ASTM D2149 covers the determination of the (a) Dielectric Constant, a measure of the amount of potential energy stored due to polarization, and (b) Dissipation Factor, a measure of energy lost during reversal of polarization.
As Dielectric Constant and Dissipation Factor are sensitive to changes in chemical composition, impurities, and homogeneity, therefore, measurement of Dielectric Constant and Dissipation Factor is useful for ensuring the quality of dielectrics and for determining the effect of environmental factors such as heat, moisture, and radiation on solid dielectrics.
There are two procedures mentioned in ASTM D2149 for the determination of Dielectric Constant and Dissipation Factor:
Test Procedure A
The sample is placed between the electrodes and the drive nut is rotated until the friction is felt to suddenly decrease. The micrometer setting is checked against the setting at which the friction first increases due to increase in the electrode spacing balance. Electrodes are opened and the sample is removed. Without changing the capacitance setting, the balance of the measuring apparatus is restored by reducing the spacing between the electrodes and adjusting the measuring circuit to balance the loss component. The new dissipation factor and micrometer setting are recorded. The capacitance and dissipation factor is determined for different temperature and frequency values
Procedure B is used in ASTM D2149 testing, when the frequency can be kept constant and when the measuring circuit is stable. For each required frequency, change in capacitance, ∆C, is determined at room temperature. Then the specimen is clamped between the electrodes and the temperature is changed to the first temperature while the measurements at each required frequency are recorded to determine the change in capacitance of the specimen.
For ASTM D2149, the samples should be disk shaped, free of bubbles and other defects. They should have a diameter of 40.00001 mm and a thickness of 2 to 3 mm. They should have finished surfaces of 1.8 pm or better and parallel surfaces within 0.01 mm.
For the two procedures, ASTM D2149 testing data is calculated as follows:
Procedure A: The capacitance Cs and dissipation factor Ds of the specimen as follows:
C s = C0– Ct + CV
CS = ΔC + CV
DS=Ct/CS (Di – DV)
Procedure B: The capacitance ( Cs) of the specimen.
C s= ΔC +CRT – CT + CV
ASTM D2149 is used to determine the Dielectric Constant and Dissipation Factor of solid dielectrics within frequency 50 Hz to 10 MHz and over a range of temperatures from 80 to 500 °C.