The functions that a pressure sensor performs within a design establish the basis for its selection. For all designs, major selection considerations are environment, cost, size, repeatability, redundancy, energy, and accuracy.
First, for each design, ask yourself: Are there temperature compensation issues, corrosive liquids, or earthquake levels of shaking involved? What type of pressure sensor (gage, absolute, differential) best matches this design? Is there sufficient time (or money, for that matter) to calibrate the sensor? How conversant am I with calibration ins and outs? How accurate must the measurement be? Are there size constraints?
Total error band (TEB): the maximum deviation in output from the ideal transfer function over the entire compensated temperature and pressure range.
How important is ease of integration?
Do you want the sensor to be as close to a plug-and-play setup as possible? If so, your choice will include factory calibration and temperature compensation, all housed in packaging that is ideal for the design. Over the past several years, many precalibrated options boasting impressive repeatability have come to market. If you don't need to add proprietary algorithms that require a lot of integration time, you can implement quickly and forget.
Matching performance to your application
Are there critical use aspects of the application, i.e., medical use where life may depend on accuracy? For the simplest solution involving unamplified, uncompensated, mV output, along with small size and board mounting, a basic sensor is the natural choice. For harsh environments, a heavy-duty solution may be the only possibility. In between the two, accuracy required typically dictates the type used. There are performance/cost tradeoffs at every junction. Considerations include accuracy, repeatability, consistency, and reliability. The performance level impacts the importance you place on each of these elements during the selection.
What are the environmental realities?
Corrosion, temperature extremes, and humidity all play an important role in pressure sensor choice. Humidity impacts the sensor materials used. For example, some sensor plastics soak up moisture before the reflow process, and they fall apart as a result. Gels inherent in some basic sensors may burn in the soldering process, and some sensors lack stability after soldering -- and may not gain it back for weeks. By carefully matching the environment and the sensor characteristics, you will ultimately save time and money.
When I was designing industrial tsting and calibration systems the two main criteria were stability and ruggedness. Because of the types of testing the machines did, the readings only needed to be accurate at the points of interest, often at a single point. So the transducer had to hold calibration for that particular pressure point, and be veryy stable. Calibrations were normally done quarterly, at least that was the target, and by selecting the right sensors we would normally only need a very small correction on a yearly basis. So it was far cheaper to spend more on a stable transducer that wound up only needing to be checked on a yearly basis. That stability won my argument for purchasing the more expensive device. Mechanical durability and freedom from mechanical aging, (diaphram work hardening), and electronic stability, wound up being the two primary parameters in our selection process, because that was what mattered to our customers.
The customer may not always be right, but they are always the ones with the money.
For the most extreme situation I think that is the case, but for markets and applications that are large enough, they drive standard solutions even if those are ruggedized solutions. One other thing that has become important is that to have the flexibility and scale to create a particular solution, it is much more efficient to have a standard product platform that the sensor supplier then takes and implements a ruggedized solution on top of the standard product. Much faster and more efficient than developing the entire sensor from scratch. This is how we have positioned our standard product platforms at Honeywell to address individual customer needs for ruggedized sensors. - AJ Smith
That can be the case, Rob, and increases as the sensor becomes more specialized. When it comes to doing something completely custom there are often times development agreements and engineering costs involved beyond just the cost of the sensor as well. - AJ Smith
Rob, What I see is that environmental resilience is much like other performance factors in that the more stringent the requirement (in this case the more extreme the environmental factor(s) the fewer options there are to choose from. And in some cases for really extreme environments the design engineer may need to work with a supplier to develop a product that can meet their particular requirements. - AJ Smith
Nice article A.J. - I appreciated your very straightforward explanation of pressure sensor selection. I can also see how your criteria can be easily adapted to selecting any sensor by using your general principles coupled with sensor specific criteria. It's so important for the design engineer to understand all of the variables involved when making a selection!
AJ, On your last point about finding sensors that can function in difficult environment, do design engineers have sufficient choices that can work under environmental stress?
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