Capsule transducers

Pressure transducers derived directly from the diaphragm gage can be used for moderate frequencies. The displacement of the diaphragm due to the pressure difference can be converted into an electrical output in different ways:

■ Potentiometric: the motion of a wiper along a resistive mechanism modifies the electrical resistance of a circuit powered by direct current; the presence of a sliding contact, which is subject to wear, limits the life of the instrument.

■ Electromagnetic: a block of magnetic material is moved in a coil so as to affect the inductance (Figure 1.8) of the circuit. It is necessary to feed the coil with a high frequency alternate current which must then be filtered out in the output signal in order to show only the changes induced by pressure oscillations; power supply and signal processing are expensive.

■ Capacitive: the diaphragm and the pressure cavity powered by direct current create a variable capacitor; the advantage in terms of natural frequency is the lack of moving masses, apart from the vibrating plate.

The output signal is the voltage if the measure is made on site; if the signal has to be sent at a distance, without taking into account the voltage drop in the transmitting wires, it is appropriate that the signal is the current flowing in the circuit: in this case, one talks about a pressure transmitter instead of a pressure transducer.

Capsule transducers of moderate size are commercially available, typically with a diameter of 4-6 cm, an accuracy approaching 0.1% FS (full scale) and a response time of the order of 5 ms. They are made entirely of steel and are particularly suitable for industrial applications in the presence of shock, vibration, or temperature changes. The measuring range extends up to 700 bars.

Electromagnetic transducer

— Iron core
Coil

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