Varian VHS-6 Instruction Manual download pdf (Page 47)

4-1

VHS-4, VHS-6 and VHS-250 Diffusion Pumps

DRAFT 1/23/08

Section 4. Troubleshooting

4.1 Leakage

Analysis of general operational experience with diffusion pumps indicates that certain

locations are more prone to vacuum leaks. The following locations should be checked first

if leakage is the suspected cause of poor system performance:

❑ Inlet and foreline connections

❑ Drain and fill plugs

❑ Other compression fittings, such as high vacuum gauges in the system

❑ Threaded connections, such as foreline gauge

4.2 Outgassing

High vacuum systems, even without external leakage, can also exhibit high gas loads due to

outgassing from internal surfaces or processes. The pressure in the system is a result of gas

load divided by pumping speed (P = Q/S). If the gas load (Q) exceeds the maximum

throughput capability of the diffusion pump, the diffusion pump will not function and the

pumping action will essentially be performed by the mechanical backing pump.

To estimate gas load:

After evacuation, isolate the system from all pumps and measure the rate of pressure

rise. The gas load created by the system can be estimated as:

Where:

V is the isolated volume

∆P is the pressure rise, and

∆t is the time period of measurement

Q = V x ∆P

∆t

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Varian VHS-6 Instruction Manual Page 47