Agilent Varian 531 Thermocouple Vacuum Pressure Measurement Gauge Tube Sensor, 1/8 in. NPT Mild Steel
Varian Part Number F0472301

The Agilent Varian 531 thermocouple gauge tube sensor provides continuous accurate indication of total pressure from 1 x 10-3 torr to 2.0 torr (1X10-3 mbar to 2.7 mbar). Developed to combine rugged dependability and outstanding performance for roughing and foreline pressure measurement. The Varian 531 thermocouple vacuum gauges are a low cost pressure measurement sensor. The Varian 531 thermocouples tube sensors offers high resistance to vibration and is nickel plated mild steel. Varian Part Number F0472301

Thermocouple Vacuum Pressure Gauges, by Wikapedia.org
Thermal conductivity gauges rely on the fact that the ability of a gas to conduct heat decreases with pressure. In this type of gauge, a wire filament is heated by running current through it. A thermocouple or Resistance Temperature Detector (RTD) can then be used to measure the temperature of the filament. This temperature is dependent on the rate at which the filament loses heat to the surrounding gas, and therefore on the thermal conductivity. A common variant is the Pirani gauge which uses a single platinum filament as both the heated element and RTD. These gauges are accurate from 20 Torr to 10−3 Torr (depending on the model of sensor and controller purchased). However, they are sensitive to the chemical composition of the gases being measured and are termed a relative pressure measurement method, that is, they determine the pressure indirectly from the thermal conductivity property of the gas medium.