Kyle, Gerardo 

Today we bagged the Y-mid vent/purge valve NW50-4.5" adapter and sprayed helium (Valve is/was closed)

Warmed up LD for > 30 minutes and calibrated (cal-gas on Tee at LD inlet) -> Helium signal 6.4 x 10-8 torr*L/sec with cal-leak valved-in, < 10-11 torr*L/sec when cal-gas valved out

1.4 x 10-8 torr*L/sec helium baseline when turbo initially valved-in to Y-mid -> Declined to 7.3 x 10-9 torr*L/sec over next 90 minutes -> bagged vent/purge valve's NW50-4.5" adapter, filled bag with 4 LPM helium flow for 100 seconds and again for 10s of seconds -> LD signal began to increase ~30 seconds after initial helium applied and peaked at 1.2 x 10-8 torr*L/sec -> Removed bag -> LD signal fell off to 6.4 x 10-9 torr*L/sec after ???? minutes -> Bagged adapter a second time only this time included the O2 sensor (with internal pump) -> Repeated exercise and achieved O2 < 1% -> Max signal 1.1 x 10-8 torr*L/sec. -> Removed bag and valved-out Turbo etc. when Y-mid helium signal < 8 x 10-9 torr*L/sec 

Conclusion: 
Turbo aperture ~75 sq. in., beam tube apertures (~1500 sq. in) x 2 = ~3,000 sq. in. >> Turbo "sees" ~ 3 % of actual signal -> Air leak rate 0.4 that for helium -> maybe can explain 1/10 of assumed air leak.