Thermoelectric Thermometers
A thermoelectric thermometer is used for the measurement of temperatures both low and high.
Principle its working is based on see beck effect thermocouples of different metals are constructed depending upon the temperature to be measured as given below.
Thermocouple Range of temperature
Copper and gold-iron alloy 1K to 50 K
Copper and constantan 50 K to 400 K
Platinum and platinum iridium alloy 400 K to 1300 K
Platinum and platinum rhodium alloy 1300 K to 2000 K
Construction the wires forming the thermocouple are welded together at one end and this end forms the hot junction. The portions of the wires near the hot junction are properly insulated from each other by enclosing them in a hard-glass capillary tube C.T. the wires are passed through mice discs D, which are fitted one above the other in a porcelain tube T. the ends of these wires are connected to the terminals Ti and T2 to these terminals are connected compensating leads Li and L2 of the same materials as those forming the thermocouple itself. By so doing the cold junction is shifted to a convenient place where a constant temperature of OC is maintained.
By keeping hot junction at different known temperatures the corresponding values of thermo are noted using distal voltmeter in the circuit of thermocouple. The digital voltmeter gives its internal resistance is very high (about 10 O). A graph plotted between the temperature of hot junction and Themo the curve so obtained is called calibration curve.
To find the unknown temperature of a given bath the hot junction is placed in the given bath. The thermo so generated is measured. The temperature corresponding to this value of thermo is determined from the calibrations curve.
Note it is important to note that thermocouple should not be used to measure the temperature above the neutral temperature.
Advantages
1. Thermoelectric thermometers can be used over much wide3er range of temperatures; ranging from – 200C to 1600C.
2. These thermometers are cheap and can be constructed easily.
3. These can be used to measure the temperature of small cavity.
4. Their thermal capacity being low they attain the temperature to be measured very quickly and are thus useful for measuring changing temperatures.
Disadvantages
1. Since these thermometers are not direct riding thermometers so they cannot be used for calorimetric purposes.
2. There is no common relation connecting thermo and the temperature of a junction in any thermocouple. Therefore the given thermo couple has to be calibrated first.
3. For the measurement of temperature over different ranges, different thermocouples are to be used.
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