MKS 41A - How a Gage Switch Works; Reverse Calibration Switch; Atmospheric Switch

How A Gage Switch Works Cha
p
ter Three: Overview
22
How A Gage Switch Works
The 41 and 42 gage pressure switches can measure pressure both above and below atmospheric
pressure (that is both positive and negative pressure). A gage pressure switch can be used as an
atmospheric switch; one that references the trip point pressure to current atmospheric pressure.
For example, a gage pressure switch with a 100 psi full scale range, enables you to set the trip
point value at 15 psi above atmospheric pressure (2 atmospheres). Therefore, even though
atmospheric pressure may fluctuate around 760 Torr (in most places) the switch would always
change when the pressure exceeded 15 psi above
current
atmospheric pressure. This switch
would require a “normal calibration” since the trip point pressure is above atmospheric pressure.
In the model code the full scale range would be “12P” to designate pressure units of psi with a
normal calibration. The trip point pressure would be “015.”
Reverse Calibration Switch
In some instances you may need to reference atmospheric pressure yet set the trip point value
somewhat below atmospheric pressure. For example, using a 100 Torr Type 41 gage pressure
switch, you may need to set the trip point pressure 10 Torr
below
atmospheric pressure. This
switch would require a “reverse calibration” since the trip point is set below atmospheric
pressure. The choice of a reverse calibration is selected in the model code, in the “Full Scale
Range” section. In our example, the full scale range would be “12D” where the “D” designates
the pressure units of Torr with a reverse calibration. The trip point pressure, specified in the last
three digits of the model code, is defined as the value below atmospheric pressure. In our
example, the trip point pressure would be “010.” Refer to
Appendix B: Model Code
Explanation
, page 31, for a complete description of the model code.
Atmospheric Switch
Some processes operate in a vacuum yet require the switch to change at atmospheric pressure. A
gage switch with a reverse calibration can achieve this effect. A typical configuration may have
a full scale range of 10 Torr or “11D” with a trip point set to atmospheric pressure (000). This
switch would trip whenever the measured pressure reached atmospheric pressure.