with the desired gas
PN: That probably doesn’t require a high vacuum.
JP: Correct. There are also many applications in the lab involving
evaporating materials, so you only have to get below the vapor
pressure of that material; for water for example, a few Torr is all
There are no firm rules on
pairing primary and secondary
you need. Conversely, when you are dealing with something
like a mass spectrometer where you need ions to travel through
the instrument, for those ions to travel any distance at all, the
environment through which they travel has to be evacuated
to a much lower pressure to avoid collisions with atmospheric
PN: This obviously calls for a secondary pump.
JP: Yes. Here you typically need to get down to pressures of 10-⁶ or
10-⁷ Torr, so you would need a primary pump to get you down
to say 10-² and then you would need a secondary pump to take
over. Typically you would have an oil filled rotary pump or a dry
pump, followed by a turbo pump.
PN: How about the pairing up of primary and secondary pumps?
JP: There are no firm rules on pairing primary and secondary pumps
but if you are going to use an oil based secondary pump (a
diffusion pump works by entraining the molecules of the gas
you are pumping in an oil stream), you might as well use an oil
sealed rotary vane pump as the primary pump. If you are going
to use a turbo pump as your secondary pump then, if a lack of
oil is important to you, it would be sensible to pair this with a
dry primary pump such as a scroll pump.
PN: What other laboratory instruments require a secondary
JP: An electron microscope is another good example, again here you
are wanting to transmit ions through the instrument and you
need to avoid collisions with gas molecules. A synchrotron [sub-atomic particle accelerator] would be an example of a very large
system that would need to be pumped down to these levels.
PN: Have there been new applications that have come out recently
presenting pump designers with new challenges?
JP: In my field, which is scientific applications, the challenge that
Figure 1. Oil sealed rotary vane pumps
we constantly have is that mass spectrometer manufacturers,
for example, are trying to make their instruments smaller and
more sensitive—and at lower cost. The vacuum system in a
mass spectrometer has a large impact on all these things so
we have the challenge of designing smaller pumps with similar
or better performance than before, and—by the way—at less
PN: Another important consideration must be the rate of pumped
JP: Certainly, how much gas you want to pump is part of the decision. In a scientific application you might be pumping very
small amounts of gas and therefore not need a very large pump,
whereas in an industrial application you might be pumping a
chamber that is big enough to fit a car into. Pumping speed is
therefore a key characteristic in choosing the size of pump you
are going to need but not necessarily the pumping technology.
Pumps, like rotary vane pumps for example, are available in a
range of sizes from 0.5 m3 /hr to several hundred m3/hr.
PN: How about turbo pumps?
JP: Turbomolecular pumps are also available in a large range of
Figure 2. Turbomolecular pumps