The RTD is a passive sensor, it requires the passage of a measurement current to produce a useful signal. This current heats the element and raises it’s temperature. Errors will result if the sensor does not absorb the additional heat.
Self Heating is expressed in mW/°C, which is the power in milliwatts (1000.RI2) which raises the internal temperatureof the sensor by 1°C. The higher the mW/°C, the less the phenomen is important. For example, suppose a measurement current of 5 mA in a PT100 sensor in a 100°C ambient temperature. The specifications indicate 50mW/°C in water displaces at 1m/sec. The quantity of heat produced is:
1000 mW * (0,005 A) * (18,5) = 3,5 mW; the self heating error is (3,5 mW) / (50 mW/°C) = 0.07°C.
Modern measuring practices use very low currents, 100µA and sometimes lower. This practice, if used in the above example, would give an elevated temperature error of only (0.00138 mW)/(50 mW/°C) = 0,000027°C, which is negligable.
The resulting error is inversely proportional to the capacity of the sensor to evacuate the additional heat. This depends on materials, construction, and environment of the sensor. The worst case for this product is there is a high resistance value in a small bodied construction. RTD film, with a small surface area to absorb the heat, is an example. Self heating depends equally on where the sensor is immersed. The error in non-moving air can be 100 higher than that of running water.