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High Vacuum technology
To create a high vacuum in a system it
is necessary to move all of the molecules of gas out of the system. The
molecules will move only if there is a pressure difference between the two
regions of the space (see Figure 4). The low pressure region is the space with
the smaller number of molecules, while the high pressure region is the space
with the larger number of molecules.
Any device which can induce a
pressure difference between the two regions in the space is called a pump. The
pump which creates the vacuum in the certain system is called a vacuum pump.
Exercise: you need only a
cup of beverage and a straw. Try to suck a drink by the straw and feel how do
the mouth muscles move. When one sucks on a straw the mouth muscles create the
region of low pressure, while atmospheric pressure on the surface of your
beverage pushes it up the straw. Pumps operate on the same principles.
Figure 4: Vacuum pump operation.
There are two categories of vacuum
pumps: transfer pumps and trapping pumps.
The transfer pumps are also
called kinetic pumps since they impart the momentum to the gas which is
being pushed in a such a way that the gas is transferred continuously from the
inlet of the pump to the outlet. This is usually done by mechanical moving parts
of the pump, as shown in Figure 5. The moving (usually rotating) part of the
pump accelerates the molecules of the gas and makes the region of lower
pressure. Therefore, the molecules from the tank will start moving towards the
region of lower static pressure, with the procedure continuously repeating until
all (or most) of the molecules are taken from the container where we would like
to have a vacuum. When we get the wanted level of vacuum we isolate the tank by
a high vacuum valve. This valve stops any exchange of gas between the container
and the pump.
Figure 5: Operation of the
transfer pump.
The trapping or
capture pumps are usually located in the container being evacuated. The
trapping pumps remove gas molecules by sorption or condensation on its internal
surfaces (see Figure 6). If the gas molecules chemically react with the internal
material of the pump, the new material created by the reaction of gas and inner
material molecules will be deposited as a thin film. This is called sorption of
the gas molecules. Furthermore, if the gas molecules come in touch with the
refrigerated surfaces of the pump, gas will be condensed and removed as a
liquid.
Figure 6: Operation of the
trapping vacuum pump.
The low vacuum levels can be
measured also with the simplest manometer; i.e.; U – tube (see again Figure 2).
In this case, instead of having a container with pressurized gas, we will have a
vacuum container. Since the atmospheric pressure is higher, the air molecules on
the open end of U – tube will push the fluid towards vacuum container. Again,
the difference in heights of fluid between the two sides of U – tube will
determine how much the pressure in the container is less than the atmospheric
pressure.
In general, the special devices
which measure the vacuum levels are called vacuum gauges.
The simplest mechanical gauge you
can imagine as a chamber divided by a membrane. One side of the chamber is
connected to the vacuum container and the other to the known pressure tank (it
can be simply open, therefore it will be under the atmoshperic pressure). The
membrane is attached to the pointer mechanism (see Figure 7). Depending on the
membrane deformation, the pointer will show the vacuum level. A gauge like this
is considered to be a coarse or rough guage.
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