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5. Monitor inlet and foreline pressures.
During operation of the diffusion pump:
The gas load at the inlet should not exceed the maximum throughput capability of
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the pump.
The forepressure should not exceed the specified tolerable forepressure.
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2.1.1 Humid Environments
Due to their inherent design, diffusion pump heaters absorb moisture. This results in
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a decrease in heater insulation resistance (higher leakage current). Under normal
conditions, the decrease in insulation resistance is a momentary condition that
occurs on startup. The amount of moisture adsorbed depends on the temperature,
humidity, and the length of time the heaters have been exposed to this
environment without power being applied. If diffusion pumps are started in high
humidity environments, the heaters may be damaged on startup at full power. To
overcome the effect of moisture absorbed in the heaters, start pumps at a low line
voltage,e.g., 50% of line voltage, for a period or time to slowly expel any moisture.
2.2 Shutdown
1. Turn off the power to the diffusion pump and continue to back the diffusion pump with
the appropriate mechanical pump.
2. Allow cooling water to flow through the diffusion pump until the pump body
temperature, located just above the boiler plate, has cooled to a temperature of
approximately 130 °F.
After isolating the backing pump, the diffusion pump can be vented to atmosphere.
If faster cooling is desired, the pump can be cooled using the Quick Cool Coil at the
bottom of the diffusion pump.
VHS-4, VHS-6 and VHS-250 Diffusion Pumps
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