Needle Probe Method According to ASTM D5334-22a

The thermal conductivity of the sample is calculated from the temperature rise with time registered by the probe. As the equation describing the temperature response of the line source cannot be solved explicitly, numerical inversion techniques and / or approximations must be used to evaluate the measuring data (ASTM D5334-22a section 10). Chosing an evaluation method is a compromise: simple methods are easier to implement, mathematically complex ones are more accurate.

The most common approximation method calculates thermal conductivity from the slope of a straight line fitted to the measuring data. An interlaboratory study conducted by ASTM International indicated an accuracy of between ± 10% and ± 15% for this method (NISTIR-89/3908, 1989).

Mathematically more complex approximations describe the form of the temperature curve more closely than a straight line. They yield more accurate results, and they can be used to detect disturbances in the data by comparing the measured temperature curve to the theoretical curve form. This improves reliability significantly, because thermal conductivity measurements are very sensitive to sample preparation problems and instable measuring conditions.

For TK04 we developed an evaluation method which combines a more complex approximation formula with techniques for detecting external disturbances, resulting in an accuracy of ± 2%.}

As hard or brittle sample materials are often difficult to prepare for inserting a conventional needle probe, our modified method uses a needle embedded in the underside of a disk-shaped probe body which is just placed on top of the sample surface (probe for plane surfaces, ISRM Suggested Method DOI 10.1007/s00603-016-1070-5 for determining thermal conductivity of rocks). TK04 can use both methods by simply changing the connected probe.

When using the modified method, part of the heat generated during measuring penetrates into the probe body instead of the sample material. A correction method developed by TeKa uses the thermal parameters of probe and sample to automatically determine the effective amount of heat entering into the sample material. This ensures consistent results for the conventional and the modified needle probe method in the complete measuring range.

The probes are calibrated once by the manufacturer before delivery. The method then yields absolute thermal conductivity values without requiring any time-consuming reference or comparison measurements. TK04 comes with a reference material for checking the operating condition of instrument and probes from time to time.