Thermocouple vacuum gauge measures the temperature change of the hot wire by means of thermocouple. And the thermoelectric potential generated by the thermocouple characterizes the pressure in the gauge. The pressure measurement range of the DL-3 thermocouple gauge is 102 to 10-1Pa.
The structure diagram of thermocouple vacuum gauge is showed in Figure 1. It consists of thermocouple gauge and measurement circuits. The thermocouple gauge is mainly composed of hot wire and thermocouple. The hot end of the thermocouple is connected with the hot wire, and the other end is used as cold end and via the lead wire outlet tube to connect to a millivoltmeter for measuring the thermocouple potential. Measuring circuit is relatively simple.
During the measurement, the hot wire of gauge is given a certain heating current. At lower pressures (λ≥r2), the thermocouple potential E is determined by the pressure p in the gauge, because the temperature change of the filament depends on the heat conduction of the gas. When the pressure decreases, the amount of heat conducted by the gas molecules decreases, and the temperature of the hot wire increases accordingly. As a result, the thermocouple potential E increases. Otherwise the thermocouple potential E decreases.
The change of heating current of gauge has affects on both the sensitivity and the measuring range. If the heating current increases, the regulatory sensitivity will increase and the higher pressure can be measured, but the measuring range becomes narrower. On the other hand, if the heating current decreases, the regulatory sensitivity will decrease, and only a lower pressure can be measured, but the measuring range is bigger.
When the heating current is constant, if the relationship between the thermocouple potential E and the pressure p has been measured in advance, the pressure of the system under test can be directly figured out according to the indication of the millivoltmeter.