Chapter 5p

TEMPERATURE MEASUREMENTTEMPERATURE MEASUREMENT

INTRODUCTIONINTRODUCTION What is Temperature?Temperature is defined as the degree of hotness or coldnessp g

measured on a definite scale. Hotness and coldness are the resultof molecular activity. As the molecules of a substance move faster,the temperature of that substance increases.p

What is Heat?Heat is a form of energy and is measured in calories or BTU'sgy

(British Thermal Units). Why temperature is measured as a process variable?

There are changes in the physical or chemical state of mostsubstances when they are heated or cooled. Themeasurement of temperature is also important for protectionmeasurement of temperature is also important for protectionof the equipment, as uncontrolled high or low temperaturescan cause structural deterioration of pipelines and vessels.

Heat Transfer The flow of heat is transferred in three ways: convection, conduction,

and radiation.) C d tia) Conduction When heat is applied to one part of a substance, it is transferred to all

parts of the substance. The movement is from molecule to molecule. pGases and liquids are poor conductors. The flow of heat by conduction takes place most effectively in solids.

b) Convectionb) Convection Heat transferred by the actual movement of portions of a gas or liquid from one place

to another is called convection. This movement is caused by changes in density due to rising temperat re For e ample in a forced air heating s stem the arm airto rising temperature. For example, in a forced air heating system, the warm air entering the room through the supply duct is less dense, and therefore, lighter than the cooler air already in the room. As the warm air-cools, it drops and moves through the cool air return and back through the heating systemthe cool air return and back through the heating system.

c) Radiation Heat energy is transferred in the form of rays sent out by the heated

substance as its molecules undergo internal change. Only energy is transferred. The direction of the flow of heat is from the radiatingtransferred. The direction of the flow of heat is from the radiating source. The radiant energy is then absorbed by a colder substance or object. Radiation takes place in any medium (gas, liquid, or solid), or in a vacuuma vacuum.

Temperature Scales

Absolute zero is the temperature at which the movement of molecules completely stops Temperature value on given scale can be converted to express on other Scales: C = ( F-32) x 5/9 F = ( C x 9/5) + 32 K = ( C + 273.2)

Temperature Measuring SensorsTemperature Measuring Sensors

The common temperature measuring sensors The common temperature measuring sensors

are:

1.Thermometers,

2.Thermocouples,

3.Resistance Temperature Detectors (RTDs)

1. ThermometersFill d Th ta)Filled Thermometers

These are thermometers filled with either a liquid such as mercury or an evaporating fluid such as used in refrigerators In both cases the inside evaporating fluid such as used in refrigerators. In both cases the inside of the sensor head and the connecting tube are completely full.

Any rise in temperature produces expansion or evaporation of the liquid Any rise in temperature produces expansion or evaporation of the liquid so the sensor becomes pressurised. The pressure is related to the temperature and it may be indicated on a simple pressure gauge.

The problems with glass thermometers are that The problems with glass thermometers are that

they are:

1.Mercury solidifies at -40oC.

2.Alcohol boils at around 120oC.

3 Accurate manufacture is needed and this makes 3.Accurate manufacture is needed and this makes

accurate ones expensive.

4.It is easy for people to make mistakes reading

themthem.

b) Bimetallic Strips

A bimetallic strip is constructed by

b di t t l ith diff t bonding two metals with different

coefficients of thermal expansion.

If heat is applied to one end of the

strip, the metal with the higher p, g

coefficient of expansion will expand

more readily than the lower one. more readily than the lower one.

As a result, the whole metallic strip

will bend in the direction of the metal

with the lower coefficient.

Bimetallic strip applications:Bimetallic strip applications:

it can be used as a fast acting

thermostat to control air

temperature.

They are usually used to measure

process parameters for local readout. process parameters for local readout.

2. Thermocouplesp A thermocouple consists of two pieces of dissimilar metals with

their ends joined together (by twisting soldering or welding)their ends joined together (by twisting, soldering or welding).

When heat is applied to the junction, a voltage, in the range of

millivolts (mV), is generated. A thermocouple is therefore said

to be self-powered. Shown is a completed thermocouple circuit.

In order to use a thermocouple to measure process temperature, one d f h h l h b k h hend of the thermocouple has to be kept in contact with the process

while the other end has to be kept at a constant temperature. The end that is in contact with the process is called the hot or measurement pjunction. Circuit emf = Measurement emf - Reference emf

If circuit emf and reference emf are known, measurement emf can be calculated and the relative temperature determined.

To convert the emf generated by a thermocouple to the standard 4-20 mA signal, a transmitter is needed.

Types of thermocouplesThermocouples exist in many different types, each with its own color codes for the

dissimilar-metal wires. Here is a table showing the more common thermocouple types and their standardized colors, along with some distinguishing yp , g g gcharacteristics of the metal types to aid in polarity identification when the wire colors are not clearly visible:

An output of 40 millivolts at 1000ºF can be compared to 30 mv for type J and 22 mV for type K Type E has more tendencies to change characteristics with time than typefor type K. Type E has more tendencies to change characteristics with time than type J, K and T.

Three Basic Types Of Thermocouple Assemblyyp p ySome thermocouple assemblies are manufactured as follw:

Ground junction Ground junction

Ungrounded junction

Exposed junction

o The exposed junction is often used for the measurement of o The exposed junction is often used for the measurement of

static or flowing non-corrosive gas temperatures where the

ti t b i i l response time must be minimal.

o The ungrounded junction often is used for the measurement of

static or flowing corrosive gas and liquid temperatures in critical

electrical applications.pp

o The grounded junction often is used for the measurement of

i fl i i d li id d f static or flowing corrosive gas and liquid temperatures and for

high-pressure applications.

T/C Operation and Installation AspectsExtension cables

In every thermocouple circuit there must be both a measurement y pjunction and a reference junction: this is an inevitable consequence of forming a complete circuit (loop) using dissimilar-metal wires.

As we already know, the voltage received by the measuring instrument from a thermocouple will be the difference between the voltages produced by the measurement and reference junctions.

Since the purpose of most temperature instruments is to accurately measure temperature at a specific location, the effects of the reference junction’s voltage must be “compensated” for by some means, either a

i l i it d i d t dd dditi l li lt bspecial circuit designed to add an additional canceling voltage or by a software algorithm to digitally cancel the reference junction’s effect.

Thermocouple reference tables

When using thermocouples, temperature reference

t bl b d t t th V i l f th tables can be used to convert the mV signal from the

thermocouple into a temperature reading.

All table values are referenced to a cold junction

temperature of 0° C. if the reference junction of a

thermocouple is not at 0°C, the tables can still be p ,

used by applying an appropriate Correction to

t f th diff b t th f compensate for the difference between the reference

junction and 0 ºC.

3. Resistance Temperature Detectors The resistance of a conductor usually increase as the temperature

increase .if the properties of that conductor are known, the temperature can be calculated from the measured resistance.

Any conductor can be used to construct an RTD, but a few have been identified as having more described characterstics than others. The characteristics which are desired includecharacteristics which are desired include.

1.Stability: in the temperature range to be measured. The material must not melt, correde, embattle or change electrical characteristics when subjected to the environment in which it will operate.

2.Linearity: The resistance change with temperature should be as liner as possible over the rang of interst to simplify readout.as possible over the rang of interst to simplify readout.

3.High resistively: Less material is needed to manufactor an RTD with a specified resistance when the matrial has a high characteristic

i ti l resistively.

4.Workability: The material must be suitable for configuring for insertion into the media.

Platinum has been accepted as the material, which best fit all the criteria and has been generally accepted for industrial measurement between –300 and 1200° F (-150 and 650 °C ).

RTDS are commercially available with resistances from 50 to 1000 ohms.

Platinum RTD known as Pt100, because it has 100 Ohm resistance at 32°F ( 0°C) and increase resistance 0.385 ohms f °C f t t i for every °C of temperature rise.

When the resistance of the RTD is found by measurement, the b l l dtemperature can be calculated:

°C = ( Ohms reading – 100 ) / 0.385

the accuracy of this calculation is determined primarily by the accuracy of the reading.

RTD Connections2 wire connection:2-wire connection:

To detect the small variations of resistance of the RTD, a temperature transmitter in the form of a Wheatstone bridge is generally used. The circuit compares the RTD value with three known and highly accurate resistors.

A problem arises when the RTD is installed some distance away from the A problem arises when the RTD is installed some distance away from the transmitter. Since the connecting wires are long, resistance of the wires changes as ambient temperature fluctuates. The variations in wire resistance would introduce an error in the transmitter To eliminate this resistance would introduce an error in the transmitter. To eliminate this problem, a three-wire RTD is used.

3-wire RTD connection:

The connecting wires (w1, w2, w3) are made the same length and therefore the same resistance. The power supply is connected to one end of the RTD and the top of the Wheatstone bridge. end of the RTD and the top of the Wheatstone bridge.

It can be seen that the resistance of the right leg of the Wheatstone bridge is R1 + R2 + RW2.

The resistance of the left leg of the bridge is R3 + RW3 + RTD.

Since RW1 = RW2, the result is that the resistances of the wires cancel and therefore the effect of the connecting wires is eliminatedand therefore the effect of the connecting wires is eliminated.

Thermistors Thermistor is resistance temperature element made from a semiconductor

material and basically do the same job as an RTD. These elementsgenerally have a negative Temperature coefficient (NTC) but positiveg y g p ( ) ptemperature coefficients are also available over a limited range. They areonly used for a typical range of -20 to 120oC and are commonly used insmall hand held thermometers for every day use.Th d t f Th i t i it i hi hl iti t t t The advantage of a Thermistor is it is highly sensitive to temperaturechanges making them useful in temperature trip alarms. Unfortunately theyposses highly non-linear resistive properties which restrict their usefulrangerange

ThermowellsThe mo ell e ed to p ote t the dete to nd o th t the dete to n Thermowells are used to protect the detector and so that the detector can be changed without interrupting the process. One downside of using a thermowell is the time delay it introduces into the measurement system due to thermal lag.

Thermowlls should be installed where a good representative sample of the process fluid temperature can be measuredprocess fluid temperature can be measured.

The optimum immersion length of a thermowell depends on the applicationpp

If the well is installed perpendicular to the line, the tip of the well should be between one half and one third of the pipe diameter.

If the well is installed in an elbow, the tip should point towards the flow.

The speed of response of a sensor in a thermowell will be slower than that of an unprotected buib. Keeping the clearance between bulb and pocket down to an absolute minimum and filling the space with oil or glycol (antifreeze) can reduce this effect.( )