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General Chapter Three: Overview
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Chapter Three: Overview
General
A complete pressure transducer system requires three components to convert pressure to a linear
DC voltage output: a sensor, signal conditioner, and power supply. An analog or digital meter is
required to display the DC output in pressure units.
The MKS 7xxC Series transducers contain two of the above components: the sensor and signal
conditioner. An MKS or MKS-compatible power supply is required to complete the pressure to
DC voltage output conversion, and an MKS or MKS-compatible display unit is required for direct
pressure readout. The display unit could be a personal computer, an MKS pressure controller, or
an MKS PDR Series power supply/readout unit.
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 3, page 26, and Figure 5,
page 27, show the design of the flow-through and single-ended versions.
The reference side of the absolute version 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 is open to the atmosphere. Since its pressure should not be altered, there is no
connector on the reference side. The reference side of the compound version is also open to the
atmosphere.
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. The resultant signal is then linearized, zeroed, and amplified via the signal
conditioner electronics, to produce a precise output signal scaled to the range of the transducer.