MKS Micro-Baratron 870B, Instruction Manual, Page: 49 / 68

Chapter Three: Overview General Information
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Chapter Three: Overview
General Information
A complete pressure transducer system requires three components to convert pressure to a linear
output: a sensor, signal conditioner, and power supply. The 870/872 transducer contains two of
the required components: the sensor and signal conditioner.
An MKS or MKS-compatible power supply is required to complete the pressure to DC voltage or
mA current conversion. For a direct readout of the pressure measurement, a meter (analog or
digital) is required.
Sensor
The 870/872 transducer is a variable capacitance sensor consisting 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 reference side is permanently evacuated below the resolution of
the instrument.
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. The diaphragm, in either
configuration, 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.
Using a precision constant frequency oscillator for excitation, the imbalance of capacitances is
converted to a DC voltage or mA current. The resultant signal is linearized, zeroed, and amplified
via the signal conditioner electronics to produce a precise output signal scaled to the range of the
transducer. The output signal, which is proportional to pressure, is provided through the connector
on the top of the unit, as either a 0 to 10 V or 0 to 5 V DC voltage, or a 4 to 20 mA current.
Signal Conditioner
The signal conditioner contains state-of-the-art balanced bridge circuitry, self-compensated for
thermal stability with ambient temperature changes. The circuit board construction uses surface
mount technology. The output is either a DC voltage or mA current, which is linear with pressure.
The transducer is then calibrated against a transfer standard to provide the selected output over
the range of the transducer.