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DIGIVACTM
Controlling
Vacuum in a System
Often, it is necessary to control the level
of vacuum in a system.
Three approaches come to mind:
1) Turn the pump or
vacuum source on and off.
2) Bleed air or some
other gas into the system.
3) Throttle the pump
suction with a proportional valve. If
turning the pump on and off will control well
enough to suit your needs and will not create other problems, all that
is needed is a gauge with control outputs such as a Digivac 200P/2c.
This gauge has two Spdt relays that can handle line voltage and 7 amps
of current.
Turning the pump on and off may be problematic. Some pumps will
surrender their oil if stopped and exposed to vacuum. Some motors
will not be able to handle frequent stops and starts. Current
surges from stops and starts may cause electromagnetic interference,
etc.
If it is necessary to control in the
millitorr region, regulation by bleeding a small amount of air or
another gas is usually best. This has the added benefit of sending purge
are through the vacuum pump and that acts as a gas ballast to purge
moisture out of the oil.
The difficulty in implementing bleed control, is that the amount
of bleed gas necessary to hold a vacuum chamber at, say 100 millitorr,
is very, very small, and the smallest solenoid valve that is readily
available (say, with a .020" orfice) is several thousand times too
large.
Digivac gauges overcome this problem with
small, but fast acting (millisecond opening and closing times) valves with
intelligent software that opens and closes the valve, give the
thermocouple sensor (which has a time delayed reaction) a chance to react.
Digivac will provide the gauge, valve and software, and will even perform
an engineering model test to gaurantee performance in a particular
application. A Digivac Model 200P/1c/sol will provide such a
solution.
IThe third approach for control is to throttle the vacuum source.
This is most useful at higher pressures. (Too much purge air will make a
vacuum pump emit smoke.)
This requires a proportional valve in series with the vacuum pump.
Digivac offers a turnkey solution with the Model 450 vacuum
gauge/controller.
This unit senses system vacuum using an
isolated strain gauge sensor and modulates a proportional valve which is
in series with a vacuum source. Well developed software modulates
the valve using 400 hz PWM, overcomes inherent proportional valve
hysteresis, displays the results.
The 450 gauge / controller is an excellent solution for the scientist
who wants to regulate the vacuum in a distillation flask to a value
between (say) 10 and 700 torr.
Its only limitation is the size of available proportional valves.
Presently a 9/32" orfice is the largest we have been able to find
but custom solutions to particular applications, using (possibly)
motor-driven butterfly throttling valves.
Remote Setpoints
It is often desirable to integrate Digivac's proven control
technology with a custom application which is controlled by an external
device such as a computer or PLC.
Digivac
gauges can accept remotely generated setpoints, either from Rs232 or an
analog voltage.
To discuss your control application, call Digivac at 732.571.2300.
Chances are, we have recently solved a problem very similar to yours.
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