1. Design of Cell - A cell for the purpose of measuring the loss characteristic and relative permittivity (dielectric constant) of electrical insulating liquids shall meet the following general requirements:
2. The design of the cell shall be such as to facilitate easy and thorough cleaning of its component parts, permit the use of the cell in a suitable temperature bath, and provide means for measuring the temperature of the liquid under test.
3. The materials used in constructing the cell shall be nonporous and capable of satisfactorily with standing the temperature to which the cell will be subjected under test. The alignment of the electrodes shall not be influenced by this temperature nor by the operation of filling the cell with the test liquid.
4. The electrodes or their surfaces shall be made of a metal capable of resisting attack by mild acids such as are found in oils of petroleum origin, particularly after prolonged exposure at elevated temperatures. Metals that have been found satisfactory from this standpoint are gold, nickel, Monel, platinum, and stainless steel. In general, plated surfaces have been found less satisfactory than solid metal electrodes when testing liquids having low loss. Plated surfaces that may be satisfactory for testing liquids having low acidity and for short periods of time are gold, platinum, nickel, chromium over nickel, or rhodium.
5. In order to achieve a high order accuracy in determining the dissipation factor (or power factor) or dielectric constant, a guard electrode shall be provided which adequately shields the measuring electrode. Use a shielded wire or coaxial cable fo connecting the guard and measuring electrodes to the bridge.
6. The solid insulation used to support the guard electrode relative to the measuring electrode shall not extend into the portion of the samplebeing tested.
7. The insulating materials used in constructing the cell shall not absorb or be adversely affected by the testl iquids or cleaning solvents. The dissipation factor of these insulating materials must necessarily below, particularly that between the guard and measuring electrodes; otherwise difficulty may be experienced in balancing the guard circuits at elevated temperatures. Insulating materials that have proven satisfactory are boron nitride, borosilicate glass, quartz, steatite, and TFE-fluorocarbon. Thermoplastic materials such as hard rubber and polystyrene, although having good electrical properties, are not suitable as they soften below 130°C. Insulating materials of the molded mica-dust type, have been found to absorb solvents and therefore are not considered as satisfactory in the measurement of low loss liquids.
8. In designing the cell, the distance across the surface of the test specimen and across the solid insulating material between the guard and the measuring electrode shall be great enough adequately to withstand the test potential used. Leakage across these paths has been found to produce an unsteady bridge balance on some bridges.
9. Fig. A1.1 The surface area of the measuring electrode and the gap spacing between the measuring electrode and high-voltage electrode shall be such that the ratio of surface area to thickness of the test sample shall be large enough to provide sufficient current for adequate operation of the measuring equipment. No minimum capacitance is therefore suggested. It is required, however, that the precision of the measurements meets the intended accuracy of this test method.
10. Fig. A1.1 - The cell shown in Fig. A1.1 is a two-terminal cell intended solely for making routine tests. The concentric cylinders are of nickel-plated brass which are mounted on a TFE-fluorocarbon base and contained in an 800-mL borosilicate glass beaker. The volume of oil required when using this cell is 500mL. The cell is suitable for use at voltages up to 5kV and temperatures up to 150°C.
