Question

State the operational ranges and limitations of platinum resistance thermometer and Fe-Constantan thermocouple.

Answer 1

Depending on the model and propose of usage Standard Platinum Resistance Thermometers (SPRTs) can be used in a wide temperature range (−200 °C to 1000 °C in the laboratory environment however good precision is preserved at the temperatures around 50K (-223 °C) ). From the low temperature side of the temperature range they are limited by the fact that at this temperatures the resistivity of a metal is mainly generated by impurities and is temperature independent. In the high temperature region the sensors are limited by their design: platinum or the other materials used in the sensor (for instance the shielding of a sensor, used to establish the temperature contact between platinum wire and the surroundings) will eventually melt.

Fe-Constantan thermocouples (type J) are used to measure temperatures in a more restricted temperature range  (−40 °C to 750 °C, however this range can be increased with the use of high precision voltmeters). At high temperatures the use of a device is restricted due to the degradation of the materials. Usage of the termocouple by itself (without a termocouple of a different type) is also problematic as the user requires a precise and stable temperature reference point (or a sensor to know the current temperature) to join with the cold end of the termocouple. The best thing about the couples of this type is the price.