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. |
