ASTM D1169 for specific resistance (resistivity) of insulating liquids
ASTM D1169 standard test method for specific resistance (resistivity) of electrical insulating liquids
PERMISSIBLE MODIFICATIONS WHEN MAKING ROUTINE TESTS
19. General
19.1 When it is desired to get an approximation of the resistivity of a test specimen or to make tests on a group of test specimens of the same type of insulating fluid to ascertain whether the resistivity is greater or less than some specified value, certain modifications to Sections 17 and 18 are permitted as outlined in the following sections.
20. Test Cell
20.1 Any test cell conforming to the requirements given in the Annex may be used. For routine tests, however, the requirements for a guard electrode may be waived provided that the two-electrode cell used is of adequate capacitance and with sufficient ratio of surface area to specimen thickness to provide proper operation of the measuring equipment. One such cell is shown in Fig. A1.1.
21. Test Chamber
21.1 An oil bath having uniform temperature distribution and provided with adequate temperature regulation may be used for routine tests, provided that, when the test cell is immersed in the bath, the temperature difference in the test cell between any part of the inner electrode and the outer electrode does not exceed 2°C. A less satisfactory method is the use of a hot plate, but variations in temperature throughout the cell may lead to questionable results. In general, the use of a forced-draft air test chamber is preferable.
22. Test Temperature
22.1 Resistivity measurements may be made when the test specimen is within more or less 2.0°C of the desired temperature.
23. Cleaning Test Cell
23.1 Since some two-electrode cells cannot be dismantled, the cleaning procedure given in Section 16 cannot always be followed. Where this applies, it is imperative that each laboratory evolves a good cleaning procedure for the cell used so that repeatable results can be obtained. Only solvents mentioned in 16.2 are satisfactory as cleaning agents.
23.2 When a number of test specimens of the same type of fluid are to be tested consecutively, the same test cell may be used without cleaning, provided that the resistivity of the test specimen previously tested was greater than the specified value. If the resistivity of the last test specimen tested was less than the specified value, clean the test cell before using for further tests.
24. Procedure
24.1 Because of the difference in construction of the various types of test cells that may be used for routine tests, no detailed procedure for preparation of the test specimen and filling of the test cell is given. Give due regard to the precautions outlined in Section 17.
24.2 Before filling, always rinse the test cell with a portion of the test specimen to be tested.
25. Electrical Connection
25.1 When making electrical connection to a two-electrode cell, the shield on the lead usually connected to the guard electrode (see Fig. 1) shall be securely clipped to the TFE-fluorocarbon insulation in order to prevent contact with any surface.
26. Number of Tests
26.1 Only a single test specimen need be tested in measuring the resistivity of a sample.
27. Calculation
27.1 Calculate the resistivity of a test specimen, ρ, in units of 10(12) ohm-centimetres, by using one of the following equations, depending on whether the galvanometer or picoammeter was connected in the test circuit:
27.1.1 Using the galvanometer for current measurement:
ρ = (E x K)/(D x Gs)
27.1.2 Using the picoammeter for current measurement:
ρ = (E x K)/I
where:
E = test voltage, V
K = cell constant, cm,
D = galvanometer deflection, in divisions,
Gs = galvanometer sensitivity, in picoamperes per division, and
I = current, picoamperes.
