Choosing sensors: Specsmanship vs. reality
Unpredictable errors, such as long-term stability, thermal hysteresis and nonrepeatability, cannot be treated mathematically to improve accuracy or precision and are far more difficult to deal with. While thermal hysteresis and nonrepeatability can be quantified at the point of manufacture under controlled conditions, long-term stability cannot.
Various statistical tools are available to help define long-term stability, but ultimately you have to make a decision that will depend in part on how critical the measurement is. Routine recalibration may be the only reliable way of eliminating the consequences of long-term deterioration in the sensor’s performance. Top tips for the specifier:
• Repeatability is the single most important sensor performance parameter; without it no amount of compensation or result correction will be meaningful.
• Consider the environmental temperature range within which the sensor will operate. Thermal errors, particularly those associated with the zero output of the sensor, will dominate.
• Do not overspecify the operating range of the sensor. Manufacturers state the sensor’s safe over-range limits, and these should be sufficient. By overspecifying your sensor, you will reduce its signal magnitude, and zero-based errors will increase as a percentage of the measurement range.
• Do not confuse resolution with accuracy.
• If the sensor is to be used long-term, consider the effect of its long-term stability. Progressive deterioration in sensor characteristics can have disastrous consequences. This emphasizes the need for periodic recalibration. Typically, 12 months is an acceptable recalibration period, but both the operating environment and the consequences of inaccurate data must be considered.
• Calculate the total error that can be expected from the sensor by referring to the data sheet performance parameters, being careful to include only those that are pertinent to the specific application.
About the author
Michael Baker is managing director at Sherborne Sensors. He earlier founded Maywood Instruments and was general manager of Schaevitz Sensors, part of U.K.-based Measurement Specialties. He led a management buyout team to form Sherborne Sensors in 2002.
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