Indian Institute of Technology GandhinagarDepartment of Electrical Engineering

EE 402 Control System Lab. B. Tech.: Electrical, Sem. : VII

EXPERIMENT 6: Process Measurements - Temperature

AIM

Temperature measurement plays a major role in control of industrial processes. The important sensors that are used to measure the temperature are thermocouple, RTD, Thermistor. The primary aim of this experiment is to study and plot the characteristics of these sensors with their signal conditioning and amplifiers circuits, understand their application and learn to use them as process measurement devices in control applications.

THERMOCOUPLE SENSORSTHEORYThe thermocouple is one of the simplest and most commonly used methods of measuring process temperatures. The operation of a thermocouple is based upon Seebeck effect which states that when heat is applied to junction (hot junction) of two dissimilar metals, an emf is generated which can be measured at the other junction (cold junction). The two dissimilar metals form an electric circuit, and current flows as a result of the generated emf as shown in Fig. I

T1>T2

The emf produced is function of the difference in temperature of hot and cold junctions and is given

by:

E = a ∆θ

Where ∆ θ = difference between temperatures of hot and cold junctions.

REFERENCE JUNCTION COMPENSATION

A factor which is important in the use of thermocouple is the requirement of a known reference temperature of the reference junction. This is because when the reference junction is not held at O°C, the observed value must be corrected by adding to it a voltage that has resulted from a temperature difference equal to the amount by which the reference junction is above O°C. (This is because the thermocouples are calibrated with temperature of reference junction as O°C). Now E T= Et + Eo where ET is the total emf at temperature T, Et is the emf on account of temperature difference between detecting (hot) and the reference junction and Eo is the emf due to temperature of the reference junction being above O°C. Since, there exists a non-linear relationship between the emf and the temperature, it is important that temperatures are determined by the above process rather than converting an emf to temperature and then adding it to ambient temperature.

i. Advantages of Thermocouple

1. Thermocouples are cheaper than the resistance thermometers.2. Thermocouples follow the temperature changes with a small time lag and as such are

suitable for recording comparatively rapid changes in temperature.3. Thermocouples are very convenient for measuring the temperature at one particular point

in a piece of apparatus.

ii. Disadvantages of Thermocouples

1. They have a lower accuracy and hence they cannot be used for precision work.2. To ensure long life on the thermocouple in their operating environments, they should be

protected in an open or closed end metal protecting tube. To prevent contamination of the thermocouple, when precious metals like platinum or its alloys are being used, the protecting tube has to be made chemically inert and vacuum tight.

3. The thermocouple is placed remote from measuring devices. Connections are thus made by means of wires called extension wires. Maximum accuracy of measurement is assured only when compensating wires are of the same materials as the thermocouple wires are used. The circuitry is, thus, very complex.

THERMOCOUPLE CHARACTERISTICS TRAINER FRONT PANEL DIAGRAM

TECHNICAL SPECIFICATIONS

1. ITB - 05CE Unit

Working Temperature - 15°C - 50°C Accuracy - 1.5% of Full scale division. Linearity - 1875% of Full scale division. Size - 370 × 280 × 90m Cabinet - Mild Steel

2. Thermocouple

Type - J type Material - Iron constantan Tube Diameter - 6mm Working Temperature - -200 to 760C Tube Length - 120mm Thermowell material - Stainless steel Coating - Nickel, Chromium Cable Length - 950mm

EXPERIMENT: THERMOCOUPLE CHARACTERISTICS USING TRAINER ITB - 05CE

AIMTo study the characteristics of thermocouple with compensation.

PROCEDURE1. Patch the two terminals of the thermocouple across T1 & T2.2. Position the switch ‘SW1' towards downwards.3. Switch ‘ON’ the unit and note the displayed temperature.4. If there is any difference in displayed temperature at room temperature, adjust the offset

knob ‘Zero’ to set 0°C in display.5. Insert the thermocouple and thermometer into the water bath.6. Place the multimeter across T7 & T87. Position the switch ‘SW1' towards the ‘NC’8. Switch ‘ON’ the water bath.9. Note the actual temperature in thermometer, voltage in multimeter and displayed

temperature simultaneously.10. Tabulate the reading and calculate %Error using the above formula.

Plot the graph for

i. Actual Temperature Vs % Error.ii. Actual Temperature Vs signal conditioner output.

Tabular Column

%E = Displayed Temperature−ActualTemperatureActual Temperature

X 10

MODEL GRAPH

TEMPERATURE Vs SIGNAL CONDITIONER OUTPUT VOLTAGE

RESULTThus the characteristics of thermocouple with compensation were studied and graph is plotted.

THERMISTORSTHEORYThermistor is a contraction of a term "thermal resistors". Thermistors are generally composed of semi-conductor materials. Although positive temperature co-efficient of units (which exhibit an increase in the value of resistance with increase in temperature) are available, most thermistors have a negative coefficient of temperature resistance, i.e., their resistance decreases with increase in temperature.The negative temperature coefficient of resistance can be as large as several percent per degree Celsius. This allows the thermistor circuit to detect very small change in temperature which could not be observed with a RTD or a thermocouple. In some cases the resistance of thermistor at room temperature may decrease as much as 5 percent for each 1°C rise in temperature. This high sensitivity of temperature change makes thermistor extremely useful for precision temperature measurements control and compensation. Thermistors are widely used in applications which involve measurements in the range of -60°C to 150°C. The resistance of thermistors ranges from 0.5 Ω to 0.75 M . Thermistor is a highly sensitive device. The price to be paid for the highΩ sensitivity is in terms of linearity. The thermistor exhibits high non-linear characteristic of resistance versus temperature.

TECHNICAL SPECIFICATIONThermistor Type - NTC Probe Material - S.S Diameter - 10mm Lead Pitch - 5mm Bead colour - Blue Resistance at 25°C - 5kΩ Temperature Range - -80°C to 150 °C Tolerance (0 - 70°C) - ±0.2°C Dissipation constants - 1mw Time constants - 10s

TRAINER FOR RESISTANCE-TEMPERATURE CHARACTERISTICS OF THERMISTOR

FRONT PANEL DESCRIPTION

FRONT PANEL DESCRIPTIONPower ON/OFF : Switch ON / OFF the unit.T1 & T2 : Provision to connect the thermistor terminals.SW : Select either Resistance mode or Voltage mode.T3 & T4 : Measure the resistance value of thermistor.Zero : Adjust this potentiometer to set 5 Volt at 30° C in EXT modeT5 : Measure the signal conditioner output voltage.GND : Common GND terminalSeven segment display : For displaying the output voltage.

CIRCUIT DESCRIPTION

The thermistor which senses the temperature from the water bath as resistance This circuit consists of three amplifiers, two gain amplifiers and one inverting amplifier. The thermistor is connected at the feedback of the first gain-amplifier which gives constant voltage at initial stage. During the time of heating the thermistor, the resistance of thermistor will be reduced. It converts the resistance into millivolts. The output obtained from non inverting amplifier voltage is given as input to the signal conditioner for further amplification where the output is tuned with the range of -5 to -0 V using the trimpot TP1 Zero and TP2 gain. This output is applied to inverting amplifier to convert the negative input into positive output of range (0-5) VDC. This signal conditioner voltage can be displayed in the display (voltage).

EXPERIMENT RESISTANCE-TEMPERATURE CHARACTERISTICS OF THERMISTOR

AIM

To study the Temperature Vs Resistance and Temperature Vs Voltage characteristics of thermistor.

PROCEDURE : Temperature Vs Resistance Interface the thermistor across T1 and T2 & switch ON the unit. For resistance measurement, SW should be in resistance mode. Connect the multimeter (in resistance mode) across T3 & T4. Insert the thermometer and thermistor into the water bath. Switch ON the water bath. Note down the temperature in thermometer and corresponding resistance output of

the thermistor. Plot the graph between temperature and resistance along X and Y axis respectively.

PROCEDURE : Temperature Vs Voltage

PROCEDURE Interface the thermistor across T1 and T2 & switch ON the unit. Switch SW is in INT mode. Connect the multimeter (in DC -Volt mode) across T5 & T6. The Zero POT is adjusted to 5V because thermistor is NTC type Before conducting the experiment, SW should be in INT mode. Insert the thermometer and thermistor into the water bath. Switch ON the water bath. Now note down the temperature of the thermometer and corresponding voltage

output. Plot the graph between temperature and voltage along X and Y axis respectively.

TABULATIONS(temperature Vs Resistance)

MODEL GRAPH

TABULATION ( Temperature vs Voltage)

MODEL GRAPH

RESULT

Thus the Temperature Vs Resistance and Voltage characteristics of thermistor was studied and the graph has been plotted.[NOTE]The type of thermistor sensor is NTC, so the output will be in reverse condition.i.e. 30°C - 5V

100°C - 0V

RESISTANCE TEMPERATURE DETECTORS

THEORY

Resistance Temperature Detectors or RTDs for short, are wire wound and thin film devices that measure temperature because of the physical principle of the positive temperature coefficient of electrical resistance of metals. The hotter they become, the larger their resistance. They, in the case of Platinum known variously as PRTs and PRT100s, are the most popular RTD type, nearly linear over a wide range of temperatures and some small enough to have response times of a fraction of a second. They are among the most precise temperature sensors available with resolution and measurement uncertainties or ±0.1 °C or better possible in special designs. Usually they are provided encapsulated in probes for temperature sensing and measurement with an external indicator, controller or transmitter, or enclosed inside other devices where they measure temperature as a part of the device's function, such as a temperature controller or precision thermostat. The advantages of RTDs include stable output for long period of time, ease of recalibration and accurate readings over relatively narrow temperature spans. They are active devices requiring an electrical current to produce a voltage drop across the sensor that can be then measured by a calibrated read-out device.

The requirements of a conductor material to be used in RTDs are:

1. The change in resistance of material per unit change in temperature should be as large as possible.2. The material should have a high value of resistivity so that minimum volume of material is used for the construction of RTD.3. The resistance of materials should have a continuous and stable relationship with temperature.

The most common RTDs are made of either platinum nickel or nickel allows. The economical nickel wires are used over a limited temperature range. They are quite non-linear and tend to drift with time. For measurement integrity, platinum is the obvious choice.

Before an RTD can be used for measurement or control, this change in resistance must be converted to a change in voltage or current. The electrical power dissipated in the RTD for this conversion must be strictly limited to avoid errors due to I2R heating of the sensor. Typically 10mW dissipation will cause the temperature rise of 0.3°C, which implies low values current (less than 10mA) and voltage (below 1V). The commonest circuits, however are based on Wheatstone bridge of Figure-4. If the measuring circuit has high impedance (so that it does not load the bridge), simple circuit analysis shows that:

The Figure above shows the non-linear output from the bridge is processed by a suitable linearising circuit to give an output voltage which is linearly related to the temperature. The linearising can be performed by an op-amp circuit.

EXPERIMENT AIMTo study the characteristics of temperature Vs Resistance and temperature Vs voltage and the accuracy of the signal conditioning circuits

PROCEDURE - TEMPERATURE VS RESISTANCE

1. Patch the wires of RTD to the T1 and T2 terminals of the RTD input block and switch ON the unit.

2. Place the RTD and thermometer into the holes provides in the water bath.

3. Keep the switch SW1 in right direction.4. Place the multimeter in the resistance mode across T3 and T4 terminals.5. Heat the water bath and note the temperature in thermometer and corresponding

resistance value in multimeter.6. Repeat step 5 for different values of temperature and tabulate the readings.7. Plot the temperature Vs resistance graph.

This gives the characteristic curve of the RTD. Refer to the model graph.

1. TABULAR COLUMN

PROCEDURE - TEMPERATURE VS VOLTAGE

2. Patch the wires of RTD to the T1 and T2 terminals of the RTD input block. 3. Switch ON the ITB -006CE Unit.4. Keep the switch in left direction and switch SW2 in external mode.5. Now adjust the ‘Zero’ Potentiometer to read 0°C at the display. This is done for initial

setup of the unit and this adjustment should be left undisturbed.6. Insert the RTD into the water bath and note the temperature without any heating at

ambient condition.7. Keep the switch SW1 in left direction and switch SW2 in internal mode.

8. Place the multimeter in voltage mode across the T6 and T7 terminals.9. Now, gradually start heating the water bath and note down the actual temperature,

output voltage of the unit and the displayed temperature simultaneously.10. Repeat step 8 for different values of temperatures and tabulate the readings.11. Plot the graph for Temperature Vs Voltage.12. Calculate the % error and plot the graph for Temperature Vs %Error

The first graph measures the linearity of the signal conditioning unit and the second graph measures the accuracy.

13. TABULAR COLUMN

MODEL GRAPH1. The graph between temperature and voltage are drawn.2. The graph between temperature and % Error are drawn

RESULT

Thus the study of Temperature Vs Voltage and the accuracy of signal conditioning board wasstudied and the graph is drawn