Reducing Contact Resistance Errors in the Measurement of Thermal Conductivity with the Single Needle Heat Pulse Technique.

The single needle heat pulse (SNHP) technique has gained widespread popularity for measuring thermal conductivity and its inverse, thermal resistivity, in soil and other porous materials.  Significant error can be introduced into SNHP measurements if the needle probe is not well coupled thermally to the sample material; a condition known as contact resistance.  This condition is most prevalent in dry soil samples and samples with grain sizes that are large compared to the diameter of the needle probe.  Since SNHP sensor accuracy is often verified in agar-stabilized water or viscous liquid samples (e.g. glycerol) which are not subject to contact resistance, the effects of contact resistance are often not observed and therefore not accounted for in the measurement.  We have recently re-visited the problem of contact resistance and have developed several strategies for minimizing its effects on measurement accuracy that have been implemented into our commercial thermal properties instrumentation. These strategies and their effects on measurement accuracy will be discussed in this paper.