ASTM D1169 for specific resistance (resistivity) of insulating liquids
ASTM D1169 standard test method for specific resistance (resistivity) of electrical insulating liquids
PROCEDURE FOR MAKING REFEREE TESTS
17. Preparation of Test Specimen and Filling Test Cell
17.1 Use only a three-terminal cell for these tests.
17.2 When insulating liquids are heated to elevated temperatures, some of their characteristics undergo a change with time, and the change, even though of the minutest nature, may be reflected in the resistivity results. It is therefore desired that the elapsed time necessary for the test specimen to attain temperature equilibrium with the test cell be held to a minimum. For optimum procedure do not exceed 20 min for this time. It is essential therefore that the procedure outlined in 17.4-17.7 be closely followed.
17.3 When the resistivity test is to be made subsequent to the ac loss characteristic measurement, it follows that the precautions to be observed in preparation of the test specimen and filling the test cell have already been met. In the event, however, that the resistivity test is to be made without making the a-c loss characteristic measurement, use the procedure for preparation of the test specimen and filling the test cell outlined in 17.4-17.7.
17.4 In order that representative test specimens may be obtained, gently tilt or invert the sample container and swirl the fluid several times. Immediately after mixing the sample, pour a quantity of fluid sufficient for four fillings of the test cell into a chemically clean, dry beaker and heat on a hot plate to a temperature 2° below the desired test temperature. During the heating period, stir the fluid frequently.
17.5 Remove the cell from the test oven, lift out the inner electrode, but do not rest it on any surface, and fill the cell with a portion of the heated sample. Replace the beaker with the remainder of the heated sample on the hot plate. Insert the inner electrode and rinse the electrodes by twice raising and lowering the inner electrode. Remove the inner electrode and hold it suspended in air; then decant the rinsing fluid and immediately fill the cell from the remainder of the heated sample. Replace the inner electrode.
17.6 Insert a mercury thermometer (see Warning), graduated in 0.25°C increments, in the thermometer well provided in the inner electrode. Immediately return the filled cell to the test chamber (adjusted to a temperature above the desired test temperature) and make the necessary electrical connections to the cell. (Warning - A spring-loaded thermocouple may be used for measuring the temperature of the inner electrode, but extreme caution shall be exercised that these wires do not come in contact with the voltage supply lead and do not pick up stray emf's.)
17.7 Perform the operations described in 17.5 and 17.6 as rapidly as possible.
NOTE 5 - After much experimenting, the above technique has been evolved to give the most reproducible results for tests at 100°C. Little experience has been obtained from tests at higher temperatures. However, individual laboratories engaged in work at temperatures above 100°C have probably developed their own technique.
18. Procedure
18.1 Make the resistivity measurements while the temperature of the test specimen or inner electrode (depending on cell design) is within more or less 0.5°C of the desired test temperature. If a power factor test has been made on the test specimen, short-circuit the cell electrodes for 1 min, then start the resistivity measurements immediately thereafter.
18.2 In making the initial measurement, apply "direct polarity" of the potential to the test specimen and at the end of 1 min of electrification, record the current and voltage measurements. Short-circuit the test cell electrodes for a period of 5 min. Calculate the resistivity (see Section 27).
18.3 At the end of the 5-min period, remove the short-circuit from the electrodes, then apply "reverse polarity" of the potential to the test specimen. At the end of 1 min record the current and voltage measurements and calculate the resistivity. Average the resistivity values obtained from the "direct" and "reverse" polarity measurements.
18.4 Pour off the liquid in the test cell and without rinsing again fill the test cell with a second test specimen from the heated sample and take another set of measurements as outlined in 18.2 and 18.3.
18.5 If the difference in the resistivity values of the two test specimens is within 20 % of the higher of the two values, no further tests are required and report the average of the two test specimens as the resistivity of the sample. If the difference in the values of the two test specimens is not within 20 %, test a third specimen. If the difference in the values between any two of the three test specimens is not within 20 %, clean the test cell and test specimens from another sample until the difference in the resistivity of two test specimens is within 20 %.
18.6 When the dissipation factor or power factor measurements on test specimens from a sample are not within the limits specified in Test Method D924, do not report the results of resistivity measurements on these test specimens (if made) and repeat the test when a fresh sample is obtained.
