5.1 The thermal diffusivity is a parameter that arises in the solution of transient heat conduction problems. It generally characterizes the rate at which a heat pulse will diffuse through a solid material.

5.2 The number of parameters required for solution of a transient heat conduction problem depends on both the geometry and imposed boundary conditions. In a few special cases, only the thermal diffusivity of the material is required. In most cases, separate values of k, ρ, and c_p are required in addition to α. This test method provides a consistent set of parameters for numerical or analytical heat conduction calculations related to heat transport through rocks.

5.3 In order to use this test method for determination of the thermal diffusivity, the parameters (k, ρ, c_p) must be determined under as near identical specimen conditions as possible.

5.4 The diffusivity determined by this test method can only be used to analyze heat transport in rock under thermal conditions identical to those existing for the k, ρ, and c_p measurements.

Note 2: The quality of the result produced by this standard is dependent on the competence of the personnel performing it, and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing/sampling/inspection/etc. Users of this standard are cautioned that compliance with Practice D3740 does not in itself assure reliable results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.

1.1 This test method involves calculation of the thermal diffusivity from measured values of the mass density, thermal conductivity, and specific heat at constant pressure. It is applicable for any materials where these data can be determined. The temperature range covered by this test method is 293 to 573 K. This test method is closely linked to the overall test procedure used in obtaining the primary data on density, specific heat, and thermal conductivity. It cannot be used as a “stand alone” test method because the thermal diffusivity values calculated by this test method are dependent on the nature of the primary data base. The test method furnishes general guidelines to calculate the thermal diffusivity but cannot be considered to be all-inclusive to capture issues related to the density, specific heat, and thermal conductivity

Note 1: The diffusivity, as determined by this test method, is intended to be a volume average value, with the averaging volume being ≥ 2 × 10⁻⁵ m³ (20 cm³). This requirement necessitates the use of specimens with volumes greater than the minimum averaging volume and precludes use of flash methods of measuring thermal diffusivity, such as the laser pulse technique.

1.2 The values stated in SI units are to be regarded as the standard. No other units of measurements are included in this standard.

1.3 This test method is intended to apply to isotropic samples; that is, samples in which the thermal transport properties do not depend on the direction of heat flow. If the thermal conductivity depends on the direction of heat flow, then the diffusivity derived by this test method must be associated with the same direction as that utilized in the conductivity measurement.

1.4 The thermal conductivity, specific heat, and mass density measurements must be made with specimens that are as near identical in composition and water content as possible.

1.5 The generally inhomogeneous nature of geologic formations precludes the unique specification of a thermal diffusivity characterizing an entire rock formation or soil layer. Geologic media are highly variable in character, and it is impossible to specify a test method for diffusivity determination that will be suitable for all possible cases. Some of the most important limitations arise from the following factors:

1.5.1 Variable Mineralogy—If the mineralogy of the formation under study is highly variable over distances on the same order as the size of the sample from which the conductivity, specific heat, and density specimens are cut, then the calculated diffusivity for a given set of specimens will be dependent on the precise locations from which these specimens were obtained.

1.5.2 Variable Porosity—The thermal properties of porous rock or soil are highly dependent on the amount and nature of the porosity. A spatially varying porosity introduces problems of a nature similar to those encountered with a spatially varying composition. In addition, the character of the porosity may preclude complete dehydration by oven drying.

1.6 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D6026.

1.6.1 The procedure used to specify how data are collected/recorded or calculated in this standard are regarded as the industry standard. In addition, they are representative of the significant digits that generally should be retained. The procedures used do not consider material variation, purpose for obtaining the data, special purpose studies, or any considerations for the user’s objectives; and it is common practice to increase or reduce significant digits of reported data to be commensurate with these considerations. It is beyond the scope of this standard to consider significant digits used in analytical methods for engineering design.

1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.