Brooks Instrument XacTorr CMX160 - Manual (Page 9)

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2.0 DESCRIPTION
2.1 BASIC DESIGN
The CMX (also referred to as a capacitance manometer (CM)) is a pressure
measuring instrument that senses a change in process pressure. It does this by
measuring the change in capacitance caused by a change in deflection of a
tensioned diaphragm.
The basic design of the CMX sensor is shown in Figure 1. A thin metal
diaphragm is welded into a sealed sensor assembly where it is maintained at a
fixed distance from an electrode. The area behind the diaphragm, including the
electrode, is known as the reference side of the sensor. It may be kept at an
ultrahigh vacuum for an absolute gauge, or it may be connected to another
source of pressure or vacuum to function as a differential gauge. The front side
of the diaphragm is exposed to the process. The difference pressure between
the process and the reference side
Figure 1: Basic Capacitance Manometer Sensor
causes the diaphragm to deflect slightly. This deflection causes a change in
capacitance between the electrode and the diaphragm. The result is a capacitive
imbalance, which is measured by a circuit, and then converted to an output
signal. This output is directly proportional to the pressure (or vacuum) in the
process chamber.
The CM reading is not affected by changes or differences in gas composition
and therefore overcomes disadvantages of specific gas calibration (due to
thermodynamic and electrical property). Typically, in other vacuum gauges such
as cold cathode ionization, Pirani, and thermocouple, each instrument must be
calibrated for each specific process gas.
Capacitance manometers measure absolute pressure (force per unit area).
They can measure to extremely precise levels and usually have quoted
accuracies of 0.25% error of reading (versus typically 10% of reading with other
indirect vacuum measurement technologies).
NOTICE!
From this point forward, the
CMX Series Heated Digital
Capacitance Manometers
will be referred to in general
terms as the CMX or CM.
2.0
_DESCRIPTION