Capacitance Manometer Auto-Zero Feature - MKS HPS 937A Series Owner's Operation And Maintenance Manual

A special calibration algorithm in the 937A allows independent
zero and atmosphere calibration adjustments for Pirani, Convection
Pirani and thermocouple sensors.

Capacitance Manometer Auto-Zero Feature

The auto-zero feature continuously resets the zero of a capacitance
manometer when the cold cathode sensor in Slot CC indicates system
pressure is low enough. The capacitance manometer and the cold cathode
sensor should be closely located on the vacuum system and exposed to
roughly the same system pressure.
To enable auto-zeroing, set DIP Switch 4 for either head on the capacitance
manometer module to On (page 29). The cold cathode sensor in slot CC must
be on and its pressure reading must be ,
less than 1.0 x 10
measurement range, and
less than 0.005% of full scale for the capacitance manometer's Torr
head (5% of its minimum reading), e.g., 5.0 x 10
If the above conditions are true, and the capacitance manometer signal is
within range, a new zero is set each time the capacitance manometer signal
is read. CM ZERO and L O on the capacitance manometer's channel line
both appear on the display.
As soon as any one of the required conditions ceases to be true, auto-zeroing
stops and the last valid zero reading is used until all conditions are again true.
Allow the Controller to stay in auto-zero mode for at least 30 sec before
taking it to a pressure above the auto-zero pressure. The capacitance
manometer needs this time to stabilize.
The voltage of the capacitance manometer must be within ±50mV of 0V. If
the sensor voltage exceeds this range, a new calibration value will not be
accepted. Refer to the zero adjustment procedure in the capacitance
manometer instruction manual.
The zero value determined by auto-zero calibration is not saved
in nonvolatile memory and is lost when the Controller is turned off.
The zero calibration which is stored in non-volatile memory is not changed by
the auto-zero feature. If a manual zero calibration is carried out during auto-
zeroing, the selected zero value will be stored as usual, but will not affect the
pressure measurement. The saved reading will be used when the Controller
power is next turned on, until auto-zeroing resumes. This allows a calibration
saved during auto-zeroing to become the default zero value.
52
-2 Torr, the high end of the cold cathode sensor's
937A Multi-Sensor System
-5 Torr for a 1 Torr head.