Maintenance for DR

We removed the dilution refrigerator from the OVC to replace the large O-ring on the top flange (see attached pictures).  After cleaning the groove we found surface damage from the early years when water used to get into the O-ring groove and stay. The top Aluminum can was sent out to Kosmos Industries and 0.018 inch was machined off to remove the damage.

 We replaced all the O-rings by new Viton O-rings instead of the Silicon O-rings that were being used. Helium gas diffuses through Silicon O-rings and gives a leak rate in the mid 10^-8 range.

Also, the booster pump oil was changed 1.5 L Apiezon 201.

Background

Pump IVC and OVC pressure down to 7 x 10-5 torr

Both LD and Edwards pump ON leak rate less than 2x10-9.

Capillary flow through

7 mbar/min @ 0.5 atm He(4). Good

Capillary leak check

No change in leak rate

IVC/OVC Window

Normal diffusion through the window 1.3 x 10-8 in 5 min @ 500 m torr.

Main bath to OVC/IVC

One psi over pressure in the main bath resulted in an increase of the leak rate to 4x 10-8 in 2 min. After the helium leaked out of the main bath we noticed the leak rate improve.  Also, the leak was to the IVC and not the OVC. We tracked this down to the three temperature and heater connectors on top of the DR, the O-rings are still Silicon, we just order some Viton O-rings to replace them. The new O-rings were installed and the leak check was good.