The thermocouple gauge is a device used to measure low vacuum. A filament is heated up by passing a current through it. When gas molecules interact with the filament, heat is carried away. Therefore higher pressure in the chamber means that more heat is taken away. The temperature of the filament can be measured with a thermocouple where the generated voltage depends on the temperature. Therefore in this system the voltage of the thermocouple is measured and converted into pressure units like millibars or pascals.
Vacuum can be understood as space from where matter (for example air) has been removed. It naturally exists in outer space but for certain applications, like materials characterization techniques, it needs to be achieved artificially. The desired level of vacuum is obtained with the help of a suitable vacuum pump. For example low vacuum (low quality vacuum with higher pressure) can be generated with a diffusion pump, scroll compressor pump, rotary vane pump, diaphragm pump or a sorption pump. High vacuum (high quality vacuum with very low pressures) however, can be obtained with high vacuum pumps such as the turbomolecular pump, ion pump, titanium sublimation pump and cryopump. The level of vacuum is measured with devices called vacuum gauges (vacuum meters) like the thermocouple gauge, pirani gauge, penning ionization gauge and the quadrupole mass spectrometer (analyzer). The working principle of vacuum pumps and vacuum gauges is explained with 3D animations in the video lecture above.