E-Zine June 2007
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Differential pressure flow transmitters generate an electrical signal that represents the differential pressure at its ports. Of importance is how well the differential pressure flow transmitter performs this function. Because performance is the prime concern in many applications, the technology used to effect the measurement is typically a secondary or tertiary matter. The following sections provide brief explanations of the principles used in differential pressure flow transmitter designs.
Capacitance
In a capacitance design, the differential pressure at the ports causes the wetted diaphragm to move an internal diaphragm located between two fixed plates. The movement of the internal diaphragm causes a capacitance change that can be converted into a signal that is proportional to the applied differential pressure.
Differential Transformer
In a differential transformer design, the differential pressure at the ports causes the wetted diaphragm (or bellows) to move the magnetic core in a transformer. The movement of the core causes an electrical imbalance that can be converted into a signal that is proportional to the applied differential pressure.
Force Balance
In a force balance design, the differential pressure at the ports causes the wetted bellows to create a force that is counteracted by a force generated by an electromagnet (or perhaps a servomotor). A measurement of the generated counteractive force can be converted into a signal that is proportional to the applied differential pressure.
Piezoelectric
In a piezoelectric design, the differential pressure at the ports causes the wetted diaphragm to apply force on a crystal. This force causes an electric signal to be generated that can be converted into a signal that is proportional to the applied differential pressure.
Potentiometer
In a potentiometer design, the differential pressure at the ports causes the wetted diaphragm (or bellows) to move the wiper of a variable resistor (potentiometer). The movement of the wiper causes a resistance change that can be converted into a signal that is proportional to the applied differential pressure.
Silicon Resonance
A silicon resonance sensor is a micro-machined semi-conductor structure fabricated on a silicon crystal. The structure is shaped such that it can oscillate and resonate at high frequencies. When a differential pressure is applied, part of the structure is under compression while another part of the structure is in tension. The compression and tension forces change the resonant frequency of the structure in a manner proportional to the applied differential pressure.
Strain Gage
In a strain gage design, the differential pressure at the ports causes the wetted diaphragm to apply a force on a strain gage. This force stretches the strain gage and causes the resistance of the strain gage to change. The resistance change causes an electric signal to be generated that can be converted into a signal that is proportional to the applied differential pressure.
Excerpted from The Consumer Guide to Differential Pressure Flow Transmitters
ISSN 1538-5280
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