This is an Agilent Varian 531 Thermocouple Gauge Simulator and Reference Tube. This aids and To Be Used When Testing or Calibrating Gauge Controllers. It simulates both 2.0 Torr pressure and 1 × 10-3 torr. The Reference Tube is permanently sealed with a vacuum inside at a Pressure of <=5 × 10-6 Torr.

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.