Numerous problems have occurred with both the StanCam and FIES CryoTiger cooling systems over the passed few months. A feature of the cooling system is that for no obvious reason the cold-head temperature jumps from the nominal -200C to approximately -160C and then after a period that can be a few hours to many weeks it returns to it correct temperature. This property has been seen with CryoTigers at both Mercator and Liverpool telescopes and no explanation is known. Various attempts to try and identify the problem have been tried, including removing the in-line filter/dryer, reverse flushing the cold-head to remove any possible blockage and replacing both the compressor and filter/dryer with new ones. Basically the only reliable thing identified is that switching the compressor off for a while usually helps.
Currently, the StanCam cooling system has all its original parts back and seems to be operating satisfactory, while FIES has a new filter/dryer installed (only recently) and also seems to be operating properly. One issue with FIES is a possible vacuum leak but because the vacuum sensor is disconnected (to remove the electrical noise it induces) the variations in pressure are not known.
Thomas Augusteijn 2010-02-09