ASTM D1169 for specific resistance (resistivity) of insulating liquids
ASTM D1169 standard test method for specific resistance (resistivity) of electrical insulating liquids
6. Instrumentation
6.1 Instrumentation listed in Test Methods D257 is suitable, with the exception of the Voltage Rate-of-Change Method. However, in order to obtain the greatest precision when making this test, use the voltage-current method with the following instruments:
6.1.1 Voltmeter, having an accuracy of 2 % or better, operated in the upper one third of its scale range for measuring the voltage supply.

6.1.2 Current-Measuring Device - Any type of instrument having adequate sensitivity and precision and with a suitable range for measurement of the wide spread of currents encountered when making this test on new or used liquids will be satisfactory. For currents greater than 10(-9) A an Ayrton shunt and galvanometer, an appropriate electrometer or picoammeter having a sensitivity of 50 pA (50 x 10(-12) A) per division has been found convenient and satisfactory. The galvanometer deflection shall be not less than 20 divisions for the applicable Ayrton shunt ratio. For currents less than 10(-9) A an electronic picoammeter has been found suitable. In using this instrument the multiplier selected shall be such as to give at least one-half full-scale deflection on the indicating instrument.

6.1.3 Time-Measuring Device, accurate to 0.5 s, for measuring the time of electrification.

6.1.4 Batteries or other stable direct-voltage supplies are recommended for the steady voltage source.

NOTE 1 - Rectified high-frequency power supplies cannot be used because the high frequency ripple in these supplies can cause the ac component of current to equal or exceed the dc current being measured. The ac component of current is equal to 2 p times the product of the ripple voltage, the ripple frequency, and the capacitance of the test cell in farads (where p = 3.14). If the capacitance of the test cell is 100 pF (10(-10)F), the ripple frequency is 100 kHz, and the ripple voltage is 5 mV (0.001 % of a 500 V test voltage), the alternating component of current is 3.14 x 10(-7) amperes. The meter would be unreadable under these conditions.

7. Test Circuit
7.1 A schematic diagram of the test circuit is shown in Fig. 1.

7.2 Construct the circuitry so that leakage is minimal. To this end, mount the transfer switches on polystyrene or TFE-fluorocarbon insulation of sufficient thickness to minimize possible leakage. Make all soldered connections with low-thermal-emf solder using a soldering flux of resin and alcohol.

NOTE 2 - The use of ordinary solder and flux can result in spurious thermal emf's that will cause erroneous indications.

7.3 Completely shield the test circuit. Make connections to the current-measuring instrument with shielded leads. TFE-fluorocarbon-insulated shielded leads are recommended for connecting the high-voltage electrode and measuring electrode of the test cell to the test circuit.

8. Sampling
8.1 Sample liquids for use in this test in accordance with Practices D923. When possible, obtain samples for testing through a closed system. If exposed to atmospheric conditions, take the sample when the relative humidity is 50 % or less. Some liquids, in certain applications, require special handling and processes in the sampling, and these will be found in the governing procedures. Consult such procedures before samples are taken.

8.2 Take a sufficient quantity of sample for this test for at least three separate resistivity determinations.

9. Galvanometer Calibration and Sensitivity
9.1 When a dc galvanometer is used to measure the current, it shall first be calibrated to ensure that it is properly balanced, that is, that the deflections on either side of zero are equal when the galvanometer is energized with "direct" and "reverse" polarities of the test potential.

NOTE 3 - Throughout this test method the terms "direct polarity" and "reverse polarity" are used to indicate when the positive and negative potential leads, respectively, are connected to the outer electrode of the test cell.

9.2 The galvanometer sensitivity, Gs, in amperes per division, is used in computing the resistivity and is derived from the following equation:
Gs = (E/R) x (S/D)
where:
E = test voltage, V,
R = calibrating resistor, Ω,
S = shunt multiplying factor (ratio of galvanometer current to total current), and
D = galvanometer deflection, in divisions.

10. Test Cells
10.1 The design of test cells that conform to the general requirements given in the Annex are considered suitable for use in making these tests.

10.2 A two-electrode cell suitable for making routine tests is shown in Fig. A1.1. A brief description of this cell is given in the Annex.

10.3 Because the configuration of the electrodes of these test cells is such that their effective area and the distance between them are difficult to measure, each test cell constant, K, can be derived from the following equation:
K = 3.6πC = 11.3C
where:
K = test cell constant, cm, and
C = capacitance, pF, of the electrode system with air as the dielectric. (For methods of measuring C, see Test Methods D150).

11. Test Chamber
11.1 When the tests are to be made above room temperature but below 300°C, use a forced-draft, thermostatically controlled oven that conforms to the requirements of Section 17 as the test chamber. For tests at room temperature the unenergized oven can be conveniently used as the test chamber.

11.2 Provide the test chamber with an opening in the wall through which two lengths of TFE-fluorocarbon-insulated shielded cable will pass to make electrical connection from the measuring equipment and voltage source, respectively, to the test cell. Use a perforated ceramic plate or disk to insulate the test cell from the metal flooring of the oven if the flooring is not insulated from the oven.

11.3 Provide a safety interlock on the door of the test chamber so that the electrical circuit supplying voltage to the test cell will be broken when the door is opened.

11.4 A cross-sectional view of the test chamber with a three-electrode test cell in place and with test cables connected is shown in Fig. 1.

12. Test Temperature
12.1 The temperature at which a referee test is made shall be mutually agreed upon between the purchaser and the seller. Resistivity measurements are made at many different temperatures. For acceptance tests, it is generally made at a temperature of 100°C, while for routine testing, it is usually made at room temperature, 85, or 100°C. In some research investigations, tests may be made at considerably higher temperatures, while in other cases, particularly for tests on cable oils in service, tests may be made over a range of temperatures.

13. Test Voltage
13.1 The average electrical stress to which the specimen is subjected shall be not less than 200 V/mm (5 V/mil) nor more than 1200 V/mm (30 V/mil). The upper limit has been set with the purpose of avoiding possible ionization if higher stresses were permitted. For acceptance testing, the stress and time of electrification should be mutually agreed upon by the purchaser and the seller. The time of electrification in general usage is 1 min.

NOTE 4 - The dc volume resistivity of new oil, particularly at room temperature, has been shown to be a function of both electrical stress and electrode spacing. The resistivity has been found to have a maximum value when the applied electrical stress is about 50 V/mil; electrical stresses either below or above this critical value yield lower values of volume resistivity.

14. Conditioning
14.1 Store the sample in its original sealed container and shield it from light. Some liquids, such as oils of petroleum origin, undergo changes when exposed to sunlight. Allow the sealed container to stand undisturbed, in the room in which the test is to be made, for a sufficient period of time to permit the sample to attain room temperature before it is opened.

15. Storing Test Cell
15.1 Clean and dry the test cell, when not in use, in accordance with Section 16. Store it in a dust-free cabinet until it is to be used again, at which time clean and dry as directed by Section 16.

16. Cleaning Test Cell
16.1 The cleanliness of the test cell is of paramount importance when making resistivity measurements because of the inherent susceptibility of most insulating liquids to contaminating influences of the most minute nature. For this reason clean and dry the cell immediately prior to making the test. It is essential that the procedures and precautions outlined in 16.2-16.5 be strictly observed.

16.2 Dismantle the cell completely and wash all the component parts thoroughly with a technical grade of a suitable solvent (such as acetone or pentane). Wash the component parts with a mild abrasive soap or detergent. Take care not to lay the electrodes on any surface. Rinse all parts thoroughly with hot tap water, then with cold tap water, followed by several rinsings with distilled water. Take extreme care during the washing and rinsing of the test cell shown in Fig. 2 to prevent any moisture from entering the thermometer well in the inner electrode. As a precaution against this eventuality, use a suitable stopper to plug this opening prior to starting the cleaning operation.

16.3 After the surfaces of the electrodes and guard have been washed, take care not to touch these surfaces during the rinsing or any subsequent operation.

16.4 Place the component parts of the test cell in an oven maintained at 110°C for a period of not less than 60 min. Do not dry test cells made of Monel at this elevated temperature for more than 90 min as oxidation will take place, causing erroneous results. Take care to place the component parts of the cell on a clean surface of the oven.

16.5 At the expiration of the drying period, assemble the cell in the oven, using clean cotton gloves to protect the hands. Observe the precaution given in 16.3.

16.6 Quickly transfer the assembled test cell to the test chamber maintained at a temperature above the desired test temperature. The exact temperature will depend on both the oven and the cell design. The test chamber shall be such that when the oil, preheated to 2°C below the test temperature, is transferred to the cell, the test temperature is attained and maintained within 20 min.