The structure of Pt100 and thermocouple sensors has characteristics that in itself can lead to inaccurate measurements. This applies regardless of brand and type. One of these often neglected sources of error is the isolation in the sensor, which, if too low, can significantly degrade the measuring result. The isolation can be impaired by heat, contamination, vibration, physical or chemical influences or radioactive influences.
Most of KROHNE’s temperature transmitters are microprocessor-based and offer several measurements and control functions beyond the standard measurements. One of these transmitter functions is to monitor the isolation resistance of the temperature sensor and the sensor leads, known as SmartSense. SmartSense offers the possibility of monitoring a Pt100 (3-wire connected) or thermocouple sensor with a low isolation resistance. SmartSense not only monitors the sensor but also the conductors from the sensor terminals to the transmitter terminals. This gives full control over the condition of the measuring chain from the measuring point to the transmitter
To accomplish the monitoring, the sensor must be equipped with an extra conductor. This conductor will have a separate terminal and go through the sensor all the way to the sensor element. When the isolation resistance is too low the output signal will go to a pre-programmed value.
For a Pt100 the detection limit for “low isolation” is adjustable between 50 kΩ and 500 kΩ. The error due to the isolation value RISO has to be added to other measurement errors. At 400 °C/752 °F the added error is 0.4 °C/0.7 °F for 500 kΩ and 3.1°C/5.6°F for 50 kΩ isolation
For thermocouples the detection limit for “low isolation” can be adjusted between 20 kΩ and 200 kΩ. The added error depends on the relation between the lead resistance and the isolation resistance. The error is also dependent on the temperature difference between the measuring point and the location of the low isolation. Under the following circumstances: measuring temperature 1000 °C/1832 °F, ambient temperature 25 °C/77 °F and wire resistance 50Ω, there will be an error of 1% if the isolation resistance in the ambient temperature area is 5 kΩ. This equals 10 °C for type K.
