Accuracy of ASTM D5470 Thermal Impedance Measurements
The fundamental accuracy of measurements performed using ASTM 5470 is influenced by five factors which are rooted in the the physics of the method and not associated with any particular engineering approach to performing the method. These factors are described below.
Temperature difference measurement: The measurement of the temperature difference across the test surfaces that clamp the sample is associated with an absolute measurement inaccuracy. The smaller the temperature difference, the greater the influence of this inaccuracy on the final accuracy-tolerance of the thermal impedance reported. Thus, for very lower thermal impedance samples, the impedance accuracy will be dominated by the temperature-difference-inaccuracy.
Heat flow measurement: The measurement of the true heat flow between the hot and cold test surfaces is subject to measurement inaccuracy primarily due to heat leakage. Regardless of how carefully heat leakage is handled, the lower the heat flow, the more the inaccuracy of the true heat flow through the sample will contribute to the final accuracy of the thermal impedance reported. Thus, the higher the thermal impedance of the sample, the more the impedance accuracy will be influenced by the heat flow inaccuracy.
Sample-thickness measurement: The measurement of the sample thickness as tested is vitally important to the overall scope of ASTM 5470. Although the sample thickness is not specifically required for the computation of apparent thermal conductivity, the thickness of the sample is key to utilizing the apparent thermal conductivity. Thickness measurement accuracy is generally dominated by an absolute inaccuracy that becomes increasingly dominant as the sample thickness becomes smaller. Generally, a stable environment temperature is conducive to lower inaccuracy tolerances.
Sample-area measurement: The accuracy with which the area of the sample is measured and laterally aligned with the test surfaces contributes to the overall accuracy of the measured thermal impedance. Type I materials present the greatest inaccuracy in this regard since there is the possibility of entrained bubbles as well as some difficulty in accurately controlling the perimeter of the test sample. The use of surface contact treatments can also contribute to the area-inaccuracy due the creation of edge-fillets comprised of the contact oil or grease.
Sample-impedance variation with time: Since many types of samples have contact resistances that vary with time and pressure, the accuracy of the measurement can be inherently tied to this phenomena. Although it is not strictly an accuracy issue the traditional sense, it can certainly make test data appear to be variable and either inaccurate or unrepeatable unless it is recognized and specifically addressed.
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