23 December, 2000

Coldhead Maintenance

Every two years, the coldheads, refrigerators that help keep the dewars extremely cold, need to be taken off the dewar and cleaned. This process involves not only cleaning the coldhead, but also the inside of the dewar that is now exposed, and replacing parts on the coldhead that might be worn. Today Ed Pernic came by to help with this process on the first of four coldheads.

Once the coldhead has been removed, there is a cavity inside the dewar that might have collected gunk in it that needs to be cleaned. This again needs to be done carefully so that the surface isnšt scratched. Here, Ed Pernic is using a piece of tissue paper with alcohol on it to clean the insides. After all of the large bits have been cleaned out, we blow pressurized helium gas through the cavity to remove the excess alcohol and other bits that might have gotten in there. The opening is then covered so that nothing accidentally falls into it while we work on the coldhead.

Once the coldhead has been removed from the dewar, it can be taken apart, cleaned and worn parts replaced. Here, you see the coldhead in three pieces, with the stem removed. Ed is carefully cleaning one of the pieces.

The coldheads are cleaned once every few years. In that time, the can collect a lot of grime, even though they are in clean environments. Here I am cleaning one of the stem pieces.

Once the coldhead has been fixed up, it needs to be put back together, and the timing set. The timing controls when valves will be open to the dewar, allowing it to be cooled. This is done just before putting the stem back on, and is a careful process that can take the better part of an hour.

Once the timing is set, the coldhead is ready to be placed back on the dewar. Here Ed inspects the completed coldhead.