Chapter Two: Installation Setu
15
Lags
Lags must be removed.
If lags (delays) are built into a system, stabilization will be difficult or impossible. Lag may be
introduced pneumatically in two areas: the tubing to the sensor and the tubing to the valve.
Both
tubing runs must be less than 6 inches long and no less than ¼ inch in diameter.
Make sure
the fittings do not introduce extra lags (internal small drilled passages). Other lags such as
antichambers off the main vacuum chamber may also cause problems and must be kept to a
minimum.
If a 270/690 type system is used for pressure measurement, the most likely cause of lags is
allowing the indicator switch on the 270 signal conditioner to be placed in the wrong position.
This switch must be in the NORMAL position.
Choking
Another important consideration, is a condition that is peculiar to diffusion pumps, called
choking. If too much gas is introduced at too high a pressure, the pumping speed becomes
erratic and changes radically. This behavior is easily recognized by lack of stability and
pneumatic noise, sometimes making control impossible. Reducing the flow to the pump is the
only solution.
Note
Diffusion pumps should never be operated at inlet pressures above which
the “top jet” will experience a pressure in excess of 5 x 10
-4
Torr.
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