thermocouple characteristics trainer -...

THERMOCOUPLECHARACTERISTICS TRAINER

(Model No : ITB - 05CE)

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, 91-44-24963142Mail : [email protected],Web : www.vimicrosystems.com02 - 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

THERMOCOUPLE CHARACTERISTICS TRAINER ITB - 05CE

Vi Microsystems Pvt. Ltd., [ 1 ]

1. INTRODUCTION

Temperature measurement plays a major role in industrial application. The various sensors whichis used to measure the temperature are thermocouple, RTD, Thermistor etc. Due to the salientfeatures of thermocouple, it is being widely used in industries. Based on the thermoelectricprinciple, it senses the temperature of the medium. The two junction temperature difference isdirectly 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 compensationtechnique, any one can calibrate the thermocouple for desired temperature measurement.

2. TECHNICAL SPECIFICATION

i. 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

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



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



3. FRONT PANEL DIAGRAM



4. FRONT PANEL DESCRIPTION

Power ON/OFF - Power ON/OFF switch is used to ON/OFF theunit.

(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 compensatedoutput.

* 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 roomtemperature. When the compensated mode.

(LED) display - Shows the temperature in Celsius



5. TEMPERATURE MEASUREMENT

In general, there are four types of sensors based on the following physical properties, which aretemperature dependent:

1. Expansion of a substance with temperature, which produces a change in length, volumeor pressure. In it's simplest form is the common mercury-in-glass or alcohol-in-glassthermometer.

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 andthermistors.

The second property is used in our analysis as a temperature sensor. Thermocouple are activetransducers which can generate voltage when subjected to a temperature source. They don't needany external excitation for the voltage generation which is in the range of millivolts. It is suitablysignal-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 processtemperatures. The operation of a thermocouple is based upon Seeback effect which states thatwhen heat is applied to junction (hot junction) of two dissimilar metals, an emf is generated whichcan be measured at the other junction (cold junction). The two dissimilar metals form an electriccircuit, and a current flows as a result of the generated emf as shown in Fig. I

Figure - 1The emf produced is function of the difference in temperature of hot and cold junctions and isgiven by:

E = a

where = difference between temperatures of hot and cold junctions.



EMF

BACK EFFECT

T2

SEE

B

T1

A

7. SEEBACK EFFECT

Using solid state theory, the above mentioned situation may be analysed to show that its emf canbe given by an integral over temperature.

Figure - 2Where,

E = emf produced in voltsT1,T2 = Junction temperature in KQA, QB = Thermal transport constants of the two metals.

This equation, Which describes the seeback effect, shows that the emf produced is proportionalto the difference in temperature and further, to the difference in the metallic thermal transportconstant. 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 QA and QB are nearly independent of temperatureand that an approximate linear relationship exists as

E = (T2-T1)

Where, = Constant in volt / KT1,T2 = Junction temperatures in K

However, the small but finite temperatures dependence of QA and QB is necessary for accurateconsiderations.



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 aresuitable for recording comparatively rapid changes in temperature.

Thermocouples are very convenient for measuring the temperature at one particular point in apiece 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 beprotected in an open or closed end metal protecting tube or well. To preventcontamination of the thermocouple, when precious metals like platinum or its alloys arebeing 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 madeby means of wires called extension wires. Maximum accuracy of measurement is assuredonly when compensating wires are of the same material as the thermocouple wires. Thecircuitry is, thus, very complex.

The application notes of some thermocouple are given below.

Type Application Notes

J For reducing atmosphere

K For oxidizing atmosphere

T When moisture is present

E When corrosion possibility

R High resistance to oxidation and corrosion

S High resistance to oxidation and corrosion



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. TheAD590 converts its ambient temperature in degrees Kelvin into an output current, IC that is IµAfor every degree Kelvin. In terms of Celsius temperatures L Ir = 273 at 0°C(273°K) and 373 µAat 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 avoltage reading to indicate temperature, such as 10m V/°F, a current to voltage converter.

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.

f

OP-177+

-0

-15V

V = (10mv/ c) x (Temp in c)

R = 10K +1%

+15V

3

2

TI-+

AD590

273 A+15V

-15V

54.9K

10K

OP-177+

-0

-15V

V = (10mv/ F) x (Temp in F)

+15V

3

2

TI+-

AD590

255 A+15V

+15V

58.8K

10K

R = 18.18Kf

a) V = 0v at 0 C and 1000mV at 100 C

b) V = 0v at 0 F and 1000mV at 100 F

AD 590 TEMPERATURE TRANSDUCER



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 keptas a reference and the other is subjected to the temperature. Depending on the difference in thetemperature of the two junctions, it develops an output voltage without the need of anyexcitation. Hence is called an active transducer on the principle of seeback effect. The outputvoltage 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 ofmillivolts. This millivolts output which is obtained from thermocouple is given to invertingamplifier 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 becompensated. 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 singlemonolithic integrated circuit. AD590 gives current as output signal proportional to thetemperature. This is suitably signal condoned and gives an output voltage of 1.5V. The currentoutput of AD590 is

IOUT = 1 × 10-6 T amps, Where T = Temperature, °K (or)

IOUT = 273 × 10-6 + 1 × 10-6 Ampere,

Where = Temperature, °C



EXPERIMENTAL SECTIONEXPERIMENT - 1

AIM

To study the characteristics of thermocouple.

APPARATUS REQUIRED

i. ITB-05CEii. Thermocouple (J)iii. Water bathiv. Thermometerv. Digital multi metervi. 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 inMultimerter.

5. Tabulate the readings temperature Vs mV and plot the graph.

Tabular Column

ActualTemperature (°C)

Thermocouple Output (mV)



MODEL GRAPH

RESULT

Thus the characteristics of thermocouple was studied and graph is plotted.

Temperature( C) Vs Thermocouple Output(mV)

Termocouple Output(mV)

Thermocouple Output : 3.8mv approximateTemperature : 100 C

Tem

pera

ture

( C

)



EXPERIMENT - 2

AIM

To study the characteristics of thermocouple without compensation.

APPARATUS REQUIRED


FORMULA

% EActual Temp Displayed Temp

Full Scale Division

100

PROCEDURE


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 offsetknob ‘Zero’ to set 0°C in display.


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.



Tabular Column


DisplayedTemperature (°C)

% Error


Full Scale Division

100

MODEL GRAPH

RESULT

Thus the characteristics of thermocouple without compensation was studied and graph is plotted.



EXPERIMENT - 3

AIM

To study the characteristics of thermocouple with compensation.

APPARATUS REQUIRED


FORMULA


Full Scale Division

100

PROCEDURE


2. Position the switch ‘SW1' towards ‘NO’.

3. If there is any difference in displayed temperature at room temperature, adjust the offsetknob ‘Zero’ to set 0°C in display.

4. Switch ‘ON’ the unit and note the displayed temperature.


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 displayedtemperature simultaneously.

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



11. Plot the graph for

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

Tabular Column


DisplayedTemperature (°C)

Signalconditioneroutput (V)

% Error


Full Scale Division

100

MODEL GRAPH



ii. TEMPERATURE Vs SIGNAL CONDITIONER OUTPUT VOLTAGE

RESULT

Thus the characteristics of thermocouple with compensation was studied and graph is plotted.

Tem

pera

ture

( C

)

Temperature : 100 COutput Voltage : 5V

Output Voltage(V)

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