Question

Why thermistor resistance, thermocouple voltage, platinum resistance are directly proportional to temperature?

Answer 1

A thermocouple is defined as an electrical device which consists of two dissimilar electrical conductors forming electrical junctions at different temperatures. A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect, and this voltage can be interpreted to measure temperature.

The electrical conductivity of a metal depends on the mobility of the conduction electrons. If the voltage is applied to the ends of a metal wire the electrons move towards the positive poles. Due to this movement ther will be faults in the crystal lattice. They include foreign or missing lattice atoms, grain boundaries, and atoms on interlattice positions. Since these fault positions are independent of temperature they produce a constant resistance. With rising temperature the atoms of the metal lattice exhibit increasing oscillations about their rest positions and thereby impede the movement of the conduction electrons. Since this oscillation increases linearly with temperature, the resistance increase caused by it depends as a first approximation directly on the temperature.

In industrial measurement the resistance material platinum has found general acceptance. Its advantages include chemical stability, comparatively easy fabrication the possibility of obtaining it in highly pure form, and electrical properties which can be reproduced. These characteristics have made the platinum resistance sensor the most universally interchangeable temperature sensor.

A thermistor is a type of resistor whose resistance depends on temperature.

Hence the thermistor resistance, thermocouple voltage, platinum resistance are directly proportional to the temperature.