Tutorial: Capacitive pressure sensors

Measure pressure: Capacitive sensing offers a broad range of advantages for process instrumentation. See diagram.

02/19/2009


When selecting instrumentation technology, the task often comes down to choosing the most favorable selection of characteristics. For pressure measurement, capacitive sensing offers trade-offs that have earned it wide acceptance.

Capacitive pressure sensor construction is very simple and robust: Control Engineering diagram.
Capacitive pressure sensors use a moving diaphragm and a stationary base as the two capacitor plates. Either the two plates are charged separately, or the diaphragm serves as a bridge across two fixed electrodes. In either case, when the diaphragm deflects due to increasing pressure, the surfaces come closer together and the capacitance value changes. This change can be quantified and converted to an analog or digital output.

Simple construction, 7 more advantages
This approach makes for a very simple mechanism, which is one of the reasons it has been used so widely. Key advantages include:

  • Wide range of diaphragm materials possible;

  • Relatively simple electronic signal conversion;

  • Low power consumption, suitable for battery power;

  • Correction for ambient and process temperature conditions is uncomplicated;

  • High degree of accuracy and stability possible;

  • Sensors can be very durable capable of withstanding heavy overpressure; and

  • Can be configured for gage, absolute, or differential pressure readings.


Drawbacks
There are some downsides to the technology compared to other sensing techniques. The extent of these varies between manufacturers, so consult with your supplier.

  • Maximum pressure range is relatively low, but within most process application limits;

  • Operating temperature range is narrow; and

  • Sensors tend to be comparatively bulky as the diaphragms require specific minimum surface area.

For more reading on pressure measurement:
Pressure sensor technologies
Understanding pressure instrumentation

Pressure instrumentation suppliers do not always indicate what the sensing technology is for a given device or family. Moreover, many companies use different technologies for subgroups within a larger line, so you may have to ask for specifics. Capacitive pressure instruments are available from a range of manufacturers, including:

ABB
Baumer
Endress + Hauser
Rosemount (Emerson Process Management)
Siemens

—Peter Welander, process industries editor, PWelander@cfemedia.com ,
Control Engineering Process Instrumentation & Sensors Monthly
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