John W., Kyle R. 

Recap-
A few weeks ago we connected a leak detector to the exhaust of the YBM turbo pump in order to revisit and quantify a known leak at GV6's lead-screw area -> we were unable to do this at that time as the helium background signal was 2 x 10-6 torr*L/sec, i.e. larger than the signal we were trying to quantify! -> We assumed that this "background" helium had entered the vacuum system via permeation through the viton O-rings of GV1's annulus (these had been sprayed a few weeks ago while hunting for a leak on GV1) -> Once in the system, helium won't go away nominally as the 80K pumps don't pump helium and the big ion pumps are poor helium pumps when unsaturated and non-helium pumps once saturated.  As such, we have been valving-in the YBM turbo pump during the day (must be "attended") in order to remove the lingering helium. This has been working but it is slower than expected.  Instrument air was then used to pressurize the air-side of GV1's outer O-ring(s) followed by venting then pumping the annulus volume to assure that helium was no longer in contact with the O-rings, i.e. remove the source if still present.  This had no observable effect.  Removal via the YBM turbo is so slow that we now need more data to have confidence that our helium source assumption is correct.

Today- 
We opened GV8 for a 1/2 hour or so and observed that the helium signal (LD backing YBM turbo) fell off by a factor of ~2 or so i.e. the soft-closed GV8 had been a barrier to the high helium background originating from the Corner Station -> Hard-closing GV1 reduced the signal some confirming what we already suspected, that being, the helium source is between GV8-GV4-GV1 -> Stroking GV1 closed-open-closed-open, i.e. exercising its lead-screw bellows didn't result in a lasting change in the equilibrium signal.  

Tomorrow we will vent/pump other annulus volumes that have been sprayed with helium at some point in the past