05-Thermocouples

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5. MEASURING WITH THERMOCOUPLE

Objective

1. Check the function of two-wire transducer XTR101 for measuring temperature with

thermocouples.

2. Check the use of measuring module Janascard AD232 with isothermal terminal board and

serial interface.

Introduction:

In the setup, there are thermocouples of K-type. It is thermocouple NiCr-Ni (nickel/chromium

- nickel), also known as Chromel-Alumel (Ch-A). There are three K-thermocouples in the

laboratory miniature furnace. Their sensitivity is approximately =&

42 V/K.

The first thermocouple is connected directly to the module Janascard AD232.

The second thermocouple is connected to transducer XTR101. The reference ends are placed

in a bulk aluminum block together with a bipolar transistor whose PN junction is used by

XTR101 for measuring the temperature of reference (cold) ends. To evaluate the quality of

reference ends temperature compensation by XTR101, the aluminum block is heated up by

current (approx. 1 A) passing through a power transistor mounted on front side. The

temperature of the aluminum block can be inspected by a digital room thermometer with

external probe. The probe is also inserted into the aluminum block. The lower of the two

displays shows the temperature of aluminum block.

Note: the third thermocouple is connected to so called electronic ice point (battery powered

device for reference ends temperature compensation).

Procedure

5.1. Two wire transducer XTR 101

5.1.1. Study the manual of XTR101

5.1.2. Influence of reference (cold) end temperatureSimply stated, the generated thermoelectric voltage is proportional to the difference of

temperatures between the measuring (hot) junction and the reference (cold) ends. Thus,

warming up of the reference ends will influence the generated voltage.

While the furnace is off (stable temperature of junction) the reference ends in aluminum block

are heated up by current in the attached power transistor. Warm up the block several degrees

above the ambient temperature and observe the change of generated voltage by a voltmeter.

The same thermoelectric voltage is connected also to the input of XTR101 circuit. The

temperature of reference ends is sensed by PN junction of an auxiliary transistor. If the

resistor network of XTR101 is properly designed and the PN junction is connected, the circuit


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compensates any changes of reference ends temperature and the resulting output voltage is

not influenced.

The output signal of XTR101 is measured by another voltmeter. This voltage should be stable

in spite of heating of the cold ends of thermocouple and thus variations of the thermoelectric

voltage in the input. (The XTR101 is able to compensate the heating up of reference ends.)

5.1.3. Influence of the wire resistance

While the temperature of the hot junction and the cold ends is stable, examine the influence of

wire resistance. The chip XTR101 output works in current loop, i.e. the output signal is the

current in circuit between the voltage supply and the XTR101. (In our case, the measured

signal is voltage drop on a sensing resistor RL). The current should be proportional only to the

measured quantity (here - temperature). However, when the resistance of supply wires is too

high, the chip is not able to regulate the current properly and signal is distorted.

Increase the resistance of the supply circuit by setting the decade resistor box from zero to

higher and higher value. Observe what is the maximum resistance before signal is distorted.

5.2. Using measuring module Janascard AD232

Apply line voltage (100-220V) to the furnace and let it heat up to 150-200 C. Measure the

generated thermoelectric voltage and also the output voltage of XTR101 (10 values). At the

same time, monitor the temperature in furnace by module Janascard AD232. The controlling

software in PC can display the voltage and recalculated temperature according to setup.

Record also the output voltage of third thermocouple compensated by electronic ice point.

Fig. 5.1 The furnace and thermocouples

Furnace

XTR101 circuit

Aluminum block

(thermostat)


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Fig. 5.2 Block diagram of XTR 101

Fig. 5.3 Cold-end temperature compensation

Fig. 5.4 Setting up the offset value (4-20mA)

Thermocouple

sensing PNunction fortemperaturecompensation


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Compensation box (electronic ice point) Omega CJ

Precission of compensation: 0,25 C at 25 C

0,5 C for 15 to 35 C

0,75 C for 10 C to 50 CSupply: lithium pill 3,6 V

Note: Turn off after measurement !!!

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05-Thermocouples