Abstract
To account for the experimental characteristic of the Platinum electric resistance thermometer that it does not follow a first order system in usual meteorological conditions, the indicial response of a model for the electric resistance thermometer is obtained by analytically solving its thermal diffusion equation. The model is so simplified as to be composed of three infinite concentric cylinders, i.e., protector sheath, filling material and electric resistance element. The theoretical indicial response is evaluated for various model thermometers with different properties in the filling material. The major results are as follows: (1) over a certain Fourier number the indicial response can be approximated with one term of the exponential function C exp (-t/λ) including two parameters, coefficient C (>1) and exponent λ; (2) when the Biot number is small, the response speed hardly depends upon the inner structure. It becomes, however, sensitive to the thickness and thermal properties of the filling material as the Biot number increases; (3) the thermal properties of protector sheath and electric resistance element have little effect on the response speed, so long as the filling material is thermally insulating.
