MKS 253B-1-2CF-1, Instruction Manual

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Chapter Three: Operation and Theory of Operation Operation
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Chapter Three: Operation and Theory of Operation
Operation
MKS products are designed and tested to provide the highest degree of safety attainable. To use
your 253 valve safely, you must always conform to the following instructions:
Caution
When operating this valve with high vacuum diffusion pumps, it is
important to make certain the pressure above the oil vapor jet does
not exceed 0.5 milliTorr, as this leads to unstable pump performance.
This unstable (oscillating) pump characteristic will manifest itself as
a seemingly unstable control loop and no amount of adjustment of
the lead and gain will correct the situation. The only solution is to
lower flow rates so that the diffusion pump may operate in its correct
pressure range.
Refer to the appropriate Instruction Manual for complete setup instructions for your MKS controller.
Theory of Operation
A butterfly valve’s flow rate is very nonlinear with respect to shaft position; that is, the percent
change in flow rate ranges from very large values when the valve is near closed, to zero when it
is near fully open. This very large change in transfer characteristic means that for optimum
performance, gain and phase lead must be reset each time a valve operates at a different position.
In a processing situation, this is intolerable and MKS has introduced a special actuator
mechanism which changes the transfer characteristics to a more linear relationship. The special
“cosine generator” actuator introduces a second nonlinear motion which cancels the butterfly’s
nonlinear characteristics by giving a very slow initial opening characteristic (theoretically zero)
rising as a sine (cosine) function and reaching a maximum when the valve is fully open. This
gives the system near constant resolution throughout the valve’s throughput range, enabling one
lead and gain setting to be used over the operating range of the valve.