The theory of the transient hot-wire method for measuring thermal conductivity

The paper contains a complete, modernized theory of the transient hot-wire method for measuring the thermal conductivity of fluids which can be employed in the form of an absolute instrument and which can be operated with a precision of 0.02% and an accuracy of 0.2%. It is a companion paper for ref. 1. The analysis demonstrates that the instrument can be designed to imitate very closely the behaviour of a finite portion of an infinite line source of constant heat flux, q, which transfers the heat radially into an infinite fluid. Expressions for the corrections are obtained by a general perturbation method which allows us to examine them one or several at a time. The principal corrections discussed in the form of nine subproblems are: finite inner cylinder, composite cylinders, Knudsen effects, radiation, outer cell circumference, compressibility and natural convection, finite cell dimensions, variable fluid properties and heating over a finite length. The last section summarizes the most important corrections for a reader who is interested in using them rather than in following the analysis itself. The main text supplies all data required by the designer of an instrument of this type.