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Oil Back Streaming

What is this thing?

Oil back streaming is an action that occurs in the transition area in the pressure ranges less than 10-3 Torr (.001mm).  What happens is the oil reaches it's vaporization point at which point it turns into a gas molecule and will actually or potentially move back into the system being evacuated as a gas.  This back streaming action can cause contamination of the system with relatively minute amounts of vacuum pump oil.  It generally happens when a pump is run at full vacuum pressure for extended periods of time.  This happens on a molecular basis, do not be confused with oil migration from a stopped pump  This happens sometimes when the pump is stopped and at vacuum on the intake side.  This happens with the pump running and things like a anti-suck back valve have no effect on this action.  

How can I stop it?

We stop this kind of action by using a molecular sieve trap.  This trap is specifically used for this type of problem.  The traps HyVac sells are called in line bake able traps.  These traps enable the user to "bake-out" the trap using the electrical connections on the trap.  The media in the trap is a zeolite.  This kind of looks like a sponge under a microscope.

The procedure is to use the trap, recognize that it is loaded with old oil, or other contaminants.   The user then valves off the system from the pump and trap, plugs in the heater and bakes the trap out with the pump running at full vacuum.  Once this is accomplished the user must change the oil in the pump. The zeolite material can be regenerated a number of times with this procedure.

Use of a quality vacuum pump oil like HyVacs' oil can assure the user a minimum of problems with this activity as it has a vapor pressure point of 10-6 Torr.

Make sure it is back streaming and not oil suck back when the system is shut down for the evening or whatever.

11/27/2005 ęCopyright HyVac Products, Inc. All Right Reserved
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