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.
A.J. Smith is director of product marketing for Honeywell Sensing and Control.