Pressure transducers utilize magnetic phenomenon in converting the elastic movement of a sensor into an electric signal including induction and reluctance types. As the pressure sensors cause a movement, this changes the inductance or reluctance of an electric circuit. Inductance is the property of an electric circuit that expresses the amount of electromotive force (EMF) induced for a given rate of change of current flow in the circuit. Reluctance is the resistance to magnetic flow, the opposition offered by a magnetic substance to magnetic flux (magnetomotive force).
Inductive Elements
In this, the process pressure moves a magnetic iron core inside a coil, which alters the self-inductance of the coil. The many turns of the solenoid coil are usually wound on a tube of insulating material. As the core is pushed into the solenoid, the inductance of the coil increases in proportion to the amount of metal inside the coil. The ratio inductance element is similar, except that there are two solenoid coils next to each other on the insulating tube and the iron core is always within them.
The core movement is detected as a change in the inductance ratio of the two coils. The sensitivity and linearity of the ratio inductance transducers are better than that of the variable inductance units. As a pressure sensor moves a conductive surface, while the transducing element remains constant, the eddy currents cause variations in impedance.
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Reluctive Elements
Electrical reluctance is the equivalent of electrical resistance in a magnetic circuit. A variable reluctance transducer, where a magnetic vane (moved by a helical Bourdon pressure detector) changes the gap in the magnetic flux path of both cores as it moves. The magnetic reluctance varies with the size of these gaps, and the result of the motion of the magnetic vane is a change in the inductance of the two coils (L1 and L2). The inductance ratio of L1/L2 is therefore related to the process pressure.
The advantages of the variable reluctance transducers include their wide temperature range, their low sensitivity to shock and vibration (due to their rugged construction), and their high output signal (40 mV/V of excitation). Their disadvantages include their temperature sensitivity (2%/100°F), their requirement for AC excitation, and their susceptibility to stray magnetic fields.
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Instrumentation Basics 