IEC 61868 Mineral insulating oils - Determination of kinematic viscosity at very low temperatures
7 Procedure
7.1 The specific details of operation vary for the different types of viscometers listed in ISO 3104. In all cases, however, proceed in accordance with 7.2 to 7.4 inclusive.
7.2 Maintain the bath at the test temperature according to the corrected readings of the thermometer within the limits given in 5.3. Locate the thermometer in the bath with the top of the mercury-thallium alloy column below the surface of the bath liquid, and the emergent stem of the thermometer above the cover of the bath.
7.2.1 Ascertain that the ice point of the thermometer has been determined recently and the corrections, if any, applied to the calibration values. For thermometers with an auxiliary ice point scale, a change in the ice point determination is an indication of a need for recalibration.
7.2.2 Select a clear, dry, calibrated viscometer having a range covering the estimated viscosity.
Viscometers used for silicone fluids, fluorocarbons, and other liquids which are difficult to remove by the use of a cleaning agent, shall be used only for the determination of the kinematic viscosity of such liquids, except when calibrating. Such viscometers shall be subjected to calibration checks at frequent intervals.
7.3 Use a sample
of insulating oil which has been filtered to remove particles greater than 2 µm. Charge the viscometer with the oil sample in the manner dictated by the design of the instrument, this operation being in conformity with that employed when the instrument was calibrated.
7.4 Flush each tube and leg of the viscometer (11) with dry nitrogen or dry air (with a dew-point below -50 °C), then charge the viscometer with the oil sample. Connect the two vinyl tubings (12) and (13) to the two openings of the viscometer. Raise the liquid level from tubing (13), with a syringe or with a bulb pipette pump connected to the third port of the valve (14), then close the valve (14) to hold the liquid level about 5 mm below the top starting mark of the viscometer. Fix the viscometer in the bath (5), fit the drying devices (15) and do not let the tubings fall into the methanol. Close the freezer for 20 h. This 20 h soak period is not applied to reach temperature stabilization of the oil sample (which will occur more rapidly), but to allow interlocking of the microcrystalline structure to take place. Soak periods longer than 20 h may be used if deemed necessary to reach the microcrystalline structure. Just before readings are taken, replace the polystyrene foam central false lid panel (4) with the clear acrylic one (3).
NOTE - The numbers in brackets refer to figure 1.
After 20 h, the contraction of the air above the capillary should have drawn the oil from 1 mm to 4 mm above the starting mark. If it has not, repeat with a fresh viscometer and aliquot. If it has, the oil is ready for its first passage through the capillary of the viscometer. It is important that the oil move very little between its cooling and its timed movement.
Measure the time required for the meniscus to pass from the first starting mark to the second mark, in seconds to within 0,2 s, by looking through the mirror, which allows a parallax-free view of the meniscus and thermometer. If this flow time is less than the specified minimum (see 7.2.2), select a viscometer with a capillary of smaller diameter and repeat the operation.
Draw the oil back to the starting mark with the syringe connected to the third port of the valve and allow for a second passage through the viscometer. If the second passage, and the subsequent ones, are similar to the first passage, Newtonian behaviour has been confirmed. Many oils will take over 20 % longer for the first passage than for the others. Note that if the oil is cooled and soaked in the bottom of the viscometer, its microstructure will be damaged when the oil is drawn up to the starting mark. Its non-Newtonian nature will not be measured.
7.5 Cleaning of viscometer
7.5.1 Between successive determinations, clean the viscometer thoroughly by several rinsings with an appropriate solvent completely miscible with the sample (e.g. cyclohexane), followed by a completely volatile solvent (e.g. acetone) then with dionized water. Dry the tube by passing a slow stream of filtered dry air through the viscometer for 2 min or until the last trace of solvent is removed.
7.5.2 In the case of used oils, where sludge or harmful suspended matter may be suspected, periodically clean the instrument with chromic acid to remove organic deposits and stains, rinse thoroughly with distilled water and acetone, and dry with clean dry air. Substitutes for chromic acid such as sulphuric acid type cleaners, can be used to meet regional regulations, provided they remove stains, particulate matter and old oil, but not glass. Inorganic deposits may be removed by hydrochloric acid treatment before the use of cleaning acid, particularly if barium salts are suspected.
Dilute sulphuric, dilute chromic, or any other mineral acid can be successfully applied to the removal of surfactant traces from glass surfaces after such detergent cleaning.
