe that the RTD Temperature Transmitter signal is atsome higher level, thermal energy being transferred to the recirculatedwater mainly from frictional resistance of the pump internal parts.

G 32. On the Heating Unit, set the manual control knob to the mid position. On thetrend recorder, observe what happens to the temperature of the water in theColumn.

Now that electrical power is applied to the heating element of the HeatingUnit, thermal energy is transferred from this element to the recirculatedwater.

Consequently, the temperature of the water should increase in the Column.Is this your observation?

G Yes G No

G 33. Let the temperature of the water in the Column increase to about 45°C(113°F), or 67% of span, then turn the manual control knob of the HeatingUnit fully counterclockwise to remove electrical power from its heatingelement.

According to the RTD Temperature Transmitter output signal on the trendrecorder, did the temperature of the water in the Column increase linearlyover time?

How long did it take for the temperature to increase from the initialtemperature to the final temperature of 45°C (113°F)?


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G 34. On the Cooling Unit, turn the manual control knob fully counterclockwise.What happens to the temperature of the water in the Column?

G 35. Allow the temperature of the water in the Column to cool down.

According to the RTD Temperature Transmitter output signal, does thetemperature of the water in the Column decrease linearly over time?Explain.

G 36. Stop the variable-speed drive of the Pumping Unit by setting the output ofcontroller FIC1 at 0%.

G 37. Turn off the Pumping Unit, the Heating Unit, and the 24-V DC Power Supplyby setting their POWER switch at O.

G 38. Open valve HV1 of the Pumping Unit completely and let the water in theColumn drain back to the reservoir. The Column can also be drained bydisconnecting the end of the hose connected to the Cooling Unit inlet portand reconnecting it to either of the auxiliary return ports on the PumpingUnit.

G 39. Disconnect the system. Return all leads, hoses, and components to theirstorage location.

CAUTION!

Hot water may remain in the hoses and components. Becareful not to allow water to enter the electrical componentsand their terminals upon disconnection of the hoses.

G 40. Wipe off any water from the floor and the Process Control Training System.

CONCLUSION

In this exercise, you familiarized yourself with the operation of an RTD temperaturetransmitter in the fixed and variable calibration modes. You learned that, in the fixedcalibration mode, the temperature measurement range is fixed and is equal to 0-100°C (32-212°F). In the variable calibration mode, the temperature measurementrange can be adjusted, and a maximum span of 30°C (54°F) can be obtained. Sincethis span is narrower than the 100°C (180°F) span of the fixed calibration mode, thevariable calibration mode provides a greater measurement accuracy for any giventransmitter output range.


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REVIEW QUESTIONS

1. What is an RTD? How does an RTD work?

2. What are three metals commonly used for RTDs? What are the advantages andlimitations of each metal?

3. Name and describe two important characteristics of RTDs.

4. How is the voltage produced across an RTD traditionally measured?

5. Why are RTDs available in three-wire version? Explain.

Sample Exercise

Extracted from

pH Process Control

39

Exercise 2-1

pH Electrodes

EXERCISE OBJECTIVES

• To explain how pH electrodes operate;• To familiarize yourself with the components of the Process Control Training

System;• To operate the pH Transmitter in both the fixed and the variable calibration

modes;• To measure the pH of different solutions using the pH Probe.

DISCUSSION

The pH probe and the pH Transmitter of the Process Control Training System

pH probes

Probes used in pH measurement are usually made of glass because electricpotential between its surfaces changes linearly with pH. Typical pH probes are madeof two electrodes, one is the reference electrode providing a stable referencepotential, and the other is the glass measurement electrode.

The glass surface of the measurement electrode is coated with a thin layer ofhydrated gel with a thickness that varies between 10 and 100 nanometers,depending on the type of electrode. The wire of the measurement electrode isimmersed in a buffer solution containing Cl- ions. This buffer solution usually hasa pH of 7.

The reference electrode provides a reference potential to compare with the potentialof the measurement electrode. The electrolyte solution of the referenceelectrode (usually potassium chloride) is in contact with the process through ajunction (usually ceramic or Teflon). Ions migrate into the junction, establishing adiffusion potential. This configuration forms a galvanic half-cell.

Since the potassium chloride solution can flow slowly through the junction, care mustbe taken to avoid it running out. For this reason, the pH probe should always bestored in a storage solution containing potassium chloride. Figure 2-9 shows theprincipal components of a typical pH glass electrode.

pH Electrodes

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Figure 2-9. Typical pH glass electrode.

CAUTION!

The pH probe of the Process Control Training System must be storedin a storage solution containing KCl to avoid damage. Refer toAppendix K for more information on the storage of the pH probe.

When the pH probe is immersed in the process solution, an electric potential, E1, iscreated between the outer surface of the glass electrode and the process solution.An electric potential, E2, appears between the inner surface of the glass electrodeand the chloride buffer solution as well as an electric potential, E3, between theelectrolyte and the internal electrode. Hence, source voltage of the system is:

Ei = E1 ! E2 ! E3 + E4 + E5

Where E1 is the potential between the outer surface of the glass electrode andthe process solution.

E2 is the potential between the inner surface of the glass electrode andthe chloride buffer solution.

E3 is the potential between chloride buffer solution and the internalelectrode.

E4 is the potential between the electrolyte and the internal electrode ofthe reference electrode.

E5 is the diffusion potential of the reference electrode.

pH Electrodes

41

To simplify, we will assume that E3 is approximately equal to E4 which is frequentlythe case. The potential E5 is very small and can be neglected. Thus:

Ei = E1 ! E2

Figure 2-10 shows an equivalent electrical circuit representing some of the potentialsand resistance values involved in a pH probe.

Figure 2-10. Equivalent electrical circuit representing a pH probe.

Where ET is the potential at the poles of the transmitter or signal converter.RT is the input resistance of the transmitter or signal converter.RG is the resistance of the electrode glass.RR is the resistance of the reference electrode.RC is the insulation resistance between the connecting cables.RS is the resistance of the process solution.

Note: For the sake of simplicity, some small electrical resistance valuesand other correction terms have been neglected.

The potential of an electrode is given by the Nernst equation, when adapting thisequation for potential E1 and E2 the result is:

pH Electrodes

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Combining the last three equations, and given the fact that

, we have:

Where R is the universal gas constant (8.314472 JAK-1Amol-1).T is the temperature in Kelvins.F is the Faraday constant (9.6485309 x 104 CAmol-1).

pHINNER is the pH value inside the glass electrode (usually equal to 7.0).pHOUTER is the pH value of the process solution.

As shown in this equation, the pH measurement is also temperature dependant.Industrial pH probes are equipped with automatic temperature compensators toautomatically correct this kind of error. For pH probes without an automaticcompensator, a pH temperature error table such as Table 2-7 must be used forprecise measurements.

Using Ei, the voltage at the poles of the transmitter, ET, can be determined. Theresistance values of RR and RS can be neglected for the calculation since they arevery small compared to RT and RG (which is typically between 10 and 1000 MS).Thus, the voltage at the poles of the transmitter can be approximated as:

pH Electrodes

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pH Transmitter

Figure 2-11. pH Transmitter.

The following is a detailed description of the pH Transmitter terminals andadjustments (refer to Figure 2-11):

Î POWER INPUT terminals: used to power the transmitter with a dc voltageof 24 V.

Ï CALIBRATION SOURCE adjustment knob: sets the probe pH value to besimulated by the calibration source signal. This pH value can be adjustedbetween 0 and 12.

Ð INPUT SELECTOR: selects between the actual probe signal or thesimulated probe signal produced by the calibration source.

Ñ CALIBRATION SELECTOR switch: places the 0-5 V and 4-20 mAOUTPUTS in either fixed or variable calibration mode.

Ò ZERO and SPAN adjustment knobs: used in the variable calibration modeto set the pH value range for which the 0-5 V and 4-20 mA OUTPUTS willpass from minimum to maximum:

– The ZERO knob sets the pH value for which the outputs will beminimum (0 V and 4 mA), i.e., the minimum pH value to bedetected. The minimum pH value can be adjusted between 2 and 10.

pH Electrodes

44

– The SPAN knob sets the pH value for which the outputs will bemaximum (5 V and 20 mA), i.e., the maximum pH value to bedetected. The maximum pH value can be adjusted between 2 and 7units above the minimum pH value set by the ZERO knob.

Ó CALibrated OUTPUT: provides a voltage proportional to the pH sensed bythe probe or to the simulated probe signal produced by the calibrationsource, depending on the position of the INPUT SELECTOR switch.

This output has a fixed calibration of 1 V per sensed pH unit. It will passfrom 0 to 12 V when the actual or simulated pH value changesfrom 0 to 12.

Ô 0-5 V and 4-20 mA OUTPUTS terminals: provide a voltage and a currentproportional to the pH value sensed by the probe or proportional to thesignal simulated by the calibration source, depending on the position of theINPUT SELECTOR switch.

The calibration of the 0-5 V and 4-20 mA OUTPUTS can be either fixedor variable, depending on the position of the CALIBRATIONSELECTOR switch:

– In the fixed calibration mode (CALIBRATION SELECTOR atFIXED), the pH value range for which the outputs will pass fromminimum to maximum is fixed at 0-12 pH unit.

– In the variable calibration mode (CALIBRATION SELECTOR atVARIABLE), the pH value range for which the outputs will pass fromminimum to maximum can be adjusted by means of the ZERO andSPAN adjustment knobs.

Õ pH probe input connector: used to connect the pH probe to the transmitter.

Ö Inlet/outlet and Probe Port: used to connect the Flow Chamber to a controlloop. The port is used to insert the pH probe into the Flow Chamber.

× pH probe: the pH probe itself.

Procedure summary

In the first part of the exercise, you will familiarize yourself with the operation of thepH Transmitter in the fixed calibration mode.

In the second part of the exercise, you will familiarize yourself with the operation ofthe pH Transmitter in the variable calibration mode.

In the third part of the exercise, you will build and operate a pH process setup andyou will use the pH Transmitter to measure the pH of the solution in the Column.

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EQUIPMENT REQUIRED


PROCEDURE

Operation of the pH Transmitter in the fixed calibration mode

G 1. Get the pH Transmitter and the DC Power Supply from your storage area.Mount these components on the Work Surface.

G 2. Power up the pH Transmitter. To do so, connect the POWER INPUTterminals of the transmitter to the 24-V DC Power Supply.

G 3. Get the pH probe from your storage location. The probe should be storedinto the Flow Chamber filled with storage solution. Leave the pH probe inthe Flow chamber and connect it to the pH PROBE input of the pHTransmitter.

Note: Refer to Appendix K for specific details on the storage andcare of the pH probe.

G 4. Make the following settings on the pH Transmitter:

INPUT SELECTOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . pH PROBECALIBRATION SELECTOR . . . . . . . . . . . . . . . . . . . . . . . . . . FIXED

This selects the pH probe signal as the transmitter input signal and placesthe transmitter OUTPUTS in the fixed calibration mode.

G 5. Connect a dc voltmeter to the 0-5 V OUTPUT of the pH Transmitter.

Since this output is in the fixed calibration mode, it generates a fixed voltageof 42 mV per sensed pH unit.

G 6. Take the pH probe out of the Flow Chamber used for storage. Immerse thetip of the probe in a 100-ml beaker filled with about 30 ml of buffersolution pH 7.0.

What is the voltmeter reading?

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G 7. Use the voltmeter reading to calculate the pH of the buffer solution.

G 8. Further experiment with the operation of the transmitter in the fixedcalibration mode:

Note: To avoid contamination between the different solutions, rinse the tipof the probe with fresh water between each measurement (if available, usedistilled or deionized water).

– Fill a clean 100-ml beaker with about 30 ml of buffer solution pH 4.0;

– Immerse the tip of the pH probe in the buffer solution. The 0-5 VOUTPUT voltage should decrease and stabilize at about 1.7 V, which,in the fixed calibration mode, corresponds to a pH value of 4.0;

– Clean the 100-ml beaker and fill it with about 30 ml of buffersolution pH 10;

– Immerse the tip of the pH probe in the buffer solution. The 0-5 VOUTPUT voltage should increase and stabilize at about 4.2 V, whichcorresponds to a pH value of 10.0.

Record your observations below.

Note: The 0-5 V OUTPUT of the pH Transmitter may stabilize ata slightly different voltage if the temperature of the measuredsolution is not 25 /C (77 /F), if the buffer solution has beencontaminated with other chemicals, or if the pH Transmitter is notcorrectly calibrated.

Note: The calibration of the pH Transmitter should be checkedregularly. Refer to Appendix F for details on the calibration ofthe pH Transmitter.

Operation of the pH Transmitter in the variable calibration mode

Note: In the following steps, you will use the calibration source ofthe pH Transmitter to calibrate its 0-5 V OUTPUT so that thevoltage at this output passes from 0.0 to 5.00 V when the signalfrom calibration source passes from a simulated pH value of 4.0to a simulated pH value of 10.0, respectively.

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G 9. Make the following settings on the pH Transmitter:

INPUT SELECTOR . . . . . . . . . . . . . . . . . . . . . . . . . . CAL. SOURCECALIBRATION SELECTOR . . . . . . . . . . . . . . . . . . . . . . . VARIABLEZERO adjustment knob . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MAX.SPAN adjustment knob . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MAX.

This selects the calibration source signal as the transmitter input signal andplaces the transmitter OUTPUTS in the variable calibration mode.

G 10. Set the probe pH value to be simulated by the calibration source of thetransmitter at a pH value of 4.0.

To do so, use a voltmeter to adjust the CALIBRATION SOURCE knob of thetransmitter until you obtain a voltage of 4.00 V at the CAL. OUTPUT of thetransmitter.

G 11. While monitoring the voltage at the 0-5 V OUTPUT of the transmitter, turnthe ZERO adjustment knob counterclockwise. Stop turning the knob as soonas the voltage ceases to decrease, which should occur around 0.01 V.Then, very slowly, turn the knob clockwise and stop turning it as soon as thevoltage starts to increase.

This sets the minimum pH value to be detected at 4.0 approximately.

G 12. Now set the probe pH value to be simulated by the calibration source of thetransmitter at a pH value of 10.0.

To do so, use a voltmeter to adjust the CALIBRATION SOURCE knob of thetransmitter until you obtain a voltage of 10.0 V at the CAL. OUTPUT of thetransmitter.

G 13. Adjust the SPAN knob in order to obtain a voltage of 5.00 V at the 0-5 VOUTPUT of the transmitter.

This sets the maximum pH value to be detected at 10.0 approximately.

G 14. Now that the pH Transmitter is calibrated, proceed to the next part of theexercise.

Preliminary setup

G 15. Get the Expanding Work Surface from your storage location and mount itvertically to the Main Work Surface (at an angle of 90°), if this has notalready been done.

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G 16. Connect the system as shown in Figure 2-12, being careful not to modify thecalibration settings just made on the pH Transmitter. Figure 2-13 shows thesuggested setup.

The speed of the variable-speed drive of the Pumping Unit will be controlledwith the Set Point Device. The Metering Pumps will be controlled manuallyusing the Metering Pump Drive.

CAUTION!

Mount the Chemical Tanks and the Column as shown inFigure 2-13. Place electrical components as far as possiblefrom them. Failure to do so may result in water entering themodules upon disconnection of the hoses, which in turnmight cause damage to electrical components.

CAUTION!

Mount the 24-V DC Power Supply and the pH Transmitter insuch a manner that water cannot enter their components andelectrical terminals upon disconnection of the hoses.

CAUTION!

Be careful, the water level in the Column can rise quiterapidly. If you are not familiar with the Pumping Unit, set thepump speed lower.

Note: Refer to Figure B-1 of Appendix B for details on how toconnect the Lab-Volt Process Control and SimulationSoftware (LVPROSIM), Model 3674, to the pHTransmitter, Model 6544, the Set Point Device, Model 6561, andthe Metering Pump Drive, Model 6560.

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Figure 2-12. Measuring pH value with a pH Transmitter.

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Figure 2-13. Suggested setup for the diagram of Figure 2-12 (see Table next page for the detail ofthe components).

Î : Chemical Tank Ó : Metering Pump

Ï : Column Ô : Tray

Ð : Expanding Work Surface (Large) Õ : Metering Pump Drive

Ñ : Flow Chamber Ö : pH Transmitter

Ò : Work Surface × : I/O Interface

Note: Make sure the hose connected at the top of the Column isconnected on the upper-right inlet.

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G 17. Make the following settings:

On the Metering Pump Drive:

S1 switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2SC 1 manual control knob . . . . . . . turned fully counterclockwiseS2 switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . continuous modeS3 switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2SC 2 manual control knob . . . . . . . turned fully counterclockwiseS4 switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . continuous mode

On the pH Transmitter:

SELECTOR switch . . . . . . . . . . . . . . . . . . . . . . . . . . pH PROBECALIBRATION SELECTOR switch . . . . . . . . . . . . . . VARIABLE

Note: The 0-5 V OUTPUT of the pH Transmitter should still becalibrated for a pH measurement range between 4.0 and 10.0.

G 18. Turn on the Metering Pump Drive.

Preparation of the CH3COOH solution

G 19. Calculate the volume of Acetic Acid 5% (Vinegar) required to make 2000 mlof a 0.08 mol/l solution of CH3COOH.

Note: Refer to the calculation made in Exercise 1-1 (steps 10to 15).

Required volume of Acetic Acid 5% (Vinegar): ml

Note: Confirm this value with your instructor before proceedingfurther.

G 20. Prepare 2000 ml of a 0.08 mol/l solution of CH3COOH.

G 21. Fill the first Chemical Tank with the CH3COOH solution.

G 22. Using the HMIG (Hazardous Materials Identification Guide) paper labels,identify the Chemical Tank with the name of the chemical (not the formula),the concentration, the date, your initials, and the possible hazard(s).

G 23. Make sure all hoses, tubing, and electrical components are connected asshown in the connection diagram B-1 of Appendix B.

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CAUTION!

To avoid water and chemical spills all over the ProcessControl Training System, make sure the pH probe is properlyinserted into the port at the top of the Flow Chamber beforestarting the Pumping Unit.

G 24. Once the equipment is set up as required, fill the Pumping Unit withapproximately 12 liters (3.2 gallons) of water.

CAUTION!

Always make sure the reservoir of the Pumping Unit is filledwith the proper amount of water (12 l/3.2 gal US) beforeturning on the Pumping Unit. Failure to do so might causethe pump to run dry, causing the pump seal to overheat andwear out prematurely.

G 25. On the Pumping Unit, adjust valves HV1 to HV3 as follows:

– Close HV1 completely (turn handle fully clockwise);– Close HV2 completely (turn handle fully clockwise);– Set HV3 for directing the full reservoir flow to the pump inlet (turn

handle fully clockwise).

G 26. Turn on the Pumping Unit by setting its POWER switch at I.

G 27. Adjust the pump speed to 60-70% of its maximum by setting the Set PointDevice output between 3.00 V and 3.50 V.

G 28. Allow the level of water to rise in the Column until it reaches 38 cm (15 in).

CAUTION!

Be careful, the water level in the Column can rise quiterapidly. If you are not familiar with the Pumping Unit, set thepump speed lower.

Placing the system in water recirculating mode

G 29. Once the proper water level is reached, rapidly adjust HV3 to stop waterflow from the reservoir and direct the full return flow to the pump inlet (turnthe handle fully counterclockwise).

G 30. The Column is now in recirculating mode. Water is pumped to the PumpingUnit outlet, passes through the Flow Chamber, goes into the Column, and

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flows out of the Column through one of the bottom outlets to be directed tothe pump inlet again.

G 31. On the Pumping Unit, open HV2 and let the water level in the Columndecrease to 15 cm (6 in). As soon as the water reaches the proper level,close HV2.

Measuring pH with the pH probe

G 32. Make sure that the water is properly circulating through the system and thatthe Metering Pump connected to the first Chemical Tank is notrunning (i.e. the SC 1 manual control knob is turned fully counterclockwise).

G 33. Have the signal at the 0-5 V OUTPUT of the pH Transmitter plotted on thetrend recorder.

Adjust the update rate of the trend recorder (sampling interval) in order tobe able to monitor the transmitter signal over a period of approximately5 minutes.

Note: Refer to Figure B-1 of Appendix B for details on how toconnect the LVPROSIM computer to the pH Transmitter. Onthe I/O Interface, make sure the RANGE switch of ANALOGINPUT 1 is set at 5 V.

In LVPROSIM, select Analog Input 1 from the Trend Recorderselection list to have the pH Transmitter signal plotted on thetrend recorder. Set the LVPROSIM sampling interval at 500 ms.Access the Configure Analog Inputs window and set the minimumand maximum range values of Analog Input 1 at a pH value of 4and 10 respectively, which corresponds to the measurementrange previously set on the pH Transmitter. Set the filter timeconstant of this input at 0.5 second. Make sure the square rootextracting function is unselected. Accept setup and return to themain screen.

G 34. On the trend recorder, observe the pH Transmitter output signal.

Since no acid has been added to the water in the column, the pH valueshould be around 7.0

Note: If water from the public water supply system is used, themeasured pH value could easily vary between 6.5 and 8.5.

Assuming that the pH value of the water is 7, the level of the pH Transmittersignal should be at 50% of span on the trend recorder, since the minimumand maximum pH values the transmitter can detect have been set to 4and 10 respectively.

If the pH of the water is not 7, you will have to keep this information in mindwhile using the Process Control Training System. It can indicate thepresence of mineral salts dissolved in the water. Those salts can sometimes

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act as buffers, giving titration curves slightly different from the onetheoretically expected.

G 35. On the Metering Pump Drive, make sure that the S2 switch is set to thecontinuous mode. Set the SC 1 manual control knob to the mid position.This allows the 0.08 mol/l solution of acetic acid to be continuously addedto the water in the Column. On the trend recorder, observe what happensto the pH of the water.

The pH of the water should decrease. Is this your observation?

G Yes G No

G 36. Let the pH value of the water in the Column decrease to 5, then turnthe SC 1 manual control knob of the Metering Pump Drive fullycounterclockwise to stop the Metering Pump and stop the flow of acetic acidsolution from the Chemical Tank.

G 37. According to the pH Transmitter output signal on the trend recorder, didthe pH of the water in the Column decrease linearly over time?

G Yes G No

G 38. How long did it take for the pH to decrease from its initial value to the finalvalue of 5?

G 39. Stop the variable-speed drive of the Pumping Unit by setting the Set PointDevice output to 0.00 V.

G 40. Open valve HV1 of the Pumping Unit completely and let the water in theColumn drain back to the reservoir.

G 41. Turn off the Pumping Unit and the 24-V DC Power Supply by setting theirPOWER switch at O.

G 42. Disconnect the hoses of the Pumping Unit from the system and safelydispose of the solution in the reservoir.

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CAUTION!

Before disposing of the reservoir contents, always neutralizethe solution to avoid acid or alkaline products from beingreleased into the environment. After neutralization, onlywater and salts should remain in the reservoir. Refer to theneutralization procedure in Appendix I for details.

G 43. Disconnect the system. Return all leads, hoses, and components to theirstorage location.

CAUTION!

Water may remain in the hoses and components. Be carefulnot to allow water to enter the electrical components andtheir terminals upon disconnection of the hoses.

G 44. Thoroughly wash the glassware.

G 45. Store the pH probe in the flow chamber filled with storage solution. Refer toAppendix K for details.

G 46. Wipe up any water from the floor and the Process Control Training System.

G 47. Remove and dispose of your protection gloves before leaving theclassroom. Carefully wash your hands.

CONCLUSION

In this exercise, you familiarized yourself with the operation of a pH Transmitter inthe fixed and variable calibration modes. You learned that in the fixed calibrationmode, the pH value measurement range is fixed and is equal to 0-12. In the variablecalibration mode, the pH value measurement range can be adjusted and a spanof 6 pH units can be obtained. Since this span is narrower than the 12 pH unit spanof the fixed calibration mode, the variable calibration mode provides a greatermeasurement accuracy for any given transmitter output range. Finally, you learnedhow to operate a pH process setup and how to use the pH Transmitter to measurethe pH of a solution.

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REVIEW QUESTIONS

1. How does a pH glass electrode work?

2. Which type of ion is usually present in the buffer solution inside the glasselectrode?

3. What is the pH of the buffer solution inside the glass electrode?

4. Why should you store the pH probe in a KCl solution instead of distilled water?

5. Referring to the Nernst equation, which physical properties influencemeasurements taken with the type of pH probe described in this exercise?

Other Sample

Extracted from

Pressure, Flow, and


59

Unit Test

1. Volumetric flow rate can be measured in

a. cubic meters per second (m3/s).b. liters per minute (l/min).c. gallons US per minute (gal US/min).d. All of the above

2. Which of the following flow meters operates by increasing the size of its orificeas the flow rate increases?

a. The venturi tubeb. The turbine flow meterc. The rotameterd. The orifice plate

3. The graduations on a rotameter used to measure the flow rate of liquids are validonly for

a. a given viscosity and operating pressure.b. a given mass density and specific gravity.c. a given pressure loss and temperature.d. a given mass density and pressure loss.

4. The pulsed signal produced by a paddle wheel flow meter has a frequency whichis directly proportional to

a. the rotation speed of the paddles.b. the intensity of the infrared light beam generated internally.c. the flow rate of the liquid.d. Both (a) and (c)

5. According to Bernouilli's principle, when a liquid enters a restriction,

a. it accelerates, causing the pressure to increase.b. it accelerates, causing the pressure to decrease.c. it decelerates, causing the pressure to increase.d. it decelerates, causing the pressure to decrease.

6. The pressure drop produced by a differential-pressure flow meter is

a. directly proportional to the area of its restricting part.b. inversely proportional to the flow rate through its restricting part.c. directly proportional to the square of the flow rate through its restricting part.d. inversely proportional to the mass density of the liquid.

Unit Test (cont’d)

60

7. Differential-pressure flow meters produce a permanent pressure loss which

a. causes the pressure at the outlet port of the meter to be always less thanthe pressure at the inlet port.

b. is due to the fact that some portion of the kinetic energy of the liquid is lostas heat from friction between the liquid and the restricting part of the meter.

c. determines the amount of power dissipated as heat by the meter.d. All of the above

8. The pressure of the liquid flowing through an orifice plate flow meter reaches aminimum

a. near the entrance of the orifice plate.b. as it passes through the orifice of the orifice plate.c. at the vena contracta.d. at the outlet port of the meter.

9. The section of a venturi tube where the pressure of the liquid is constant andminimum is

a. the convergent inlet section.b. the throat section.c. the divergent outlet section.d. the outlet pipe section.

10. The permanent pressure loss caused by a venturi tube

a. is much lower than that caused by an orifice place of equivalent size.b. is as low as 10 to 25% of the pressure drop it produces.c. is typically 60 to 80% of the pressure drop it produces.d. Both (a) and (b)

Instructor Guide Sample

Extracted from

Pressure, Flow, and


Pressure, Flow, and Level Process Control

63

EX. 5-1 PRESSURE PROCESS CONTROL

ANSWERS TO PROCEDURE QUESTIONS

G 16. Since the proportional band is large (proportional gain is low), the voltagetakes a relatively long time to stabilize (overdamped response). Moreover,a large residual error remains at equilibrium, as Figure 5-1 (a) shows.

Figure 5-1.

G 19. The Pressure Transmitter output voltage stabilizes quicker and the residualerror has decreased, as Figure 5-1 (b) shows.

G 20. Small continuous variations should appear in the response of the PressureTransmitter output voltage at a proportional band of about 15%, asFigure 5-2 shows.


64

Figure 5-2.

G 22. The response of the Pressure Transmitter output voltage is overdampeddue to the long integral time setting.

No residual error remains once the process has reached steadystate, dueto the addition of integral action. This is illustrated in Figure 5-3.


65

Figure 5-3.

G 23. The Pressure Transmitter output voltage should start to overshoot thesetpoint at an integral time of about 0.0125 min/rpt, as Figure 5-4 shows.


66

Figure 5-4.

G 24. Derivative action does not decrease the overshooting of the PressureTransmitter output voltage. Worse still, it tends to increase the small randomfluctuations in the controller output, as Figure 5-5 shows. This indicates thatderivative action is unnecessary with this process.


67

Figure 5-5.

G 25. With a proportional band of 30%, an integral time of 0.03 min/rpt, and aderivative time of 0.000 min, the controller tuning remains acceptable overa broad range of setpoints, as Figure 5-6 shows.


68

Figure 5-6.

G 27. With a proportional band of 30%, an integral time of 0.03 min/rpt, and aderivative time of 0.000 min, the closure of valve HV1 results in oscillationof the Pressure Transmitter output voltage, as Figure 5-7 (a) shows. Thisproblem can be remedied by increasing the proportional band at 60% andthe integral time at 0.04 min/rpt, as Figure 5-7 (b) shows.


69

Figure 5-7.

ANSWERS TO REVIEW QUESTIONS

1. The integral action of the controller should be increased.

2. The proportional and/or integral action of the controller can be decreased.

3. Decreasing the proportional band will increase the integral action of thecontroller.

4. Because the pressure process is inherently a fast responding process.Consequently, derivative action does not speed up the response of the process,it does not reduce overshooting of the controlled variable, and it tends to amplifythe small random fluctuations in the controller output.

5. The trial and error tuning method has the following advantages: it does notrequire that the characteristics of the process be known, and it does not requirethat the process be brought into sustained oscillation. However, the trial anderror method can be difficult to achieve for inexperienced operators because a


70

change in tuning constant tends to affect the action of all three controller modes.For example, increasing the integral action will increase the overshooting, whichin turn will increase the rate of change of the error, which in turn will increase thederivative action.

Bibliography

Byron Bird R., Stewart W.E, and Lightfoot E.N. Transport Phenomena, New York:John Wiley & Sons, 1960

ISBN 0-471-07392-X

Chau, P. C. Process Control: A First Course with MATLAB, Cambridge UniversityPress, 2002.

ISBN 0-521-00255-9

Coughanowr, D.R. Process Systems Analysis and Control, Second Edition, NewYork: McGraw-Hill Inc., 1991

ISBN 0-07-013212-7

Liptak, B.G. Instrument Engineers' Handbook: Process Control, Third Edition,Pennsylvania, Chilton Book Company, 1995

ISBN 0-8019-8542-1

Liptak, B.G. Instrument Engineers' Handbook: Process Measurement and Analysis,Third Edition, Pennsylvania, Chilton Book Company, 1995

ISBN 0-8019-8197-2

Luyben, M. L. and Luyben, W. L. Essentials of Process Control, McGraw-HillInc., 1997.

ISBN 0-07-039172-6

Luyben, W.L. Process Modeling, Simulation and Control for Chemical Engineers,Second Edition, New York: McGraw-Hill Inc., 1990.

ISBN 0-07-100793-8

McMillan, G.K. and Cameron R.A. Advanced pH Measurement and Control, ThirdEdition, NC: ISA, 2005.

ISBN 0-07-100793-8

McMillan, G. K. Good Tuning: A Pocket Guide, ISA - The Instrumentation, Systems,and Automation Society, 2000.

ISBN 1-55617-726-7

McMillan, G. K. Process/Industrial Instruments and Controls Handbook, Fifth Edition,New York: McGraw-Hill Inc., 1999.

ISBN 0-07-012582-1

Metcalf & Eddy, inc. Wastewater Engineering - Treatment and Reuse, Third Edition,McGraw-Hill Inc., 1991.

ISBN 0-07-041690-7

Perry, R.H. and Green D. Perry's Chemical Engineers' Handbook, Sixth Edition, NewYork: McGraw-Hill Inc., 1984

ISBN 0-07-049479-7

Raman, R. Chemical Process Computation, New-York: Elsevier appliedscience ltd, 1985.

ISBN 0-85334-341-1

Ranade, V. V. Computational Flow Modeling for Chemical Reactor Engineering,California: Academic Press, 2002.

ISBN 0-12-576960-1

Shinskey, G.F. Process Control Systems, Third Edition, New York: McGraw-HillInc., 1988.

Bibliography (cont'd)

Soares, C. Process Engineering Equipment Handbook, McGraw-Hill Inc., 2002.ISBN 0-07-059614-X

Weast, R.C. CRC Handbook of Chemistry and Physics, 1st Student Edition, Florida:CRC Press, 1988.

ISBN 0-4893-0740-6

Wildi, T. Metric Units and Conversion Charts, Second Edition, Piscataway, NJ: IEEEPress, 1995.

ISBN 0-7803-1050-0.

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