MKS Baratron 121A, Инструкция по эксплуатации, Страница: 39 / 62

Chapter Three: Overview Type 121 Absolute Pressure Transducer
29
Chapter Three: Overview
Type 121 Absolute Pressure Transducer
The Type 121 Absolute Pressure Transducer converts pressure to a linear DC output voltage
using a variable capacitance measurement technique. The instrument has a total pressure
measurement range of over four decades and a 0 to 10 VDC output linear with pressure. The
Type 121 transducer consists of a sensor, a remote electronics unit (signal conditioner), and an
8 ft. shielded cable with a Type “D” connector.
Sensor
The absolute sensor contains a tensioned metal diaphragm, one side of which is exposed to the
process gas. The other (reference) side of the sensor contains an electrode assembly placed in a
factory sealed high vacuum reference cavity (10
-7
Torr) and is maintained at high vacuum (low
pressure) by an integral chemical getter system. The diaphragm deflects with changing absolute
pressure - force per unit area - independent of the composition of the measured gas. This
deflection causes a capacitance change between the diaphragm and the adjacent electrode
assembly. The electrode assembly also serves as an overpressure stop for pressures up to 35 psia
or 120% of F.S., whichever is greater.
Electronics Unit
The electronics unit contains the signal conditioner and is housed in a flange-mounted aluminum
box. The signal conditioner operates with a fixed frequency oscillator, amplifier, demodulator,
and calibration circuitry. The capacitance change that occurs in the sensor generates an AC
voltage which is sent to the electronics unit where it is amplified, demodulated, and converted
into a high level DC output voltage. The output is linear with pressure and provides a 10 Volt
DC output at sensor F.S. pressure.