Problems achieving low vacuum pressures in the
sub 500 micron range can often be traced to contaminants (assuming no air leaks). These
contaminants can be present in the product the vacuum is being pulled on, in the system,
evacuation lines, filters, or in the vacuum pump lubricant. The problem
is that different products have different vapor pressures. Solvents and water
are two common roadblocks to achieving low
pressures (higher vacuum). The vacuum pump generically can achieve, for talking sake, 10 micron pressure.
The vacuum oil out of the bottle is rated for .001 micron. If a contaminant is present in
the system that "out-gasses" at 200 micron then the best pressure attainable on
the system will be 200 micron and will be maintained at 200 micron until the contaminant
is evacuated from the system and exhausted from the pump and it's oil sump. If this
contaminant is present in the system and is removed under vacuum you can be assured it is
now in the vacuum pump oil. Once contaminants are in the pump, use of the
gas ballast
valve may help purge them from the pump oil. Only then will the system be able to
perform down to 10 micron pressure the pump is rated at. If a real low pressure is not
needed to accomplish a given task consider running the vacuum pump with gas ballast
cracked open to throughput condensable vapors through the exhaust port during evacuation
process. Make sure adequate ventilation is present as throughput
of condensable vapors will
end up in the exhaust stream of the vacuum pump and may pollute your work area and could
even pose fire hazards depending on the makeup of this exhaust. If you
are evacuating gasoline vapors these vapors will end up in your workspace
barring adequate ventilation.
Other things that can outgas include sealants used in sealing the systems, types
of elastomers including hoses, and seals. Special attention paid to materials
on the front end of
designing a system saves time trouble shooting the system. Some out gassing
can go on for very long periods of time until the contaminant is removed from the system. System solutions to
help include, liberal use of the vacuum pumps gas ballast valve, baking the system using
an oven or heat gun to help "burn off" the contaminants and identification of offending
products and elimination from the system. Adding heat to a system adds
kinetic energy to molecules that can outgas exciting them from their state
of inactivity and allowing them to be "pumped".
Cleanliness of the system, piping and traps are the most important things
you can control. Final cleaning with an Acetone rinse and letting dry is a good procedure.
Petroleum solvents such as mineral spirits, xylene etc should NOT be used as the
they have much greater residual ability. Base cleaning using 111 Dichloromethane, or other
chlorinated solvent is okay again using a final acetone rinse. In high vacuum applications
less than .001 micron, chlorine contamination may need to be addressed also. Full drying
between all steps. These are subjects that are better left to the
cleaning pros. Consideration of cleaning materials needs to be
seriously addressed when attacking pressures less than .001 micron.
