Brooks Instrument CMC Series - Manual (Page 7)

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1.0 INTRODUCTION
The Brooks CMC Series Capacitance Manometer is a temperature compen-
sated, absolute pressure transducer designed for accurate and repeatable vac-
uum measurements. Various full scale ranges are available from 10 Torr up to
1000 Torr. The units of measurement may be specified in Torr (mmHg), mBar, or
kPa.
The CMC Series operates from a 12-30 VDC power supply and provides a 0 to
10 VDC signal output that is linear with pressure and independent of gas compo-
sition. The CMC Series is pin for pin compatible with other competitive capaci-
tance manometers. Superior EMI/RFI performance is achieved by the use of a
metal case in conjunction with surge and ESD suppression components and RFI
filtering on the inputs and outputs. The CMC Series has easy access to multi-
turn potentiometers for fine Zero and Span adjustments. Inconel
® is used for all
wetted materials for compatibility with corrosive gases. A wide range of pres-
sure/vacuum fittings are available.
The high accuracy pressure sensing element used in the CMC Series is a pat-
ented variable capacitance sensor. A centrally located feed through assembly
supports a circular electrode in close proximity to the back surface of the dia-
phragm. Together, the electrode and diaphragm form a variable capacitor within
a small reference vacuum chamber. As the pressure increases, the diaphragm
deflects and the gap between the electrode and diaphragm reduces, causing an
increase in the capacitance. This change in capacitance is detected and con-
verted to a highly accurate linear DC electronic signal by a unique custom inte-
grated circuit utilizing a patented charge balance principle.
Excellent zero stability and barometric insensitivity is achieved through the pat-
ented sensor design. The CMC Series sensor contains no fragile or complex
parts found in ceramic based capacitance manometers. The all-welded con-
struction eliminates stability issues inherent in other designs due to frictional
contact between dissimilar materials.
1.0
_INTRODUCTION