Strain in Standard Platinum Resistance Thermometers

When an SPRT is strained, the platinum is being stretched and thinned (like a strain gauge) and the resistance increases. By heating the thermometer (annealing) and then cooling it in a controlled way, the crystal lattice of the platinum returns to its original condition and the resistance of the thermometer reduces. Annealing should be seen as part of the maintenance of a thermometer.