Vacuum & Cooling

Over the past few years we are having problems when filling NOTCam with LN$_2$. It manifests itself in that the LN$_2$ tank seems to be full, i.e., it is impossible to get more LN$_2$ in, thought the tank should be empty because of boiling off, and the temperature sensors show that the system is not as cold as it is when the dewar is full. It is impossible to get an inspection camera into the tank, so we can only speculate what is going on. Our hypothesis is that over the years water vapour has entered into the LN$_2$ tank. After a longer period of storage on the telescope floor, this water freezes to a lump of ice that limits the available space for LN$_2$ entering through the fill tube. It has repeatedly been seen that as soon as NOTCam gets mounted on the telescope and moved around for a while, the cooling recovers and it is again possible to fill the full amount of LN$_2$ into the tank. It was suggested to warm up NOTCam and to try to remove water from the LN$_2$ tank, and by the end of May NOTCam was warmed up in the dome while being all the time on the pump. The LN$_2$ tank was first dried by blowing nitrogen gas through it and then pumped. With this procedure the charcoal getter was not baked, but still the resulting vacuum turned out to be very good and reached 1$10^{-5}$ mbar. This showed that such a simplified procedure can be used in the future to improve the vacuum. After having cooled NOTCam down to -80 degrees with the PTR, the filling with LN$_2$ was started. During this procedure, however, the good vacuum was lost as soon as LN$_2$ was poured into the tank. This points again to thermal stresses on the fill tube O-rings. These O-rings had been exchanged earlier, and it was realized that it is probably time to exchange them again, as they could have become stiff and deformed.

The flexible steel hoses that carry helium gas in the closed-loop PTR cooler are exposed to continuous stress. We have used the same hoses since 2001, and both hoses (supply and return) have been repaired a couple of times. Repairing hoses is time-consuming and a spare hose, being pressurised and ready for exchange, was purchased from Kelvin International. The new spare PTR hose will replace the worst of the current hoses, i.e. the supply hose.