204
5 Vacuum System
Calibration valve and vent valve
Calibration valve and vent valve
Calibration valve
The calibration valve is an electromechanical valve with a vial to hold the tuning
compound. The calibration valve is held onto the front end plate by two screws. A
small O-ring provides a face seal. The valve is controlled by the MSD ChemStation.
When the calibration valve is opened, tuning compound in the vial diffuses into the
vacuum manifold. The valve has a restrictor that controls the entry of the tuning
compound into the vacuum manifold. The turbo pump and the diffusion pump
MSDs have different calibration valves designed to allow the correct diffusion of
calibrant for each vacuum system.
Perfluorotributylamine (PFTBA) is the most commonly used tuning compound. It
is required for automatic tuning of the MSD. Other compounds can be used for
manual tuning.
Vent valve
The vent valve knob screws into a threaded port in the front end plate. An O-ring
is compressed between the knob and the end plate to form a seal. The threaded
end of the knob has an air passage through its threads, allowing air to flow into the
manifold when the knob is partially unscrewed. If you turn the knob too far, the
O-ring can come loose.
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