As atmosphere is removed during evacuation the ability to transfer heat by conduction to surrounding air becomes weaker and weaker. Therefore vacuum is easily and accurately read with thermocouple gauges with a preheated sensing element. As the vacuum gets deeper and deeper the sensor simply gets hotter and hotter because there are fewer molecules to conduct away the heat. The change in temperature of the thermocouple can then be related to the level of degassing and dehydration of the piping and components of the system. Accutools and BluVac both use thermocouple technology for precise vacuum measurement.