THERMOCOUPLE CHARACTERISTICS TRAINER (Model No : ) User Manual Version 2.0 Technical Clarification /Suggestion : / Technical Support Division, Vi Microsystems Pvt. Ltd., Plot No :75,Electronics Estate, Perungudi,Chennai - 600 096,INDIA. Ph: 91-44-24961852, 91-44-24963142 Mail : service@vimicrosystems.com, Web : www.vimicrosystems.com 02-13 - 11-14
CONTENTS 1. INTRODUCTION 1 2. TECHNICAL SPECIFICATION 1 3. FRONT PANEL DIAGRAM 3 4. FRONT PANEL DESCRIPTION 4 5. TEMPERATURE MEASUREMENT 5 6. THEORY OF THERMOCOUPLE 5 7. SEEBACK EFFECT 6 8. ADVANTAGE AND DISADVANTAGE 7 9. AD590 TEMPERATURE TRANSDUCER 8 10. CIRCUIT DESCRIPTION 9 11. EXPERIMENTAL SECTION 13
1. INTRODUCTION Temperature measurement plays a major role in industrial application. The various sensors which is used to measure the temperature are thermocouple, RTD, Thermistor etc. Due to the salient features of thermocouple, it is being widely used in industries. Based on the thermoelectric principle, it senses the temperature of the medium. The two junction temperature difference is directly proportional to the generated emf, which is a measure of temperature. This unit helps to study the characteristics of thermocouple with and without compensation. Temperature compensation is performed by AD590 temperature sensor. From this compensation technique, any one can calibrate the thermocouple for desired temperature measurement. 2. TECHNICAL SPECIFICATION i. 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 ii. Thermocouple Type - J type Material - Iron constant Tube Diameter - 6mm Working Temperature - -200 to 760 C Tube Length - 120mm Thermowell material - Stainless steel Coating - Nickel, Chromium Cable Length - 950mm Vi Microsystems Pvt. Ltd., [ 1 ]
iii. LED Display Size - 50 20mm Type - Common anode Display - 3.5 Digit Segment - 7 Segment Colour - Green iv Power Supply Input - 230V AC / 50Hz Output - +5V / 1A -5V / 500mA +12V / 500mA -12V / 500mA Vi Microsystems Pvt. Ltd., [ 2 ]
3. FRONT PANEL DIAGRAM Vi Microsystems Pvt. Ltd., [ 3 ]
4. FRONT PANEL DESCRIPTION Power ON/OFF - Power ON/OFF switch is used to ON/OFF the unit. (Push Button) T/C sensor (T1, T2) - Used to connect the thermocouple terminals. T3, T4 - Used to measure the thermocouple output (mv). T5 - To measure the amplified output. T6 - To measure the AD590 sensor output. T7 - To measure the signal conditioner output. T8 - GND SW1 - To select either uncompensated or compensated output. * Compensated output : Place the switch SW1 towards NC. * Uncompensated output : Place the switch SW1 towards NO. Zero - Adjust this knob to set 0 C in display at room temperature. When the compensated mode. (LED) display - Shows the temperature in Celsius Vi Microsystems Pvt. Ltd., [ 4 ]
5. TEMPERATURE MEASUREMENT In general, there are four types of sensors based on the following physical properties, which are temperature dependent: 1. Expansion of a substance with temperature, which produces a change in length, volume or pressure. In it's simplest form is the common mercury-in-glass or alcohol-in-glass thermometer. 2. Changes in contact potential between dissimilar metals with temperature; thermocouple. 3. Changes in radiated energy with temperature; optical and radiation pyrometers. 4. Changes in electrical resistance with temperature, used in resistance thermometers and thermistors. The second property is used in our analysis as a temperature sensor. Thermocouple are active transducers which can generate voltage when subjected to a temperature source. They don't need any external excitation for the voltage generation which is in the range of millivolts. It is suitably signal-conditioned and gives an output voltage of (0-5) volt for a temperature of (0-100) C. 6. THEORY OF THERMOCOUPLE The thermocouple is one of the simplest and most commonly used methods of measuring process temperatures. The operation of a thermocouple is based upon Seeback 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 a current flows as a result of the generated emf as shown in Fig. I Figure - 1 The emf produced is function of the difference in temperature of hot and cold junctions and is given by: E = a where = difference between temperatures of hot and cold junctions. Vi Microsystems Pvt. Ltd., [ 5 ]
7. SEEBACK EFFECT Using solid state theory, the above mentioned situation may be analysed to show that its emf can be given by an integral over temperature. SEE BACK EFFECT A T2 EMF T1 B Where, Figure - 2 E = emf produced in volts T 1,T 2 = Junction temperature in K Q A, Q B = Thermal transport constants of the two metals. This equation, Which describes the seeback effect, shows that the emf produced is proportional to the difference in temperature and further, to the difference in the metallic thermal transport constant. Thus, if the metals are the same, the emf is zero and if the temperatures are the same, the emf is also zero. In practice, it is found that the two constants Q A and Q B are nearly independent of temperature and that an approximate linear relationship exists as E = (T 2 -T 1 ) Where, = Constant in volt / K T 1,T 2 = Junction temperatures in K However, the small but finite temperatures dependence of Q A and Q B is necessary for accurate considerations. Vi Microsystems Pvt. Ltd., [ 6 ]
8. 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. Thermocouples are very convenient for measuring the temperature at one particular point in a piece of apparatus. ii. Disadvantages of Thermocouples 1. The 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 or well. 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 material as the thermocouple wires. The circuitry is, thus, very complex. The application notes of some thermocouple are given below. Type J K T E R S Application Notes For reducing atmosphere For oxidizing atmosphere When moisture is present When corrosion possibility High resistance to oxidation and corrosion High resistance to oxidation and corrosion Vi Microsystems Pvt. Ltd., [ 7 ]
9. AD590 TEMPERATURE TRANSDUCER An electronic thermometer can be made from a temperature transducer, and op amp and registers. We shall select the AD590 manufactured by Analog Devices, as the temperature transducer. The AD590 converts its ambient temperature in degrees Kelvin into an output current, IC that is IµA for every degree Kelvin. In terms of Celsius temperatures L Ir = 273 at 0 C(273 K) and 373 µa at 100 C (373 K). In terms of Fahrenheit temperatures, Ir = 225 µa at 0F and 310 µa at 100 F. Thus the AD590 acts as a current source that depends on temperature. If however, we need a voltage reading to indicate temperature, such as 10m V/ F, a current to voltage converter. AD 590 TEMPERATURE TRANSDUCER -15V 10K + AD590 I T - R f = 10K +1% +15V +15V 54.9K 273 A 2 3 - OP-177 + -15V V 0= (10mv/ c) x (Temp in c) a) V = 0v at 0 C and 1000mV at 100 C +15V 10K - AD590 I T + R = 18.18K f +15V +15V 58.8K 255 A 2 3 - OP-177 + -15V V 0= (10mv/ F) x (Temp in F) b) V = 0v at 0 F and 1000mV at 100 F Also, the AD590 requires a supply voltage exceeding 4V to bias internal transistor circuitry. Let s use this device to build a Celsius or Fahrenheit thermometer. Vi Microsystems Pvt. Ltd., [ 8 ]
10. CIRCUIT DESCRIPTION This module is designed to study the characteristics of J type thermocouple described below; i. J-type thermocouple This active transducer is made of Iron and Constantan metals. There are two junctions, one kept as a reference and the other is subjected to the temperature. Depending on the difference in the temperature of the two junctions, it develops an output voltage without the need of any excitation. Hence is called an active transducer on the principle of seeback effect. The output voltage is in millivolts. This voltage is suitably signal conditioned to give an output in volts. The thermocouple senses the temperature from the temperature source (Water bath) in terms of millivolts. This millivolts output which is obtained from thermocouple is given to inverting amplifier for further amplification. This amplifier amplifies the given millivolts in the range of (0-3.5)V. Note: The main disadvantages of T/C is that it gives 0 millivolt for room temperature. It must be compensated. Due to this reason the temperature compensated circuit is used. ii. Temperature Compensated Circuit It have temperature sensing element (AD590) with signal conditioning electronics all in a single monolithic integrated circuit. AD590 gives current as output signal proportional to the temperature. This is suitably signal condoned and gives an output voltage of 1.5V. The current output of AD590 is I OUT = 1 10-6 T amps, Where T = Temperature, K (or) I OUT = 273 10-6 + 1 10-6 Ampere, Where = Temperature, C Vi Microsystems Pvt. Ltd., [ 9 ]
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EXPERIMENTAL SECTION EXPERIMENT - 1 AIM To study the characteristics of thermocouple. APPARATUS REQUIRED i. ITB-05CE ii. Thermocouple (J) iii. Water bath iv. Thermometer v. Digital multi meter vi. Power Chord. PROCEDURE 1. Connect the two terminals of the thermocouple across T1 & T2. 2. Insert the thermocouple and thermometer into the water bath. 3. Place the Multimeter (millivolts mode) across T3 and T4. 4. Switch ON the water bath and note the temperature in thermometer and mv in Multimerter. 5. Tabulate the readings temperature Vs mv and plot the graph. Tabular Column Actual Temperature ( C) Thermocouple Output (mv) Vi Microsystems Pvt. Ltd., [ 13 ]
MODEL GRAPH Temperature( C) Vs Thermocouple Output(mV) Temperature( C) Temperature : 100 C Thermocouple Output : 3.8mv approximate Termocouple Output(mV) RESULT Thus the characteristics of thermocouple was studied and graph is plotted. Vi Microsystems Pvt. Ltd., [ 14 ]
EXPERIMENT - 2 AIM To study the characteristics of thermocouple without compensation. APPARATUS REQUIRED i. ITB-05CE ii. Thermocouple (J) iii. Water bath iv. Thermometer v. Digital multi meter vi. Power Chord. FORMULA % E Actual Temp Displayed Temp Full Scale Division 100 PROCEDURE 1. Connect the two terminals of the thermocouple across T1 & T2. 2. Position the switch SW1' towards NO. 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. Switch ON the water bath. 7. Note the actual temperature in thermometer and displayed temperature simultaneously. 8. Tabulate the reading and calculate %Error using the above formula. 9. Plot the graph actual Temperature Vs% Error. Vi Microsystems Pvt. Ltd., [ 15 ]
Tabular Column Actual Temperature ( C) Displayed Temperature ( C) % Error % E Actual Temp Displayed Temp Full Scale Division 100 MODEL GRAPH RESULT Thus the characteristics of thermocouple without compensation was studied and graph is plotted. Vi Microsystems Pvt. Ltd., [ 16 ]
EXPERIMENT - 3 AIM To study the characteristics of thermocouple with compensation. APPARATUS REQUIRED i. ITB-05CE ii. Thermocouple (J) iii. Water bath iv. Thermometer v. Digital multi meter vi. Power Chord. FORMULA % E Actual Temp Displayed Temp Full Scale Division 100 PROCEDURE 1. Connect the two terminals of the thermocouple across T1 & T2. 2. Position the switch SW1' towards NO. 3. If there is any difference in displayed temperature at room temperature, adjust the offset knob Zero to set 0 C in display. 4. Switch ON the unit and note the displayed temperature. 5. Insert the thermocouple and thermometer into the water bath. 6. Place the multimeter across T7 & T8. 7. 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. Vi Microsystems Pvt. Ltd., [ 17 ]
11. Plot the graph for i. Actual Temperature Vs % Error. ii. Actual Temperature Vs signal conditioner output. Tabular Column Actual Temperature ( C) Displayed Temperature ( C) Signal conditioner output (V) % Error % E Actual Temp Displayed Temp Full Scale Division 100 MODEL GRAPH Vi Microsystems Pvt. Ltd., [ 18 ]
ii. TEMPERATURE Vs SIGNAL CONDITIONER OUTPUT VOLTAGE Temperature( C) Temperature : 100 C Output Voltage : 5V Output Voltage(V) RESULT Thus the characteristics of thermocouple with compensation was studied and graph is plotted. Vi Microsystems Pvt. Ltd., [ 19 ]