Chapter Three: Overview
General
35
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.
MKS Types 622B, 623H, and 626D 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.
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 an elastic 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 reference side is permanently evacuated (10
-7
Torr) and its vacuum is maintained with a
chemical getter system.
The diaphragm deflects with changing absolute 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.
The imbalance of capacitances is converted to a DC voltage in the bridge. This bridge is excited
by a precision constant frequency oscillator. The resultant signal is then linearized, zeroed, and
amplified via the signal conditioner electronics, to produce a precise 0 to 10 VDC signal scaled to
the range of the transducer.
Signal Conditioner/Electronics
The signal conditioner contains state-of-the-art, low impedance balanced bridge circuitry, self-
compensated for thermal stability with ambient temperature changes. Output is a DC voltage
which is linear with pressure. The transducer is then calibrated against a pressure standard to
provide a 0 to 10 Volt DC output over the range of the transducer.
