MKS 41A - Trip Point Label; Sensor; Figure 8: Trip Point Label

Sensor Cha
p
ter Three: Overview
18
Trip Point Label
Each mini-pressure switch also carries a label to identify the action of the trip point. The trip
point action is set at the factory to energize when the pressure is either above or below the trip
point.
Energized Above
Trip Point
Figure 8: Trip Point Label
Sensor
The variable capacitance sensor consists of a pressure inlet tube (port) connected to a small
chamber in the transducer body. One wall of this chamber is a metal diaphragm. The front side
of the diaphragm is exposed to the gas whose pressure is to be measured. The back, or
reference
side of the diaphragm faces a rigidly mounted ceramic disc containing two electrodes. The
diaphragm in the single-ended version is positioned opposite the inlet port. The diaphragm on
the flow-through version is positioned above the gas stream. Figure 1 page 8, and Figure 2, page
9, show the design of the single-ended version; and Figure 3, page 10, shows the design of the
flow-through version.
The reference side of the absolute version (Type 51 or 52) is permanently evacuated below the
resolution of the instrument and its vacuum is maintained with a chemical getter system. The
reference side of the gage version (Type 41 or 42) is open to the atmosphere. Since its pressure
should not be altered, there is no connector on the reference side.
The diaphragm deflects with changing pressure (force per unit area) independently of the gas
type or composition of the measured gas. This deflection causes an imbalance of the sensor
electrode capacitances since the distance to the diaphragm is now different for each electrode.
Using a precision constant frequency oscillator for excitation, the imbalance of capacitances is
converted to a DC voltage representative of pressure and is supplied as one input to a comparator
circuit. The circuit compares the pressure signal to the factory set trip point signal and
appropriately activates a relay when the two signals differ. The form C relay contacts are
available on the Type “D” connector or the flying leads cable so you can connect the pressure
switch into your system.