24 December, 2000
This morning, we cooled the dewar whose coldhead we finished maintenance on yesterday. This process is the same as when we cooled the 230 GHz receiver on December 20. Except this time there was a small glitch: we put too much liquid nitrogen in the dewar, and it cooled too much. Since the coldhead was already on the dewar and working, it continued to cool the dewar, and the remaining liquid nitrogen remained in the dewar instead of boiling off. We thought that there was no more nitrogen in the dewar and started to fill with liquid helium, but soon found the hose was frozen to something- nitrogen ice. There was a little nitrogen remaining in the dewar, which froze solid when we filled it with liquid helium. So, the only thing to do was to warm the dewar up again, and wait for the ice to melt and evaporate.
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