There appears to be a favorable slope change starting ~ Aug 8 (65 days on this log/log plot) Browsing the aLOG I find that near that time there were ring heater operations, and one week prior to this Kyle disconnected a small turbo pumping the BSC6 annulus. I don't think either of these explain the slope change. I also looked at the big ion pump voltages during this period - there were no step changes anywhere near this time.
GV18 was cycled on August 7-8 -Rai might have been making RGA measurements?
John, You are correct I was making measuremants and moving the gate valve on both August 7 and 8. It seems that the slope became 1/t after the measurements. It is not easy to understand this. The first thought would have been a leak in the valve between the RGA and the chamber with a poor vacuum in the RGA. This is not the case as the 2 liter/sec ion pump was holding the pressure in the RGA volume below that in the chamber. Another possibility is that the annulus system was leaking but you rule this out from Kyle's measurements. I am stumped. RW
Kyle decoupled the annulus turbo pump on July 31 - probably not this.
A signal we do not yet have is the BSC 6 annulus ion pump current as this chamber and it's pump were brought from the mid station. - I'll ask Dave about this.
Kyle recalls that he disconnected the annulus turbo pump for noise reasons at the request of Robert rather than for vacuum reasons. Some days later he was surprised to find that the annulus ion pump had "come on scale" as if the pressure had fallen in the annulus space to the point where the ion pump could start.
This suggests that there may be an inner oring leak from the annulus to the BSC6 chamber which might explain the slope change. Once the annulus ion pump began pumping, this gas load was removed from the chamber.
Richard has our electrician working at connecting up our new annulus ion pumps to the CDS system so that in future we will be able to trend these signals. At the time we did not assign any priority to this work.