Capacitive Pressure Sensors

Introduction

The capacitive type pressure sensor is a pressure measurement technology that is now finding many applications including absolute pressure (e.g. barometric), gauge, relative and differential pressure measurement.

How It Works

The capacitive pressure sensor is based on the properties of one or more capacitors.

Instead of two fixed plates as in the standard capacitor element, the sensor has one fixed plate and one flexible diaphragm.

A secondary isolating diaphragm is used to protect the sensing diaphragm.

Pressure applied to the flexible diaphragm causes it to move, consequently changing the distance between it and the fixed plate. This in turn proportionally changes the capacitance of the unit.

The change in capacitance can be measured by making it part of a bridge circuit consisting of the capacitive sensor plus an inductor.

The capacitive sensor as described above measures absolute pressure. Another adaption of this sensor measures differential pressure. Differential pressure measurement has useful application in industrial process control.

As shown in the diagram below this type of sensor is housed in a hollow metal cylinder. Fixed plates are attached at each end of the tube. In the centre of the tube is attached a flexible diaphragm.

If the input pressure at both ends of the cylinder are equal, the diaphragm will not suffer any deflection and the capacitances are equal.

If there is a difference in input pressures between the two cylinder ends, the flexible diaphragm will be deflected. This deflection can be measured as a difference in capacitance between the two capacitors.

Advantages

It has many advantages including:

• Low power consumption as there is no DC current flow through the sensor element.
• Simple structure with durable materials give high reliability and longevity even in some harsh environments.
• High sensitivity and accuracy over a wide temperature range (with temperature compensation).

Disadvantages

Even good things usually have some disadvantages:

• Sensitive to ambient temperature, mitigated with temperature compensation.
• Often require more complex signal conditioning circuits and calibration algorithms.
• Excel in low-frequency measurements but are less suitable for situations that require ultra-fast data collection.
• Can be sensitive to vibration which may make them unsuitable for some applications..