4.1.3 Experimental principles
A closed loop composed of two different metals is shown in Figure 4.1.1.When the temperatures at the two junctions A and B are different,a thermo-electromotive force is generated in the loop.Such a device composed of two metals is called a thermocouple.It has been empirically proved that when the temperature difference between the two junctions is not large,the thermo-electromotive force in the thermocouple is related to the temperature difference between the hot and cold junctions.This relationship can be approximately described as:
Where t stands for temperature of the hot junction and t0 stands for the temperature of the cold junction,α is often called thermoelectric coefficient or thermocouple constant which is determined by the material of the thermocouple.In the experiment,by putting in the ice-water mixture,the temperature of the cold junction is usually constant.On the contrary,the temperature of the hot junction usually changes.
The magnitude of the thermo-electromotive force can be measured by a digital multimeter.In addition,the thermocouple thermometer can be formed by combining the thermocouple with a digital multimeter to measure temperature(Figure 4.1.2).And the experimental method is used at first to determine the relationship between the thermo-electromotive force Et and the temperature difference Δt=t-t0.This experimental measuring process is known as the calibration of the thermocouple thermometer.The thermocouple calibrated in this way is a thermocouple thermometer that can be used to measure temperature.When it is being used,one junction of the thermocouple is still placed at a constant temperature(0℃)while the other junction is inserted into the temperature field for measuring.If the measured thermo-electromotive force is Ex,the temperature tx to be measured can be checked on the calibration curve.(https://www.daowen.com)
Figure 4.1.2 Measuring wiring diagram
The thermocouple can be used in a temperature range of-200～+2 000℃,with high sensitivity up to 10-3℃.In addition,the heat capacity and heating surface of the thermocouple are very small.Still,the thermocouple is widely used in production and scientific researches.
