Vacuum Standards & Calibrations

Most calibrations of vacuum gauges are based on a known gas flow rate that is generated by one of two flow meters that operate at constant pressure. The "piston flow meter" generates a flow by allowing gas at a known pressure and temperature to leak out of a small vessel. The pressure in the vessel is held constant by slowly inserting a piston into the vessel. Combining the gas pressure and temperature with the piston cross section and insertion rate yields the molar flow rate. The "bellows flow meter" uses an oil-filled chamber that contains both a piston and a gas-filled metal bellows. Inserting the piston into the chamber causes an equal change in the volume of the bellows.

High Vacuum Standard
This vacuum standard is used to calibrate ion gauges at pressures as low as 10^-7 Pa (10^-9 Torr). A known gas flow enters the top of the vacuum chamber, passes through an orifice in the middle, and exits at the bottom. Kinetic theory allows the conductance of the orifice to be calculated from its known diameter, which in turn allows the pressure drop to be calculated accurately from the conductance and flow rate. The dark color of the chamber is due to its routine baking at 260 °C.


Transition Range Vacuum Standard
This standard is used to calibrate spinning rotor gauges at pressures in the transition range, most typically at 10^-2 Pa (10^-4 Torr). As with the high vacuum standard, a known flow of gas through a calculable conductance is used to generate
a known pressure in the chamber.