EXERCISE 2: THE VACUUM SYSTEM

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EXERCISE 2: THE VACUUM SYSTEM
Because vacuum system techniques are important to all of experimental physics, before the
end of their second year. Everyone should have had their hands on a vacuum system. This
exercise ensures that you will!
In this exercise, you will pump the bell jar (chamber) down to 10-6 Torr and then bring it
back up to atmospheric pressure again, recording the chamber pressure and the backing
pressure. You will also do some pumping speed calculations to understand what is involved
in the design of a vacuum system.
Later, you may wish to use this system for the Evaporation of a Silver Film experiment.
The Vacuum System and How To Use It
The pumping station you will use consists of a bell jar pumped by a very small diffusion
pump (or pumping speed 50 litres/s) backed by a rotary mechanical pump. There are four
pressure gauges, each useful in a different pressure range, measured on different parts of the
system, and there are five valves for directing the gas flow.
 For descriptions of the workings of the rotary pump and the diffusion pump, see chapter 3
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of either of the references: Chambers et. al. or Dushman et. al.
 For descriptions of the gauges see chapter 4 of Chambers et. al. or chapter 5 of the
Dushman et. al.
 For use of the overall system, see chapter 8 of the Chambers et al. or chapter 4 of the
Dushman et al.
Note the various pressure units used. The SI unit of pressure is the Pa = N/m2. The bar is
also used: 1 bar = 105 Pa, as is the Torr: 1 Torr = 1 mm Hg = 1.333 mbar. Thus atmospheric
pressure is approximately 100 kPa = 1 bar  760 Torr.
The mechanical (roughing or backing) pump rotates an eccentrically mounted vane inside
a cylinder to compress and expel a volume of air in each revolution. It is used either to
rough out the bell jar or to back the diffusion pump. It becomes ineffective at low
pressure 10-2 Torr.
The diffusion pump is effective down to pressures 107 Torr, but must have a backing
pressure of 10-2 Torr in order to operate. Its high pressure limitation derives from the
necessity for the gas being pumped to be at molecular flow pressures. The pump works by
boiling oil with a heater and back-jetting the oil vapour with a system of jets. The oil vapour
collides with the molecules of the air being pumped and pushes them forward. The upper
walls of the pump are water cooled to recondense the oil, while the air finds its way to the
exit. Diffusion pumps can be gigantic in size but yours is the among the smallest we could
find, having a 38 mm (1 inch) or 63 mm (2 inch) opening.
The cold trap is cooled by liquid nitrogen (boiling point = 77K) to both condense any oil
that might back-stream from the diffusion pump into the bell jar and to increase the pumping
speed for condensable vapours. (Vapours that condense on the baffle of the cold trap are, in
essence, pumped from the vacuum chamber.)
The valves: Referring to the figure: the Mechanical Pump Bleed-up Valve, when opened,
permits the mechanical pumps (low) vacuum line to be vented to air. Similarly, the
Chamber Bleed-up Valve, when opened, permits the bell jar to be vented to air. The
Chamber Roughing Valve is normally kept closed when the diffusion pump is being used to
pump the bell jar. However, because diffusion pumps cannot be used to pump pressures
higher than 10-1 Torr, the chamber roughing valve may be opened to allow the mechanical
pump to rough-out the bell jar. The Diffusion Pump Backing Valve, which connects the
diffusion pump to its backing pump, will normally be open whenever the diffusion pump is
operating. However, it must be closed whenever the mechanical pump is being used to
rough-out the bell jar in order that the diffusion pump not be subjected to higher
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pressures. The Chamber Diffusion Pump Valve is opened whenever the bell jar is to be
pumped by the diffusion pump.
Cautions:
 The first law of vacuum systems is: Do not leave the mechanical pump
under vacuum when not running. For many types of mechanical pumps, oil
may be sucked back from the pump body. On final shut-down, (a) close all
valves to the system, (b) turn off the pump, ( c ) open the mechanical pump
bleeder valve to bring the mechanical pump up to atmospheric pressure.
 The second law of vacuum systems is: Keep the backing pump and
cooling water on while the diffusion pump is hot. Otherwise the oil will
backstream into the bell jar and/or be oxidized. Never let air into the
diffusion pump while it is hot, otherwise the oil will burn!
Gauges: The thermal conductivity of a gas is proportional to the pressure. The Pirani
Gauge has a hot wire in the vacuum space. Its electrical resistance, measured in the control
unit, depends on the wire temperature and therefore the pressure. It gives useful readings
from 10-2 to 10-4 Torr. In the present apparatus there are two Pirani gauges. One indicates
the diffusion pump backing pressure and the other indicates the pressure in the chamber.
The Penning Gauge monitors the current in the gas from a cold emission cathode to an
anode. The electrodes are located in the field of a permanent magnet to give long
trajectories. It overlaps the pirani gauge and goes to lower pressures - down to 10-7 Torr.
Before you start-up the pumps, examine the system and locate all the parts. There are four
pressure gauges - you should figure out what part of the system each gauge samples. Check
the positions of all the valves (which are open, which are closed). Try to ascertain the state
of vacuum in the various parts of the system.
To start the system you must:
 be certain that the mechanical pump bleed-up valve is closed
 be certain that the valve joining the bell jar to the diffusion pump (Chamber Diffusion
Pump Valve) is closed
 start the mechanical pump and check that it pumps down to about 20  10-3 Torr
reasonably quickly
 open the valve between the diffusion pump and the mechanical pump (Diffusion Pump
Backing Valve) and pump the diffusion pump down to better than 20  10-3 Torr
 turn on the diffusion pump water
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 turn on the diffusion pump heater.
After about 15 minutes, the diffusion pump should be operating.
 cool the cold trap with liquid nitrogen.
To pump out the chamber you must:
 make sure that the plastic implosion shield surrounds the bell jar
 be certain that the chamber bleed-up valve is closed
 rough out the chamber by opening the valve to the mechanical pump (Chamber Roughing
Valve) (Be careful to temporarily close the diffusion pump backing valve to protect the
diffusion pump. This valve closing admittedly is a transient violation of the 2nd Law, but
it protects the diffusion pump from the major surge of air that would certainly violate part
2 of the 2nd law.)
 when the pressure is about 10-1 Torr close off the roughing valve to the mechanical pump
(Chamber Roughing Valve) (and, of course, re-open the diffusion pump backing valve.)
 you may now open the valve between the bell jar and the diffusion pump (Chamber
Diffusion Pump Valve)
Note the rapid decrease in pressure as the diffusion pump grabs the bell jar. It will finally
go to 10-5 Torr or better.
To access the bell jar you must let it up to air. (Why cant you just pull it off?)
Valve off the diffusion pump from the chamber and then vent the chamber to air.
To shut down the system:
 close the valves to the bell jar (Chamber Diffusion Pump Valve) and turn off the
diffusion pump heater.
 after the diffusion pump has cooled and the cold trap has warmed up, close all valves and
turn off the diffusion pump water
 turn off the mechanical pump and vent it to air with mechanical pump bleed-up valve.
Leave the apparatus with both the diffusion pump and the bell jar each under vacuum. This
helps keep clean the high vacuum parts of the apparatus.
Things To Do
1. Pump the bell jar to better than 10-5 Torr and then return it to atmospheric pressure. (How
good a vacuum can you achieve?)
2. Insert the metal plate with the small aperture into the neck of the diffusion pump (above
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the valve) and pump the bell jar again.
3. Close the valve between the bell jar and the diffusion pump and, using the bleed-up valve,
carefully raise the pressure in the bell jar to approximately 0.3 Torr. (If you raise the pressure
too high, rough-out the bell jar to the desired vacuum.) Now open the diffusion pump
valve and observe the bell jar pressure as a function of time. Continue pumping for 30
minutes. Plot a graph of pressure versus time on a semi-logarithmic scale (see 7. below).
4. Familiarize yourself with the terms molecular flow, viscous flow, throughput, in reference
to gas flow; also pumping speed of a pump and conductance of an aperture. (See the
references.)
5. Calculate the mean free path in air at the ultimate vacuum that you achieve.
6. Calculate (from its dimensions) the conductance (in litres per second) of the pumping
aperture that you have inserted.
7. If the bell jar of volume V is pumped with a pumping speed S (in the molecular flow
regime), you can show that the pressure would vary as
S
p = po e- V t  p b
From a straight-line region in the graph you plotted in 3., estimate the conductance of the
aperture.
8. Explain why the pressure settles to an ultimate value of pb and not zero.
REFERENCES:
 Chambers, R.K. Fitch & B.S. Halliday; Basic Vacuum Technology (1989)
 S. Dushman, J.M. Lafferty; Scientific Foundations of Vacuum Technique, 2nd
edition (1966)
 Vacuum System Detail Book, (available in room 229)
gmg-1984, jbv-1993,1994, td-1996
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