Matching requirements with specs
Temperature ranges, power requirements, and pressure range specifications are fairly straightforward and easily matched. Where variations enter the picture are in such elements as accuracy levels and potential error rates. Apples-to-apples measurement is critical. Does your potential sensor for a high-accuracy application indicate a total error band? This is a comprehensive, clear, and meaningful measurement that indicates true accuracy over a compensated range, versus specifying a variety of different potential errors separately. It provides confidence to the design engineer that all the potential errors have been considered. Be sure to break down the specifications that you require, and consider potential problems that errors may create.
Stability in an unstable world
Sensor stability is extremely important to achieve performance over time. It is often considered the most important aspect of sensor selection. If the offset drifts or changes due to application conditions or over time, it is often necessary to implement auto-zeroing algorithms, which require an expensive valve to shut off pressure when establishing a new zero point. If sensor output changes over time, cost and complexity automatically increase.
Depending on the nature of the application, the sensor may be called upon to do more than measure. It may also on some level provide worthwhile feedback that allows the application to make safety decisions. A sensor may even report on its own health, or it may provide diagnostics on itself, so that the critical application can ensure it is operating correctly.
Does low-cost mean the least expensive?
As with every other aspect of your design, what's important is the bottom line. Tally the total cost if, for instance, you perform sensor calibration yourself or if the packaging you choose doesn't hold up to the reflow process. What might begin as a fairly low-cost sensor may end up costing more than a top-of-the-line sensor. Ask yourself, "What comparative cost would an alternative product represent?" Consider true costs in time and actual expense, and compare them for each solution before making a final choice.
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!
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