MKS Baratron 121A, Instruction Manual, Page: 49 / 62

Chapter Five: Theory of Operation Basic Design
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Chapter Five: Theory of Operation
Basic Design
The MKS Type 121 Pressure Transducer is a high accuracy general purpose absolute sensor that
utilizes a single-sided, dual-electrode/AC bridge circuit design. In this design, two capacitance
electrodes are deposited upon a ceramic disc in a concentric “bull’s-eye” arrangement. This disc
is positioned close to an Inconel
®
diaphragm to form two capacitors which are part of an AC
bridge circuit. When pressure is equal on both sides of the diaphragm, the bridge is balanced.
As pressure deflects the diaphragm toward the electrode, the center capacitance changes more
than the outer capacitance, causing the bridge to become unbalanced. By amplifying and
demodulating the output of the AC bridge, a useable high-level DC output signal is generated.
Overpressure Considerations
The Full Scale of any transducer is the highest pressure at which valid measurements can be
made. At pressures of roughly 30% over Full Scale, a high (12.5 VDC) output signal is
produced to warn of invalid readings. At pressures somewhat beyond this, the diaphragm
touches the fixed capacitor plate. The capacitor plate serves as a protective overpressure stop to
prevent damage to the diaphragm up to 35 psia or 120% of sensor F.S., whichever is higher.