Bissell HS-20 Troubleshooting Leakage, Outgassing, Poor Pump or System Performance

SECTION IV

TROUBLESHOOTING

4-1 LEAKAGE

Analysis of general operational experience with diffusion pumps indicates that certain locations are more prone to the incidence of leaks. The following locations should be checked first if leakage is the suspected cause of poor system performance.

a.Inlet and foreline connections.

b.Drain and fill plugs.

c.Other compression fittings, such as high-vacuum gauges in the system.

d.Threaded connections, such as a foreline gauge.

4-2 OUTGASSING

High-vacuum systems, even without external leakage, can have 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 diffu- sion pump, the diffusion pump will not function and the pumping action will essentially be due to the mechanical backing pump.

To estimate the gas load, isolate the system from all pumps after evacuation and measure the rate of pressure increase. The gas load can be estimated as follows:

V x ∆P

Q = ______

∆ t

where V is the isolated volume, ∆P is the pressure rise, and ∆t time period of measurement.

4-3 POOR PUMP OR SYSTEM PERFORMANCE

Before proceeding with a program of step-by-step troubleshooting, check the performance and accu- racy of the vacuum gauges used on the system. Table 4-1 shows common faults, and lists the proba- ble cause and corrective actions to correct for poor performance.

Table 4-1 Troubleshooting

Fault	Probable Cause	Corrective Action
	Leaks in system, virtual or real	Locate and repair.
	High process gas load	Measure gas load, eliminate
		cause.
	System dirty	Clean system to reduce
		outgassing.

4-1