Capacitance Pressure Sensors

Pressure sensors can also use capacitance and reluctance to provide an electric signal that varies from minimum to maximum (0-10 volts). This type of sensor uses two plates that are separated by a dielectric to create the capacitance. In most cases the dielectric is a silicone oil filling. One of the plates is stationary, and the second plate is connected to a diaphragm so that when the diaphragm moves, the plate will move and the amount of capacitance will change. Since this type of circuit uses changing capacitance, it's most usable with ac voltage. When the capacitance changes in an ac circuit, the capacitive reactance changes and the amount of voltage drop across the capacitor can be detected.

The capacitive sensor can be enhanced so that two capacitors are used with two diaphragms called the sensing diaphragm and the isolation diaphragm. When the sensor is exposed to high pressure on one side and lower pressure on the opposite side, the sensing diaphragm moves. This moves the plate in the first capacitor and increases its capacitance. Since the two capacitors are linked, when the capacitance of one increases, the capacitance of the other decreases. The high and low pressures allow this sensor to measure the difference in pressure and the sensor is called a differential pressure sensor (ΔP). When pressure difference changes, the capacitance between the two capacitors changes and this change can be converted to a change in capacitive reactance in the ac circuit. The capacitive sensors can also be used to change the frequency of an oscillator circuit and the change in frequency can be used as an input to a variable-frequency circuit, which includes a frequency-to-voltage converter.

For a more technical discusion on Capacitance Pressure Sensors, click here.

Prev. Page: Piezoelectric Pressure Sensors | Next Page: Variable Reluctance/Inductance Pressure Sensors

Related pages: Load Cells | Flow Meters