The flexible steel hoses that carry helium gas in the closed-loop PTR cooler are exposed to continuous stress during mounting and dis-mounting and also when the telescope moves. We have used the same hoses since 2001. In spite of trying to minimise the stress on the hoses, several times we needed to repair one of the hoses because of helium leaks. The cracks were always found near the instrument end, which allowed for a ``simple´´ fix by cutting 10cm off from the end of the hose. This procedure takes some hours of work as the hose needs to be evacuated and then refilled, and the exact location of the crack needs to be found. To make this simpler, it was decided to purchase an extra hose so we can simply swap the hose and repair the broken hose off the telescope.
A spare hose was ordered which arrived by the end of February, but it had a wrong diameter, 30cm instead of the 20cm we had specified. The manufacturer have told us it is better to use this diameter because it allows for better flow, but the larger tubes are actually much stiffer which is a problem. After some time the company finally agreed to replace the tubes, and at the moment we are awaiting their arrival.
We also had some problem with overheating of the cooling water of the PTR making it at times impossible to stabilise it within the allowed range of 10-30 degrees. Investigating the water cooling circuit it was found that the water was dirty. Proper cleaning of the circuit, and exchanging the water has improved the behaviour somewhat. However, it looks like the main problem is the inter-cooler which is very rusty. This unit was hand made for use with SIRCA, and we are trying to find a similar system.
Thomas Augusteijn 2013-05-10