Holding time

After a lot of efforts the work to improve the vacuum behavior finally gave fruit. The vessel had suffered from micro-leaks for a long time. It is clear that most of these came from temperature stresses on o-rings during LN2 filling and spilling. But also the large vessel o-ring needed to be taken out and be thoroughly cleaned. It was found to be contaminated with metallic dust, probably coming from the bolts holding the vessel lid to the cold table. After the last opening in January 2007 the cryostat has shown to behave very well. NOTCam can now be stored cold relying only on the PTR cooling. No vacuum leak has been seen. This has not been the case since beginning of 2002. Furthermore, the causes of the previous micro-leaks have been fully understood and both procedures and materials used have been changed accordingly.

The pressure sensor should be replaced or looked at since it sometimes jumps between modes and show values that are wrong by a factor of 100.