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Working with resistive sensor elements
February 27, 2010
Dear Control Engineering: I’ve read that RTDs (resistance temperature detector) are often the most precise temperature sensing devices. Given the relatively narrow range of resistance involved, how is it practical to get precision with two, three, or more decimal places on a Celsius scale?
Sensors that measure a process variable using changes in resistance extend beyond RTDs. Thermistors also use resistance, as do many types of strain gages that are used in pressure and weight sensors. RTDs that use platinum wire can, at least in theory, measure temperature changes as small as 0.00001 °C. (Of course saying that one technology or another is the most accurate needs to be qualified in the context of specific types of application because few of those evaluations are true universally.) The kind of precision is only possible when coupled with highly sophisticated signal processing.
Modern electronics are capable of reading very small changes in resistance which makes this sort of thing possible. Interestingly enough, one of the basic elements of precise resistance measuring circuits dates back more than 150 years. The Wheatstone bridge is still the basic approach for quantifying very small changes in resistance that are characteristic of these sensing elements.
The traditional approach of four resistors arranged in a diamond formation is able to measure very small changes in resistance by looking at resistance differences. Explaining the physics of the process is a bit beyond this forum, particularly when others have done a much better job. Recently Dataforth published a six-page application note on the topic Basic Bridge Circuits. This goes well beyond basic high-school physics discussions and explains uses in industrial applications.
–Peter Welander, process industries editor
Posted by Ask Control Engineering on February 27, 2010