Vacuum & Cooling

-3 times wellded NOTCAM PTR helium tubes. Cleanned and re-filled.

NOTCam PTR hoses (new) ---------

We have a very good experience with the new Helium hose for the Iwatani PTR from Kelvin International Corporation (KIC) installed in November 2013. It is more flexible than the old hoses. We are using a pair of hoses for the supply and return flow of helium, and initially, the idea was to buy one extra hose, to always have a spare ready to exchange. Our experience over the last half year is, however, that the old hoses tend to break more and more easily, up to once per month, requiring exchange and time-consuming repair. It was decided that we should buy another of the new ultra-flex helium hoses, to have two new hoses mounted in pair. The old hoses have probably become more brittle with time due to the constant wear and tear.

Done:

- Order sent to KIC on August 28th, again on September 12th, but was apparently lost in their system. New purchase order sent October 28th. Confirmation that it would take about 10 days ...

TBD:

- Install new hose when it arrives.

- We still need to ship back an old wrongly made hose. Pending information from KIC on how to ship this.

NOTCam Cryostat health (ongoing, updated) -----------

The cryostat has given various challenges over the years. The current status is:

1) a not very good vacuum (not critical) 2) LN2 filling problem possibly solved

The filling problem was that the tank seemed to be full and it was impossible to fill more, although it was expected to be empty from boiling off after a given time. Also the temperature sensors showed higher than nominal temperatures. The hypothesis was that over the years water vapor has entered and after a longer storage time on the telescope floor, the water freezes to a sheet of ice that divides the tank and minimizes the available space for LN2 entering through the fill tube. As soon as NOTCam got mounted on the telescope and moved around, the cooling recovered and it was again possible to fill the adequate amount of LN2 into the tank. NOTCam was therefore in May 2013 warmed up while being pumped, in order to pump the LN2 tank itself.

It is noted that over the last 1.5 year the problem of filling the dewar with LN2 has practially disappeared. It was difficult to interpret the cold table temperature logs, however, because another problem mixed in: the PTR did not function optimally due to a dirty cooling water. After having both of these problems fixed (pumping the tank and filtering the water) the problem has not re-appeared. Since the previous report, every LN2 filling has resulted in a cold table temperature of -200 deg or less, except for a period in August, which was when the PTR was not in use due to unrepaired helium tube leaks.

Done:

- We conclude that pumping the LN2 tank in May 2013 has solved the problem we had with LN2 cooling. This gives support to Carlos' hypothesis of the formation of ice sheets in the tank.

As part of opening NOTCam, several parts in the filling and venting tubes were polished and refitted, and also several O-rings were changed. This to reduce the recurrent problem when filling the dewar with LN$_2$ if NOTCam stays off the telescope for a longer time.

The flexible steel hoses that carry helium gas in the closed-loop PTR cooler are exposed to continuous stress. A new, more flexible hose was bought to replace the one that had to be repaired the most (the supply hose). Since then there has been a much lesser need of re-filling the system with Helium.