2013/04/12

Parts from Pfeiffer arrived today, including new anode. Reassembled pressure sensor with new anode and ignition aid. Attached pressure sensor to helium leak checker: background helium leak rate is 3.7E-7 mbar l/s. It is a bit high, but cannot find any leak rate information for sensor, so this may be fine. Pressure read by sensor is 3E-5 mbar, but this may have to be recalibrated.

Leak rate rose to 1E-6 mbar l/s when testing the seal between the electronics and magnet units. This is not a vacuum tight seal, but the part of the sensor being pumped on should lie behind two FPM O-rings. A leak rate below 1E-7 mbar l/s would be acceptable, but it is uncertain if 1E-6 mbar l/s is catastrophic.

This may have been the reason why the pressure in the line running to the IVC tends to increase at quite a large rate than what could be explained by simple outgassing. Upon opening the sensor, at least the FPM seal for the Pirani element was fairly loose. Unfortunately, I do not believe we have ever leak tested the pressure sensor so there is no data to compare it with. Isolated leak checker and pressure appears to have risen to 6.3E-3 mbar. It may be worth reopening the pressure sensor and trying to tighten the screws securing the pressure piece further.

Opened pressure sensor and tightened aforementioned screws as much as possible. Reconnected pressure sensor to leak checker: backgrond helium leak rate 3.5E-7 mbar l/s, which is probably within margin or error as previous background rate. Helium leak rate does not increase when checking seal between electronics and magnet units. In fact, helium can be heard escaping through seal after flooding the area. Unsure if this would cause damage so ceased leak check. Confusingly, leak rate actually dropped suddenly twice to around 2E-7 mbar l/s during leak check, and then rose to 3.3E-7 mbar l/s: unsure of reasons why, but may not be worth worrying about. Additionally, pressure sensor appears to have reset itself at some point as display on pumping stage went back to default reading: unsure of when this happened as pressure was not noted before leak check started.

Pressure is currently 4E-5 mbar and helium leak rate has increased to 3.7E-7 mbar l/s, perhaps due to higher helium concentration in atmosphere after leak check. Isolate leak checker and pressure has risen to 2.5E-2 mbar. Not much else to be done - this is probably fine, especially as leak rate did not rise during spray test.

Vent sensor and turn it off. Isolate leak checker and remove sensor. Place sensor on line to IGH and ensure all openings to atmosphere have been sealed. Start turbo pump. Ten minutes after pressure reached 1E-5 mbar, adjust <HV> potentiometer from 8.6E-4 mbar to 5.4E-4 mbar (manual states 5E-4 mbar, but this is closest possible as scale does not read values between 4.6E-4 mbar and 5.4E-4 mbar).

Vent line with nitrogen gas to atmospheric pressure, ensuring that check valve is open. Wait ten minutes. Adjust <ATM> potentiometer from >1E3 mbar to 1E3 mbar. Close all openings to atmosphere. Start evacuating line overnight to observe performance of sensor.