The Transducer Technologies for Pressure Transmitter

A pressure transmitter consists of several components including the sensor, signal-processing electronics, and signal-conditioning electronics. The sensor translates force from pressure acting on an area into an electrical signal--usually a millivoltage or voltage--that is amplified and converted by signal-processing circuitry The conditioning electronics convert the sensor output to a standard signal such as 4-20 mA, 0-10 VDC, etc.

The strain-gauge pressure transducer converts pressure (a force) to a resistance change proportional to the applied strain. The strain gauge circuit consists of one, two, or four variable-precision resistors in a Wheatstone-bridge configuration. When the resistors are subjected to pressure they deform, changing the resistance of the circuit. A strain gauge sends a millivolt output signal.

Some advantages of the strain-gauge sensor are that it is widely available from many vendors and has good immunity to shock and vibration. Principle limitations include sensitivity to swings in temperature (more sophisticated designs have temperature compensation) and limited temperature and pressure ranges.

The output of a capacitive sensor is proportional to the change in capacitance of two diaphragms (plates) inside the transducer. Applied pressure changes the distance between the two plates, causing a change in the capacitance that is detected by an oscillator circuit. Analog transmitters convert the frequency to a voltage, while digital transmitters work directly from the sampled frequency.

The principal limitation of capacitive sensors is that they are somewhat sensitive to vibration. They provide excellent response and stability, and are widely applied in many pressure transmitter manufacturers´ designs. This large installed base allows the instrument engineer to draw on a great deal of installation experience when choosing this type of transducer.

In a quartz resonator sensor, pressure results in a change in frequency across the resonator that is directly proportional to the applied force. These transducers can be scaled over a wide range of pressures, and typically contain a quartz-crystal temperature sensor for temperature compensation. This type of sensor can offer increased immunity to temperature, vibration, and acceleration.

The vibrating wire and cylinder sensor has a wire excited by a magnetic coil. The wire vibrates at its resonant frequency in a phased locked loop (PLL) electrical circuit. A pressure-sensing diaphragm changes the tension in the vibrating wire, causing a change in resonant frequency proportional to the applied pressure.