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.

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 practice provides a consistent set of parameters for numerical or analytical heat conduction calculations related to heat transport through rocks.

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

The diffusivity determined by this practice 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 3—Notwithstanding the statements on precision and bias contained in this test method; the precision of this test method 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 D 3740 are generally considered capable of competent and objective testing. Users of this test method are cautioned that compliance with Practice D 3740 does not in itself assure reliable testing. Reliable testing depends on many factors; Practice D 3740 provides a means of evaluating some of these factors.

1.1 This practice 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 practice is 20 to 300°C.

Note 1—The diffusivity, as determined by this practice, is intended to be a volume average value, with the averaging volume being ≥ 2 x 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.

Note 2—This practice is closely linked to the overall test procedure used in obtaining the primary data on density, specific heat, and conductivity. It cannot be used as a "stand alone" practice because the thermal diffusivity values calculated by this practice are dependent on the nature of the primary data base. The practice furnishes general guidelines but cannot be considered to be all-inclusive.

1.2 The practice 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 practice must be associated with the same direction as that utilized in the conductivity measurement.

1.3 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.

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

1.4.1 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.

1.4.2 Variable Porosity—The thermal properties of porous rock 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.

1.5 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D 6026.

1.6 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.