2.c Measurement of temperature using Thermocouple PDF

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Experiment no: -----------------

Date: -------------

MEASUREMENT OF TEMPERATURE USING THERMOCOUPLE AIM: 1. To plot the thermocouple output voltage versus temperature rise for the given thermocouple and hence determine the constant for the given thermocouple. 2. To design the amplifier gain so that a millivolt digital meter will directly read the temperature in °C. 3. To calibrate a given thermocouple based thermometer using an ordinary glass thermometer and plot the error versus temperature characteristic curve. APPARATUS REQUIRED: S No. Apparatus 1 Thermocouple 2 Heat Source 3 VARIAC 4 Thermometer 5 Amplifier with digital readout 6 Multimeter

Type & Range Iron/Constantine Soldering Iron heated Al block (0-270) V, 2A 0°C to 200°C (0-2) V digital

Quantity 1 1 1 1 1 1

TEST SET-UP FOR THERMOCOUPLE BASED THERMOMETER

THEORY: Thermocouples are active transducers and operate on the principle of See back effect i.e. when two dissimilar metals are connected together to form a junction called the sensing junction, an e.m.f is generated proportional to the temperature of the junction.

e.m.f generated is given by E= aΔθ Where Δθ = difference in temperature between hot thermocouple junction (sensing junction) and reference junction of thermocouple a = constant of thermocouple (or) sensitivity    

If the reference junction is kept at room temperature and hot junction is also kept at room temperature, E=0. Such a system will produce an output voltage proportional to temperature rise above room temperature. If this output voltage E can be measured, it will be indicating a temperature rise of E/a. Since constant ‘a’ is very small, emf produced will be too small to measure accurately. Hence it is amplified by a D.C. amplifier and fed to a voltmeter. For e.g., consider constant of Iron-Constantine thermocouple as 55μV/°C. Let the temperature rise to be measured be 1°C.The thermocouple will produce 0.055mV. If this emf of 0.055mV is amplified by a factor of 1/0.055 =18.18 and fed to milli voltmeter, it will read as 1mV. It will read 0, if temperature rise is 0, i.e. if hot junction is placed at room temperature.

Instead of indicating temperature rise, if actual temperature is required to be indicated, then D.C. amplifier can be given an output offset voltage of 30mV for 30°C ambient temperature. Now if temperature rise is 1°C, it will read 31°C. PROCEDURE: 1) 2) 3) 4) 5) 6)

Set the VARIAC to minimum output voltage position. Switch OFF the main switch of the kit. Short the input terminals (yellow and black on the kit) of the amplifier in the kit. Plug in the input of the kit to 230V A.C. source. Switch ON the main switch of the kit. Check with a multimeter if +5V and -5V are present in the kit (by connecting a DMM in 20V DC range between ground (black) and the blue, and black and green terminals). 7) The digital meter with input 0 Volts will now read the ambient temperature because of output offset provided in the amplifier. 8) Plug in the input of the heater, heating the Al block to the autotransformer output. Place thermocouple and the glass thermometer in the holes of the Al block in the box provided. 9) With the thermocouple in the ambient condition, note the output voltage E of thermocouple using a DMM in 200mV DC range. Remove the short, put on the input of the amplifier and place the output terminals of thermocouple at the input of the amplifier. Note the reading of the glass thermometer. 10) Switch on the supply to the auto-transformer. 11) Set the output at 25V. Wait for 5 minutes, so that the temperature attains steady state. Note the i. Voltage at the input of the amplifier using DMM in 200mV. ii. Temperature on the digital display. iii. Output voltage of amplifier using DMM in 2V DC.

iv. Glass Thermometer reading. 12) Repeat step 11 with 50V, 75V and 100V. 13) Plot the graphs of i. Output volt of thermocouple versus temperature and calculate sensitivity of thermocouple. ii. Calculate gain of amplifier required for direct reading of temperature by millivolt. iii. Calculate gain of amplifier in the kit. iv. Find the % error and plot the error versus temperature curve. RESULT: The experiment was conducted and thermocouple output voltage versus temperature curve is plotted. 1. Sensitivity of thermocouple = 2. Gain = 3. Gain of amplifier in kit = 4. Maximum % error of digital thermometer =

INFERENCE: OBSERVATION: Ambient temperature read from digital display with amplifier input shorted=

°C

Table 1: CALIBRATION OF THERMOMETER: Ambient temperature= °C S.No Thermocouple VoltageVt(mV)

Glass Thermometer Reading- θg

Temperature from digital readout- θλ

Output of AmplifierVa

Amplifier Gain= Va/ Vt

% error θg- θλ ---------- X100

θg 1.

0

For the graph, Sensitivity of the thermocouple=Slop of the thermocouple voltage Vs temperature graph = μV/°C.

Amplifier gain required Sensitivity

=Voltage required per °C =

Report Submission Date: …………… Assessment

Max. Marks

Submission Delay: …........ Initial Preparation and Viva Observation & Conduction

20 20

Result and Inference

20

Total

60

Graded Marks...