ASTM D748-00 pdf free.Standard Specification for Natural Block Mica and Mica Films Suitable for Use in Fixed Mica-Dielectric Capacitors.
X1.1 General- -This Appendix contains a description of a rapid, direct-reading, resonant-circuit method for meas uring dissipation factor or Q value of sheet insulating materials perpendicular to laminations at a frequency of 1 MHz. It has been found equally suitable for such purpose over a frequency range from 50 kHz up to 100 MHz. It is not intended as a precision method. Its chief virtues are the speed and simplicity with which a useful dissipation factor or Q value indication can be obtained. No special preparation of the test specimen is necessarily (other than conditioning, prior to or during testing, where necessary), such as the application of tin-foil electrodes. Likewise, no calculations are necessary. X1.2 Theory of Operation- -This method makes use of a comparison between the max imum resonant voltage measured across a low-loss capacitor in parallel with an inductance and the maximum resonant voltage across the same circuit when a piece of block mica is introduced in series with the low-loss capacitor, To accomplish this, resonance is re-established each time a test specimen is placed in series with the low-loss capacitor by adding capacitance to duplicate the initial capacitance of the resonant circuit, by means of a variable vemier air capacitor in parallel with the coil. This provides a test method requiring but a single dial adjustment and this, combined with a direct-reading meter, greatly facilitates rapid testing. Greater
testing speed can be realized by adapting this method to full automatic means. X1.3 Calibration- -It is possible to calibrate the indicating scale of the vacum-tube voltmeter directly in terms of dissipation factor or Q value by using test specimens of block mica of known dissipation factor.5 The resulting scale then can be used to give a direct indication of dissipation factor or Q value over a useful range. As the meter indication is a function of both the dissipation factor and the thickness of the test specimen, the latter also shall be taken into consideration in the calibration. It has been found that if the inductance coil used in such a test circuit has a Q value of approximately 300, the Q range will extend ftom somewhere in the vicinity of 10,000 at full scale to about 50 at quarter scale of the indicating meter. It further has been found that using inductances with Q’s of 300 or more, the calibration is fairly independent of frequency and a frequency range fom 50 kHz to 50 MHz can be covered by merely changing the inductances and the oscillator test frequency.ASTM D748 pdf download.