Thermo-Electric Thermometer

What is Thermo-Electric Thermometer?

The principle of the Seebeck effect and the concept of thermo-electricity are used to construct very accurate thermometers called thermo-electric thermometer which is a device used for measuring low as well as high temperatures.

Principle of Thermo-Electric Thermometer:

Its working is based on Seebeck Effect. In this thermometer, different thermocouples are used for measuring different range of temperature. Example-

• Constantan-Copper couple is used for temperature range -200°C to 400°C.
• Constantan-Iron couple is used for temperature range 0°C to 800°C.
• Copper-Iron couple is used for temperature range 0°C to 260°C.
• Chromel-Alumel couple is used for temperature range 0°C to 1200°C.
• Platinum-Platinum rhodium couple is used for temperature range 0°C to 1600°C.

Construction of Thermo-Electric Thermometer:

The wires forming the thermocouple are welded at junction A and junction A functions as host junction. The portions of the wires near the hot junction A are placed in hard-glass capillary tubes C.T. The wires are then passed through mica discs DD, which are fitted in a glass tube T. The ends of the wires are connected to terminals T₁ and T₂. Also, terminals T₁ and T₂ are further connected to compensating leads L₁ and L₂.

Keep the temperature of a cold junction at 0°C. Now, keep the hot junction at different temperature and note the corresponding values of thermo EMF, with the help of a voltmeter. Then, a graph is plotted between the temperature of the hot junction and thermo EMF, as shown in the figure. The curve so obtained is called the calibration curve.

Now, the hot junction A of the thermocouple is placed in a hot bath, whose temperature is to be measured and note the thermo EMF. Then the temperature corresponding to this thermo emf (from the calibration curve) is the temperature of a hot bath.

Advantages of Thermo-Electric Thermometer:

• These thermometers are used for measuring a wide range of temperature from -200°C to 1600°C.
• The hot junction attains the temperature of the hot body very rapidly due to its small thermal capacity. Thus, it can be used to measure slowly varying temperatures also.
• These are very cheap.
• These are easily constructed.

Disadvantages of Thermo-Electric Thermometer:

• These thermometers are not direct reading thermometers.
• We have to draw a calibration curve first for measuring temperature.
• Different thermocouples are used for measuring different range of temperature.