Determining Protection And Control Set Points - MKS HPS 937A Series Owner's Operation And Maintenance Manual

Choosing between the protection and control set points depends upon several
factors, including the need for automatic start-up, and the possibility of the
pressure rising into the rollback range.
The reading from a cold cathode sensor will "roll back", or
indicate a false low pressure, when operated above 5.0x10
This prevents a rotection set point from switching the sensor
off if the spressure rises too quickly or when the sensor is
turned on above 5.0x10
cathode sensor can be turned off automatically by a control
set point only.
Both types of set points may be used with one exception. No control
set points are available if a Cold Cathode Module is in Slot B .
Determining Protection
and Control Set Points
The range for the protection set point
is 1x10 -5 to 1x10 -2 Torr.
For the Pirani, Convection Pirani or
thermocouple sensors, the lower limit
of the control set point is the same as
the lower limit of the relay set point.
For a capacitance manometer, the
lower limit of the control set point is
one-half of the relay set point lower
limit, or 0.5% of full scale. The upper
limit for the control set point for any
-1
sensor is 9.5x10 Torr.
937A Multi-Sensor System
58
-1 Torr. Above this pressure, a cold
Comparison of Control Set Point and Protection Set Points
Operation CC sensor turns itself off
Advantage a)No additional sensor requied
b) Lower minimum set point
Disadvantages
a)User must detmine when to
restart the sensor.
b)User must re- started the
sensor using the front panel or
other external signal
c)CC sensor may be affected by
rollback, which makes it unable
to turn itself off.
Set Point 1x10
Range
-1 Torr.
Protection
-5 -2
to 1x10 Torr
Control
CC sensor is turned off by a
Control sensor
a)CC sensor will be turned
on automatically at an
acceptable pressure as
detemined by the control
sensor
b)Control sensors not affected
by rollback.
c)Higher maximum set point.
a)Separate control sensor is
required.
b)If the two sensors are not
exposed to the same pressure,
the cold cathode sensor could
beturned on accidentally at a
pressure above its operating
range. This could lead to
rollback.
-3 to -1
2x10 9.5x10 Torr
(with suitable sensor)