A designer's primer on pneumatic sensors and switches

29 April, 2015

• Wide choice of output formats and ranges.

• Modern sensors can include signal conditioning electronics to provide highly accurate linearisation and compensation.

Disadvantages of pressure sensors

• Generally higher cost than a switch which meet equivalent environmental and reliability specifications.

• Analog outputs can be more susceptible to radiated emissions and other electrical noise than on/off signal.

• Integration of pressure sensor would mean the control system needs to be able to accept continuous output.

Matching your application

When your design does indeed require a pressure sensor, the trick to selecting the right device is to only buy the performance and capabilities you need, and avoid paying for the ones you don’t. In practice, that's not always as simple as it sounds, but the list of selection considerations below should make things easier. Once you've selected the criteria that are relevant to your application, add the values that will meet your requirements.

In terms of the Pressure range, for example, what are the maximum and minimum pressures you expect to see in your application? Do you need to expand the range requirements above and below your formal requirements to cover unanticipated conditions? And what are the units of measurement you'll use (psi, bar, inches of water, etc.)?

How accurate does the pressure measurement need to be? What is the maximum Total Error Band (TEB) your application can tolerate? Should the sensor output its readings as a voltage, current, or resistance? What's the output range you need from the sensor and what units is it expressed in?

In terms of its electrical and EMI protection, what are the levels of the worst extraneous system voltages the sensor could be exposed to and what is maximum level of electromagnetic interference (EMI) you'll need your application to operate reliably in? What are the highest levels of EMI and extraneous voltage your application's sensors must be able to survive without damage?

Depending on where your device is being used, what is the required operating temperature range? What levels of shock and vibration must the sensor be able to tolerate? What is the ingress protection that a sensor is required to meet to work in application conditions?

And when it comes to media compatibility, what's the temperature range of the media being measured and does it have corrosive properties?

Packaging options

The other big consideration is a practical one: the packaging and mounting options. The environmental, media compatibility and mechanical issues you've already considered will provide plenty of guidance on the packaging requirements for a stand-alone sensor. But for some applications it may be worthwhile to consider a board-mounted sensor that's co-located with the application's other electronics. Board-mounted pressure sensors can provide space-saving solutions with specialised port options that give the sensor access to the gas or fluid being monitored while it resides within the electronics module's relatively benign environment.

Lastly, designers need to consider the requirements of the mechanical and electrical interface, and electrical connectors: does the application have any specific mounting holes, brackets or other hard points the sensor needs to attach to? If so, what holes, flanges or other features does the sensor housing need to have to mount where it's needed? What types of pressure input fittings and electrical connectors will the sensor need?

You’ll also need to ask yourself a few basic questions: Will the operating environment pose any additional challenges in relation to vibration, rapid temperature cycling, or corrosive vapours etc.; what's the type of pressure you need to measure (i.e. gage, absolute or differential); and what are the application's size and price constraints?

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