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Subject: Vacuum
Measurement
The atmosphere
in which we live is air. This air is affected by the earths
gravity which causes it to be at a pressure. Around sea level
this pressure is known as 1 atmosphere, often called 1 Bar,
i.e., 1,000 millibars (where 1 millibar is 1 thousandth part
of a bar). This pressure of our atmosphere decreases as we
go further from the surface of the earth and varies with weather
conditions.
We define
a vacuum as air (or gas) at a pressure below that of the atmosphere.
Consider the idea of a perfect vacuum in a vessel in which
there is no atmosphere, not even 1 atom. (This cannot occur
in practice, there is always something present).
We now
consider what is the actual pressure inside any vessel. As
we have said above, if it is the atmosphere at sea level then
our pressure is 1,000 millibar. If we removed some of this
air, then we may only have 10 millibar or 1 millibar or less.
We express our vacuum in terms of how many millibars of pressure
are present. By using various types of vacuum pump, we can
remove atmosphere from a suitable vessel down to a tiny fraction
of 1 millibar even as low as 1 part in 1,000,000,000. Such
a figure is written mathematically as 10-9 millibar.
In most
industrial applications we are using vacuum levels down to
1 millibar. Some examples of useable vacuum levels are as
follows. Very approximate levels are indicated.
Vacuum
cleaner: about 800 millibar
Vacuum
forming of plastics: 200-500 millibar
Water
boils: about 12 millibar at room temperature
Resins
and RTV Degassing: about 5 millibar at room temperature
Dry out
a refrigeration system: about 1 Millibar or better
Freeze
drying: 10-1 or better
Evaporate
aluminium in Decorative metalising
processes: 10-4 to 10-5 millibar
Vacuum
Furnace processing of
metals: 10-5 millibar
Inside
of a TV tube: 10-7 millibar or better
Vacuum
bag veneering: 100-500 millibar
Specialised
research in Ultra High Vacuum: 10-9
or better
Even our
atmospheric pressure varies at different places on the earth=s
surface, e.g., top of Snowdon, top of Everest.
Other
Units of Vacuum Measurement
When vacuum
was first investigated, a glass tube and a bowl of mercury
was used. The vacuum was measured as the height to which the
mercury could be lifted up the tube. A Barometer had a closed
tube and a complete vacuum. This measured 30 inches at about
sea level. This practical level of vacuum would be up to 30"
Hg (mercury), in metric units this would be 760mms of Hg.
The measure was how high the mercury column could be lifted.
A vessel
or container under vacuum will have a difference in pressure
across its surface. If the container is for example a flexible
bag as used in veneering, the user is interested to know what
thrust is being applied to the veneers. Thus another useful
unit of vacuum measurement can be 1bs per square inch,.
Our normal
atmosphere is at 14.7 PSI. Perfect vacuum would be 0 PSI.
Thus the pressure on the surface of a vessel under vacuum
equals the difference in pressure between inside and atmosphere.
Suppose we have a vacuum of say 500 millibar, i.e., half an
atmosphere then the pressure on every square inch of surface
is 7.35lbs.
If a vacuum
of 250 millibar then the pressure on the surface increases
to about 10lbs for every square inch.
Simple
calculation shows that the thrust, for example an Island Scientific
Chamber 12" nominal diameter is about 3/4 of a ton. This is
why the lids need to be so thick and strong.
Vacuum
Measuring Methods
1) Dial
Gauge
Simple
dia gauges (Bourdon Gauges) can be used to indicate level
of vacuum down to about 40 millibar. These gauges read
the difference in pressure between the atmosphere
around them and the pressure inside them, i.e., the system
to which they are connected. As the atmospheric pressure
changes so will the reading. Cost - a few pounds each.
2) Glycerine
Filled Gauge
These
are simple dial gauges (Bourdon Type) as 1 above, but
where the entire mechanism is contained in liquid, i.e.,
glycerine. This enables the gauge to be used in positions
where vibration and/or vacuum pulsation occurs. They must
be mounted vertically and the small plastic vent plug
removed once in position. Replace this vent plug if the
gauge is to be moved to a new location when it can be
removed again.
3) Barometrically
Compensated Vacuum Gauges
Looking
similar to the dial gauge and about 100mm diameter, these
have inside a more delicate mechanism and a partly evacuated
sealed capsule. This mechanism is all exposed to the actual
vacuum system. Such gauges will read down to 1 millibar.
Reasonably accurately provided that they are clean internally.
Cost over ,200.00 each.
4) Baratron
Gauges
These
gauges are much more expensive. They have a sensor unit
plus electronics. They are available to cover a wide range
of vacuum from 1,000 millibar to 10-3 millibar. Costs
hundreds of pounds.
5) Pirani
Gauge
This
is an electronic gauge capable of reading usefully from
10 millibar to 10-3 millibar or 1 millibar to 10-4 millibar
depending upon the model selected. These are sensitive
to contamination from the vacuum system.
6) Penning
and Ionisation Gauges
Are
able to read much lower pressures and are specialist electronic
instruments.
7) Mercury/Tube
Gauges
Various
types of gauge comprising glass tubes and mercury based
on the McLeod principle can be used to measure from about
10 millibar to 10-3 millibar. They are prone to contamination
and false readings unless special care is taken.
A chart
showing the comparison of different vacuum measuring units
is shown at the end.
Maintenance
of Vacuum Gauges
Gauges
should be mounted where they are free from vibration, rapid
fluctuations in pressure, at room temperature, dry and visible
to the user. Regular calibration checks should be made if
the process depends upon accurate pressure measurement in
the vacuum system. The simple dial gauge is cheap enough to
consider a throw away item if it goes wrong or is suspect.
Barometrically
compensated gauges. Some models are sealed and cannot be repaired.
Other models can be opened up to be cleaned and re-set.
Electronic
gauges repair is specialised and a task for the manufacturers
or a gauge repair/calibration specialist.
Vacuum
Switches
Most types
of vacuum gauge mechanism can be obtained to operate an electrical
switch at the pre-determined level of vacuum. Some will read
as well as switch. The actual switch mechanism will only be
light duty and needs to be connected to an electronic relay
to operate other equipment.
Fail
Safe
1) If
your vacuum system is ever likely to be pressurised by
accident or as a process procedure then the vacuum gauges
must be protected. This an be done by an isolation valve
plus a pressure relief valve (leak tight under vacuum)
in case leakage past the shut off valve occurs.
2) If
your process is likely to create blockages in the pipes
leading to vacuum gauges and controls then extra precautions
need to be taken depending upon the nature of the process
and layout of the equipment.
3) If
part of your system could possibly crack or break causing
a large leak, this will cause air to be drawn through
the rotary pump. The change in sound may not be noticeable.
The gauges may fail, but a technique used in Cathode Ray
tube production was to fit a whistle, yes, the
type used on a kettle, to the rotary pump exhaust. You
could hear that.
Copyright
1993
NB: The
information in this pamphlet is offered in good faith
and is based on our general experience. The method
of use of Island Scientific Ltd equipment and materials
are outside our control, the responsibility to ensure
that the equipment is properly used and suitable for intended
application rests with the user. No responsibility will
be accepted by this company nor any person or other company
associated with this leaflet for loss or consequential
losses as a result of them using this information.
For advice,
information, general literature, prices or a discussion of
your needs, telephone 01983 855822, fax 01983 852146 or
E-Mail: sales@island-scientific.co.uk.
A full
list of Technical Notes is available free on request.
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Technical
Note No: 1055
