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KNF N920 VACUUM PUMP IN BRUKER CRYOPROBE: FAULTY VACUUM

The cryo-probe section of Bruker NMR system complains about vacuum failure following a building power-outage. Bruker's customer-service suspects roughing pump problems. Yep, removing the pump and applying a vacuum gauge, we see that the pump only pumps down to 7.5 Torr. The spec sheet on the pump says 1.5mBar (=1.1Torr.)

Expensive pump! Online is KNF N920 G Pump, Operating Instructions, p17 shows that the pump head easily comes off, exposing the various replaceable parts. Of the three diaphragms, I found that one is wearing, which creates black powder on the small rubber flap-valves. Clean out the black crud with IPA alcohol, wipe down the small rubber valve flaps (turn them over for new flatter surface.) Put it back together, and it pumps right down to 1.1 Torr.

No need to buy a new pump ($3000!)