This test method is applicable for both intact and reconstituted soil specimens and soft rock specimens. This test method is suitable only for homogeneous materials. No other units of measurements are included in this 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.
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USA info metergroup. The KD2 Pro measures thermal conductivity, resistivity, and diffusivity of samples using a single heated needle. The needle contains a heater and a temperature sensor. The temperature of the needle is monitored as heat is applied. The thermal properties of the sample are found using a model for the thermal behavior of a line heat source.
The model assumes good thermal contact between the probe and the sample under test. Liquid samples have excellent thermal contact with the probe. Thermal property measurements on these samples, as well as on granular materials with high water content, are accurate.
In dry granular materials, especially materials with large-grain size, the minute contact points between the probe and the sample under test give rise to a contact resistance which impedes the flow of heat away from the probe. Contact resistance is not included in the line heat source model and can therefore contribute to errors in measuring thermal properties of these materials. The purpose here is to assess the magnitude of errors in thermal resistivity resulting from contact resistance in dry granular materials and to test the use of thermal grease as recommended in ASTM D for reducing these errors.
Characteristics of the materials for test are given in Table 1. The actual thermal resistivity of these materials was determined by measuring the heat flow and temperature difference in a radial, steady-state heat-flow cell.
The apparatus consisted of a 25 cm-long heater 1 cm in diameter and a 3 cm i. The heater was made by wrapping A 40 chromel-constantan thermocouple was attached to the center of the heater and another to the outside of the copper tube at the same height. The space between the heater and the copper tube was packed with the test material.
The bulk density of the test material was determined by dividing the total mass of the material by volume between the heater and the tube. After steady state was obtained, the heat input and temperature difference were measured. Thermal conductivity is computed from. Tests were run for at least three hours, and the results were monitored to be sure that the system was at steady state. Results of these measurements are given in Table 1.
We tried two types of grease: Thermal Cote, a white thermal grease with a conductivity of 0. Measurements were made with a KD2 with and without grease. For those measurements where grease was used, a thin coat of grease was applied to the entire needle. Thermal conductivity tests were performed in two ways.
For one, a layer of test material was packed into the sample container, the heated needle was placed on top of the test material, and an additional layer of material was packed on top of the needle.
For other measurements, the coated needle was inserted into the already-compacted material. Materials tested were mesh quartz sand and four sizes of glass beads. Values for the measurements are shown in Table 1. The thermal resistivity values obtained with the KD2 improved with increasing moisture content and decreasing particle size when compared with the values obtained using the steady state apparatus. Measurements without grease showed substantial error.
The use of thermally conductive grease to improve the contact between the soil and the probe made it possible to obtain thermal conductivity values that were comparable to, but still slightly below, those obtained with the steady state apparatus.
The results of the tests are shown in Table 1. The agreement between the needle and steady state measurements is likely within the limits of error of the experiment, since it was not possible to duplicate the bulk density for the two measurements, and the steady state analysis assumed one-dimensional radial flow, while the measurement cell lost heat out the ends.
The differences between the silver grease and the white grease were not statistically significant, though the means for the silver grease were generally a little higher. Increasing the particle diameter increased the measurement error in the glass bead samples when no grease was used, but the thermal conductivity of even 6 mm diameter beads was fairly accurate with the thermal grease.
Sample to sample variation not shown increased with the larger bead size. Contact resistance can produce large errors in KD2 measurements of thermal resistivity of granular materials, especially when the particle diameter is large. These errors appear to be largely eliminated by use of a high conductivity thermal grease to coat the probe.
Use of the thermal grease decreases errors, even in these materials, to a few percent. Silver-filled grease gives only slightly lower resistivity values than a thermal joint compound, even though the silver-filled grease has 20 times lower thermal resistivity than the joint compound.
For over four decades, scientists and engineers have relied on our analyzer for measuring thermal properties in almost anything—and we mean anything. You need your data to be accurate and reliable.
In this webinar, soil moisture expert Chris Chambers discusses 6 common ways people unknowingly compromise their data and important best practices for higher-quality data. Register now. Language English. Contact USA info metergroup. Conclusions Contact resistance can produce large errors in KD2 measurements of thermal resistivity of granular materials, especially when the particle diameter is large.
KD2 values shown are means of three or four measurements. Accurate readings at top speed For over four decades, scientists and engineers have relied on our analyzer for measuring thermal properties in almost anything—and we mean anything.
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Historical Version s - view previous versions of standard. Work Item s - proposed revisions of this standard. More D Agencies that meet the criteria of Practice D are generally considered capable of competent and objective testing.
How to reduce contact resistance errors in thermal properties measurements
Active view current version of standard. Other Historical Standards. Work Item s - proposed revisions of this standard. More D This test method is applicable for both undisturbed and remolded soil specimens and soft rock specimens.