When measurement instruments reach their specified performance limits, one comes to a gray zone separating reality from fantasy. Thanks to sensors, this zone is widening.
This region is approached in digital multimeters (DMMs) as the quantity being measured approaches zero. One major DMM company forces a zero reading on the display when the measurement is within a certain small band around zero. A similar issue exists in data acquisition (DAQ) systems, which are essentially multichannel DMMs.
The encroaching fantasy is being exacerbated by sensors, particularly thermocouples (TCs), which are conceptually simple sensors. When two dissimilar metals are joined, a voltage appears across their junction that is approximately proportional to temperature. Two wires -- one chromel and one alumel, for instance -- can be spot welded together at the sensing end by process chemists (who like to use TCs) and connected to the DAQ screwdown terminals, shown in orange below.
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