Introduction to RTDs
The most common material used is platinum because
it has a very predictable positive temperature coefficient over a wide temperature range and is very inert to many hostile environments.
Because the resistance of an RTD is a function of temperature, the specified resistance in a data sheet is always that at 0 C. Thus, a standard 100 ohm (by far the most common) RTD has a resistance of 100.00 ohms at 0 C. There are several other standard values that range from around 10 ohms to 10,000 ohms.
Table 1 shows the temperature coefficient for platinum in general and the resistance of a 100 ohm RTD as a function of temperature. Note that the temperature coefficient
decreases with temperature thus the RTD has a non-linear response.
Temperature 100 Ω RTD
Temperature Coefficient Resistance
-100 C 4100 ppm/C 60.26 ohms
0 3850 100.00
100 3800 138.50
200 3700 175.84
300 3600 212.02
400 3500 247.04
500 3400 280.98
600 3200 313.71
700 3100 345.28
800 3000 375.70
Measurement Concept
The basic concept is to apply a constant current to the RTD and the resulting voltage across it is directly proportional to the resistance. Then the offset voltage (because of the resistance at 0 C) is subtracted and gain is applied to scale the voltage to a common value of 10 millivolts per degree C. Other commonly used scale factors are 5 and 2 millivolts per degree C which are often used if very high temperatures are going to be measured and instrumented with a low voltage A/D converter.
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