Why use isolated transmitters for non-grounded RTD applications?
This question is often being raised by users of RTD sensors. Since Pt-100 thermal elements are not usually grounded, it is often assumed that they do not require isolated transmitters for proper operation. If the measurement environments were ideal, indeed this assumption could be, at least, partially correct.
Unfortunately, industrial environments are often ridden with various types of airborne contaminants in solid, liquid and gaseous forms. These may precipitate and settle inside, and around the instruments' and the sensors' terminals. Add just a little bit of humidity and you have created several potential parasitic leakage current paths, which could seriously effect the device measurement accuracy as well as the signal integrity.
Consider the following schematic diagram:
Diagram 1: A most prominent parasitic current path could occur between the 24VDC power supply line and the sensor's terminal or wiring. A parasitic resistance value of 20MΩcould cause ~1μA error in the excitation current. Assuming a standard 1mA excitation current, for a common input range of 0-100°C (273°K-373°K) this will result in a ~0.26°C zero shift error (~0.26%) and ~0.1°C gain error (totaling 0.36%). A 10MΩparasitic resistance will double the error to ~0.52°C and ~0.2°C gain error (totaling 0.72%).
Needless to say, the lower the parasitic resistance the higher the error. A 1MΩresistance for instance, not entirely uncommon in tropical and non-clean environments, will cause as much as ~5.2°C zero error and a total of up to 7.2°C error at full scale. A smaller input range will cause a proportionally larger % output error, naturally.
Parasitic resistance paths may also be caused as a result of metal migration internal to the sensor structure, but these are not related to environmental conditions and are more common in sensors used at elevated temperatures.
Diagram 2: Isolated transmitters break the path of the parasitic resistance and prevent a leakage current from flowing through the transmitter's circuitry, hence avoiding the errors almost entirely.
Isolated transmitters in general also provide for a far superior common noise rejection as well as far superior protection from electrical transients and surges in electrically noisy environment or during weather extremes.
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