Sealing a 6 torr closed system vacuum

  • #1
I wish to design a two chamber stainless steel vacuum system where the two vacuum chambers are connected via a flexible tube with a valve. The system needs to be able to be pumped down to 6 torr and must be able to maintain that vacuum for at least a year without pumping. The size of the chambers are 2" x 4" x X 12" X 0.12" (Height, Width, Length, Wall Thickness) and 6" x 0.065" (Diameter, wall thickness). They will be connected with stainless steel tubing and KF-25 connectors with Viton gaskets.

Question:
I have an air conditioning service pump-- will that be able to bring the vacuum to 6 torr? Otherwise, would it be possible to create at least a 6 torr vacuum by heating (to the point of boiling) the vacuum chamber that contains water and then sealing the system? The condensing water vapor should create a powerful vacuum, correct?

If an vacuum pump must be used, what is a good way to seal off the system? I have a gate valve (with elastomer seals), but it has a couple of 1/4" threaded holes in it (currently fitted with teflon and stop plugs). My concern is that system will leak through the threaded holes. Is there a recommended way to seal threads to retain vacuum?
 

Answers and Replies

  • #2
UltrafastPED
Science Advisor
Gold Member
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You can check the leak rating for your valve with the manufacturer.

For example here is an ISO butterfly valve designed for vacuum systems:
http://www.pchemlabs.com/product.asp?pid=3572

"Butterfly Valves have the advantages of having a small footprint, being turned completely off to on in one 90 degree rotation of the handle, while offering a high gas throughput and conductance. These vacuum butterfly valves are designed to have a low leak rate, typically on the order of 1x10-9 std. cc/sec. Helium. These butterfly valves have a NW-25 vacuum flange for connection to the vacuum system, are manually operated, and made of stainless steal." Price: $345.

Your vacuum chamber will also "leak" from the interior walls (out-gassing), and from any "virtual leaks" in the welding ... "vacuum welding" is a technique which eliminates these virtual leaks:
http://www.vacuumlab.com/Articles/Understanding Virtual Leaks.pdf
 

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