[time-nuts] Maser info (vacuum levels)

jimlux jimlux at earthlink.net
Fri Sep 3 04:45:09 UTC 2010

J. Forster wrote:
> Two things helped a lot:  Big pumps and an LN2 cold trap.
> The LN2 trap (as long as it is kept filled) will condense most everything
> except a few "permanent" gases. It also stops the backflow of pump oil.
> However, if something goes wrong, you would not believe the mess.
> A technician that worked for me years ago told of a vacuum chamber that
> was used to test some Apollo instruments, maybe 6' long and 5' diameter
> with a pair of 18" - 24" oil diff pumps.

Yep, that sounds pretty typical for our thermal vacuum test setups..
(JPL has some bigger chambers.. 25 foot with a solar simulator, for 

The one I like is the RF test chamber.. a 6 foot bell jar in an anechoic 
chamber for testing antennas under high power.. I think that one has a 3 
foot diameter aperture to the pump (52,000 liters/second.. yes indeed 
that is a *fast* pump).

Actually, it's kind of interesting at the lab because there's all this 
old stuff not being used anymore (giant roots roughing pumps), but it's 
still connected up to the walls, even if the insides of the lab has had 
the chamber removed.  It's probably more expensive to remove the pumps 
and dispose of them as surplus than it is to just leave them in place 
(where they've probably been since the 60s).  I do know that getting rid 
of an old small pump is a huge pain.. someone has to come and certify it 
as not being hazardous, and then they take it to some disposal facility, 
and then it has to be listed for recovery, and some scrap dealer bids on 
it (probably as a lot weighing a ton or more)  (e.g. no dumpster diving 
for employees..)

> One night the AC power went off and the emergency sequence failed. The
> diff pump oil was sucked back into the system. It took them weeks to take
> the whole thing appart, clean everything (think 55 gallon drums of
> Trichlor) and get it back together.

Oh yes... venting the chamber when the pump is hot is a BIG no-no..
Even moreso when there's hardware under test in the chamber.  (we had a 
piece of gear going through thermal vac with a cold plate using a glycol 
loop to the chiller.. and the glycol leaked..)

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