System Design Elements
Chapter Three: Overview
92
Pressure Control
The speed of pressure response is relative, and depends on chamber size and the pressure range in
which the system is operating. Lower pressures (less than 10 mTorr) are usually slower because
of the slower molecular flow (and reduced pumping speed). The maximum rate of rise (or fall)
of pressure is determined by the following formula:
dP = Fin -Fout
dt V
where:
dP
= rate of pressure rise (or fall) in Torr/second
dt
Fin
= gas flow into the chamber in Torr-liters/second
Fout
= gas flow out of the chamber (to the pump) in Torr-liters/second
V
= chamber volume in liters
In systems with small flows and relatively large chamber volumes, the pressure rises (or falls)
slowly. This occurs even when the control valve is fully opened or closed. With a fast upstream
control valve, the maximum rate of pressure drop is a function of pump speed, and the chamber
volume, but the maximum rate of pressure rise is a function of the pump speed, chamber volume,
and valve size.
Pump Choking
Choking is a situation that sometimes occurs, and is the result of a peculiarity of all diffusion
pumps. If too much gas is introduced at too high a pressure (that is, too great a flow rate), the
pumping speed can change radically and erratically. This occurs for several reasons. For
example, the pump can fill up (since the evacuation rate is less than the incoming flow rate), and
as a result it momentarily cannot take up any more gas. Also, it may be that the pumping
mechanism is slowed down by high friction.
In any case, the situation is easily recognized by lack of stability at the set point, and by
pneumatic noise. Closing down the inlet to the pump is the only solution. Diffusion pumps
should never be operated at pump inlet pressures higher than the maximum pressure set by the
pump manufacturer. Refer to the specifications for your pump to determine the maximum
allowable inlet pressure. A restriction between the pump and chamber may be necessary to
prevent pump choking.
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