Sensors Mag

A Twelve-Step Sensor Selection Checklist

November 1, 2006 By: G. Raymond Peacock, Inc. Sensors

Buying a sensor for the first time? Follow these steps to greatly reduce your chances of overlooking something important.

Selecting a device to measure a physical property, such as temperature, can be a confusing process. But shortcuts and inattention to detail can get you into a real jam. Here, in chronological order, are the steps I try to use most of the time, especially when the measurement is really important. The underlying concept is akin to the carpenter's maxim: Measure twice; cut once.

1. Establish Measurement Span or Range Requirements

Find out your measurement range and any allowable variations. Many vendors specify accuracy or uncertainty as a percentage of measurement span. Be sure you understand what those specs mean. Many suppliers play fast and loose with the definition of terms as used in their literature. Some do a better job. And yet others omit some of those critical to your application: effects of ambient temperatures, especially the residual temperature coefficient; response time constant; or effects of vibration, radio frequency interference, or magnetic fields.

2. Agree on Accuracy and Uncertainty Requirements

What roles do precision and resolution play? Do you need the temperature to within ±0.1°C, or ±1°C, or ±10°C (or °F)? You might need to look at the various sensor options and perform an error analysis on each candidate type. You might need to do some extra detective work to get realistic and realistically acceptable tolerances. Knowing the accuracy and precision you require will pay real dividends, allowing you to verify and do a reality check on what someone else tells you.

Everyone involved in the selection process, including your superiors and the operations staff, needs to be in agreement on your requirements. For instance, a thermocouple will measure just fine for a 10°F accuracy case, but certainly will not cut it for 0.1°F. On the other hand, a resistance temperature detector (RTD) will handle 0.1°F if the environmental or measurement context conditions are favorable.

If you need to use connecting wires on an RTD, for example, you have to be sure that the wire's resistance will not contribute significantly to the overall measured resistance of the detector plus lead wires. If that's likely, you might need a 3- or 4-wire configuration. That, in turn, will influence your choice of a readout, transmitter, and/or data logger unit. Which one will handle your 3-wire or 4-wire RTD connections?

3. Identify Influence of Measurement Conditions

Know all your measurement conditions and the effects they might have on your sensors. This is an area that can make or break a measurement, particularly in industrial and utility process applications. Thermocouples in particular can produce variable bias errors if connected to compensating cable that runs through large temperature gradients, or can experience very high temperatures along the heated length, as in the case of metal-sheathed, mineral-insulated devices. There are not a lot of test data on sources that influence thermocouple readings, but a visit to and should prove helpful.

If you can't use a contact device, consider your noncontact options. Also, if ambient conditions look too severe for the sensor you like, be sure to take advantage of the accessories that vendors provide to handle them. In industrial applications, heat, cold, dust loading, and vibration are the principal bad guys, but humidity, rain, sleet, and snow can sometimes be even worse.

Keep in mind that measuring any surface temperature with both contact and noncontact sensors is a hard task with many potential error sources. Achieving good thermal contact is sometimes difficult, and failure to do so can contribute to measurement errors due to reduced speed of response of thermocouples and RTDs. IR thermometers, however, can follow surface temperature changes faithfully, assuming their response time constant is small enough, and they can be superior to contact sensors for some applications. Their major limitation (in theory) is not time response, but rather dealing with surface emissivity and reflectivity, particularly when the temperature being measured is close to ambient.

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