Frequently Asked Questions
The most likely causes are -
- vacuum pumps not working correctly
- leak in vacuum chamber - this could be a 'real' or a 'virtual' leak - see below
- vacuum gauges are faulty (rare)
To test the vacuum pumps, isolate them from the chamber and check their ultimate vacuum (best vacuum reached). If this vacuum is about the same as the best achieved in the chamber, the pumping system is faulty; if it is much better than the chamber vacuum, there is a leak in the chamber or there is a substantial outgassing problem.
The most common faults with a pumping system are -
- lack of oil in one or more of the pumps, e.g. backing, roughing, mechanical booster, diffusion pumps
- rotary oil-sealed pumps (backing, roughing) require gas ballasting to remove contamination from the oil
- one or more of the diffusion pump heaters has failed
These items should be checked by a qualified service engineer. If a leak is suspected, it could be real, e.g. a damaged door seal, or virtual, e.g. excessive outgassing from a component to be welded. To test which it is, pump the chamber to its ultimate vacuum, isolate it from the pumping system and then plot the rise in pressure against time using a logarithmic scale for pressure. If after the initial rise, the pressure stabilises to an almost constant level, the leak is virtual. If the pressure continues to rise in a straight line, the leak is real, see diagram.
If the leak is virtual, check the chamber, tooling and components to be welded for contamination, e.g. water or solvents from cleaning, or excessive surface rust or grease. If the leak is real, locating it depends on the type of leak-testing equipment available. If the vacuum gauges are suspected, these can only be checked by re-calibration or substitution.
FAQ: How do I measure the pressure in an electron beam (EB) welding vacuum chamber?
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