November 2000
Intermediate to advanced
608 pages
16h 12m
English
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3.1 MEASUREMENT OF RESISTANCE
The general equation for a sensor whose resistance changes by a fraction x in response to a measurand is R = R0 f(x), assuming f (0) = 1. For linear sensors we have
Figure 3.1 The Thévenin equivalent for the circuit seen from the terminals of a resistive sensor permits us to determine that the maximal dissipation is at R = Ro.
The range of values for x depends strongly on the type of sensor and on the measurand span. For linear sensors, x varies from 0 to –1 for linear potentiometers and can be up to 10 for conductive polymers and as small as 10–5 to 10–2 for strain gages. RTDs and measuring thermistors have intermediate values for x. The ratio between sensor resistance for extreme measurand values can be higher than 1000 in LDRs and humistors, and is less than 100 in magnetoresistors, gas sensors, and liquid conductivity sensors. Switching PTC thermistors increase their resistance by more than 10,000 for temperatures above the switching temperature.
There are two requirements for all conditioners for resistive sensors. First, they must drive the sensor with an electric voltage or current in order to obtain an output signal, because a change in resistance is not by itself a signal. Second, this supply, whose magnitude affects that of the output signal, ...
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