1. Sensors Transducers Presentation
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Transcript of 1. Sensors Transducers Presentation
Introduction to process Introduction to process measurementmeasurement
• The purpose of measurement• Kinds of signals• Remote vs local display• Error in measurement system• Calibration• Calibration laboratory• Field calibration
Function of sensorFunction of sensor
1. An instrument must sense the process to be measured. part of the instrument must stretch bend, or change position or characteristics when affected by process.
2. The instrument must convert this response into an electrical or pneumatic signals.
Function of transducerFunction of transducer
• The process sensing element is usually a bellow, a diaphragm or a bourden tube.
• The primary conversion element converts the mechanical action of a process sensing element to an electrical signal, usually an electrical resistance or a voltage.
• The secondary conversion element is usually an electronic circuit that produces a standard signal according to the need of the control system.
Sensors transducersSensors transducers
• Pressure sensors.• Flow sensors.• Thermometers.• Liquid level sensors.• Vibration and speed sensors.• Proximity and displacement sensors.• Fire, Flame and smoke detectors.• Analyzers.
Transducer principleTransducer principle1. Capacitive. 2. Inductive.3. Piezoelectric.4. Resistive.5. Potentiometer.6. Strain gage.7. Photoconductive.8. Thermoelectric.
PRESSURE
PRESSURE SENSOR
PRESSURE TRANSDUCER
STRAIN GAUGE
DIAPHRAGM
BELLOWS
MAGNETIC
BOURDEN TUBE
INDUCTIVE
POTENTIOMETRIC
CAPACITVE
PIEZOELECTRIC
PHOTOELECTRIC
Working on transmitterWorking on transmitter
• A measurement device is made up of two parts• Primary element• Transmitter
• Converts signal (Transducer)• Transmits signal
flow metersflow meters
• Flow is the motion of fluid.• Flow rate is the time rate of motion fluid.• Total flow is the rate integrated over a specific
time interval.• Flow meter is a flow rate transducer.• Laminar flow.• Turbulent flow.• Reynolds no.
FLOW
DIFFERENCIAL PRESSURE MECHANICAL FLOWMETER ELECTRONIC FLOWMETER
POSITIVE DISPLACEMENT VORTEX FLOWMETER
ULTRASONIC FLOWMETER
MEGNATIC FLOWMETERTURBINE FLOWMETER
ROTARY FLOWMETER
PRIMARY ELEMENT
Types and use of orifice plate
1. Concentric
opening in center. It is for normal use.
2. Eccentric Opening slightly off
center. Used for the fluid or gas
which has solid particles.
Positive displacement Positive displacement flow metersflow meters
• Trap a known volume of fluid and allow it to pass from meter inlet to outlet.
• Total flow measurement.
• They have accurate,precise,and have a wide flow range.
• They are expensive and object to corrosion.
• They are used for water meters and gasoline pumps.
PD flow meters typesPD flow meters types
• Piston flow meter• Notating-disc flow meter• Lobed impeller and oval flow meter• Rotary vane meter• Turbine flow meter
temperature
Bimetallic and liquid filled system
Resistance temperature detector
thermister
thermocouple
Temperature
Hotness or Coldness of a Substance
Temperature Units
– Celsius °C
– Fahrenheit °F
– Kelvin °K
Temperature Measuring Instruments
Located at Distance
Thermometers
Pyrometer
Instrument Inserted or Attached
Basic Types Of Thermometers
Bimetallic
– From –75 °C to 540 °C
Liquid In Glass
– From -120 °C to 320 °C
Filled System
– From -273 °C to -185 °C
– From 650 °C to 760 °C
Metals expand on heating and contract on cooling.
Each metal has a coefficient of thermal expansion
Bimetallic strip bends toward metal of lower rate of
thermal expansion, on heating.
Advantages :
Inexpensive, Rugged and Easy to Read
Reasonably Accurate, Handle Carefully
Best for Local Use
Bimetallic Thermometers (Cont.)
RTDsSome Metal’s Resistance Increase on Increase of Temperature Metals Used in RTD Must Be
Pure
Uniform Quality
Stable With Given Temperature Rage
Able to Reproduce Resistance Temp. Reading
Temperature, °C
Metal Resistance
0 Platinum Copper Nickel
100 100 100
200 Platinum Copper Nickel
175 186 250
400 Platinum Copper Nickel
250 275 485
RTDs
RTD act just like a variable resistor Used in resistance bridge circuit Output voltage resistance of RTD Heating or cooling cause bridge unbalance Lead wire error 250 ft copper wire has 1 ohm resistance. If resistance 139 ohm temp. will be 100°C exclude 2
ohm then R=137 which show 94.4 °C error 5.1 %
RTDs
Compensating Lead Error
– Use of Third Wire, RTD Wire Should Be Opposite Arm
– Error With 2 Lead Wires 4°C, With Three Lead 0.1 °C and by Fourth Wire Error 0.03°C
RTDs
Advantages of RTD– Extremely accurate – Measure temperature within 0.15 °C– Fast in response and small in size
Disadvantages of RTD– Expansive and complex– Require bridge circuit, power supply, meter
RTDs
Thermistors
Special Type Of Resistors
High temp. resistance coefficient
Negative as well as positive Temperature Coefficient
Resistance Increases Temp. Decreases
Advantages
– Three & four wire bridge circuit not required
– Two lead wire provide sufficient accuracy
– Less expansive and less complicated
Disadvantages
– Lack RTD accuracy.
Thermocouple Two Different Metals or Alloy
– Two Junction
• Reference Junction ( to be at constant temperature)
– Measuring Junction
• Heating measuring junction produce voltage across MJ
& reference junction
Thermocouple
Extension Wires Thermocouple wires Same as thermocouple ( iron constantan, copper constantan Other material for platinum thermocouple Copper or copper alloy
Advantages
– In Expansive, Rugged, Accurate
– Do not use bridge circuit
– Wide temperature range –273 to 2800 oC
Disadvantages
– Low voltage.
– Reference junction kept to be constant
Thermocouple
Level
Pressure head instruments
Electrical instruments
Differential pressure
displacer
Sight glass
vibration
ultrasonic
Float switches
Definition of Some Points
Datum point: A fixed reference point from which level is measured
Buoyancy: Upward force exerted by liquid on an object submerged & floating in it
Buoyant force: Net upward force exerted by liquid on submerged or floating object equal the weight of the displaced liquid.
Displacer: A nearly stationary object whose purpose is to displace liquid, used for liquid measurement.
Level Measurement by Float A float, cable, sliding pointer arrangement. A fixed scale is mounted on the exterior of the tank A cable connects the float with the pointer through two pullies. Pointer move up and down and also serve as counter weight. Float type level indicators are installed on the open tanks and the
tank of the atmospheric pressure.
Level Measurement by Displacer
Displacer do not ride on the surface of the liquid but they are design to submerge as the level rises.
Displacer mechanism transfer the slight mechanical motions due to loss of weight which can be used in instruments to indicate level.
Torque Tube DisplacersTorque Tube Displacers
The most common type of displacer mechanism is a torque tube displacer.
Torque tubes are useful when level measurement is needed on the liquid under pressure.
As the liquid level changes, the buoyant force moves the displacer slightly and twist the torque tube.
The motion of the torque tube makes the pointer move to indicate the liquid level on the scale.
Level Measurement Using Differential Pressure Instruments
Differential pressure measurement is suited to level measurement in closed vessels
When liquid level is measured in an open tank, piping from the lower end of the tank is connected to the high pressure side of the transmitter and the low pressure side is vented.
When liquid level is measured in a closed tank the effect of tank pressure is compensated by connecting a line from the top of the tank to the low pressure side of the differential.
The pressure due to the height of the liquid is a measure of its
level.
analyzers
Hydrazine monitor
Sodium monitor
Dissolve oxygen monitor
Flue gas oxygen monitor
Silica monitor
Conductivity monitor
Ph monitor
Dissolved Oxygen SensorDissolved Oxygen Sensor• Oxygen molecules diffuse
through the membrane based on the partial pressure of oxygen.
• ORP Reaction– Oxygen is reduced to
hydroxide ions (OH-) at the gold cathode.
– Silver is oxidized to silver bromide (M-4) or silver chloride (M-4HD) at the silver anode
• The current flow is directly proportional to the concentration of Dissolved Oxygen in solution
Measuring principleMeasuring principle
Conductance is a measure ability of electric circuit to pass current.
Na+
Cl-
G = 1/R(siemense)
Variables affecting conductivityVariables affecting conductivity
• Temperature• Concentration• contamination
Typical Conductivity ValuesTypical Conductivity Values
0.055
0.5
1
50
50,000
355,000
0 1 100 10,000 1,000,000
Ultra Pure Water
Distilled Water
Boiler Feed Water
Mains Water Supply
Sea Water
10% NaOH
pH TheorypH Theory• pH is a Unit of Measurement
– pH = Power of Hydrogen (H+)– Defined as the Negative Logarithm of
Hydrogen Ion Activity• pH = log (1/H+)
• Used for Determining the Acidity or Alkalinity of an Aqueous Solution– Practical pH Scale for Industrial
Instrumentation• 0 - 14 pH
pH Sensor ComponentspH Sensor Components• pH Measuring Electrode
– Purpose is to Develop a Millivolt Potential Directly Proportional to the Free Hydrogen Ion Concentration in an Aqueous Solution
• Reference Cell– Purpose is to Maintain a
Constant Reference Potential Regardless of pH Change or Other Ionic Activity in the Solution
• Reference Cell Liquid Junction– Purpose is to Maintain
Electrical Contact Between the Reference Electrode and the Measuring Electrode by way of the Solution
Ag/AgCl Element
pH Measuring Electrode Reference Cell
pH Sensitive Glass Liquid Junction
KCl Gel
KCl Buffered to 7 pH
pH is a Potentiometric pH is a Potentiometric MeasurementMeasurement
• The Measuring System consists of a pH Measuring Electrode and Reference Electrode
• The Potential Difference Between the Two Electrodes is a Function of the pH Value of the Measured Solution
• The Solution Must Be Conductive and is Part of the Electrical Circuit
pH Measuring Electrode Reference Electrode
Ion ConcentrationIon Concentration
pH Hydrogen Ion (H+) Hydroxyl Ion (OH-)Acid 0 1 0.00000000000001
1 0.1 0.00000000000012 0.01 0.0000000000013 0.001 0.000000000014 0.0001 0.00000000015 0.00001 0.0000000016 0.000001 0.00000001
Neutral 7 0.0000001 0.00000018 0.00000001 0.0000019 0.000000001 0.00001
10 0.0000000001 0.000111 0.00000000001 0.00112 0.000000000001 0.0113 0.0000000000001 0.1
Alkaline 14 0.00000000000001 1
Typical pH ValuesTypical pH Values
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0
4.9 % H2SO4
Lemons
Oranges
Tomatoes
Beer
Milk
Pure Water
Blood
Crackers
4.0% NaOH
pH
Fire and smoke detectors
Ionization sensors
Infrared and ultraviolet sensors
Thermal sensors
Photo sensors
Fire and smoke detectors
Ionization sensors
Infrared and ultraviolet sensors
Thermal sensors
Photo sensors