IEC 60247 Insulating liquids - Measurement of relative permittivity, dielectric dissipation factor (tan δ) and d.c. resistivity
6 Cleaning solvent
Solvents for cleaning the test cell shall have a purity at least corresponding to the requirements for technical grades and shall have been shown to have no influence on test results. Solvents shall be stored in brown glass bottles.

If the solvent is delivered in drums, any particulate matter shall be removed by filtration and the filtered solvent stored in properly marked brown glass bottles.

Hydrocarbon solvents such as petroleum spirit (boiling range 60 °C to 80 °C), n-heptane, cyclohexane and toluene are suitable for hydrocarbon oils. Ethanol is recommended for organic esters and toluene for silicone liquids. Other insulating liquids may require special solvents.

7 Cleaning the test cell
The cleanliness of the test cell is of paramount importance when measuring dielectric properties because of the extreme susceptibility of insulating liquids to the influence of the most minute contamination.

The cell shall always be cleaned immediately before use in a reference test.

It is strongly recommended that cells in continuous use for routine measurements are regularly cleaned.

Cleaning need not precede a routine measurement made subsequent to one in which the property measured was within the limiting value specified and the liquid was of a similar chemical type. Prior to the next test, the cell shall be flushed with a volume of the next sample corresponding to at least three fillings of the cell.

When cells are to be regularly used for testing fluids with similar chemical type and having similar electrical properties, they may be stored filled with a clean sample of the fluid and simply flushed with a volume of the next sample corresponding to at least three fillings of the cell.

Many different types of cleaning procedure can be used provided that they have been proved to be efficient.

Examples of alternative procedures are given in Annexes A and B.

The following reference procedure shall be used in the case of dispute between laboratories.

CAUTION: Appropriate precautions against fire and toxic effects on personnel shall be observed when using solvents.

7.1 Trisodium phosphate cleaning procedure
Dismantle the cell completely.

Wash all the component parts thoroughly with two portions of the solvent (see Clause 6). Rinse all parts with acetone and then wash them with a mild abrasive soap or mild abrasive detergent.

Abrasive particles and rubbing action should be such as to cause no deterioration of the smooth finish of metal surfaces.

Boil all parts for at least 5 min in 5 % trisodium phosphate solution in distilled or de-ionized water, followed by several rinses with distilled or de-ionized water.

Boil all parts in distilled or de-ionized water for at least 30 min.

Thoroughly dry the component parts in an oven heated at 105 °C to 110 °C for no longer than 120 min because certain materials may deteriorate. Drying time depends on the design of the cell but usually 60 min to 1 20 min is satisfactory to remove any moisture.

Reassemble the test cell while hot, ensuring that any surfaces that will be wetted by the liquid are not touched with bare hands.

7.2 Storage of cell
When the cell is not in use, it is recommended that it remains filled with either a clean sample of the insulating liquid normally used, or, where different liquids are tested, with a solvent which has no detrimental effect on the cell.

Cells not in regular use shall be cleaned, dried, assembled and stored in a dry and dust free cabinet.

Alternatively, follow the manufacturer's recommendations.

8 Sampling
Insulating liquids for use in these tests shall be sampled in accordance with the method described in IEC 60475.

Samples shall be stored and transported in their original containers and shielded from light.

9 Preparation of samples
Unless specifically required by the specification relevant to the liquid under test, no treatment, such as filtration, drying, etc., shall be applied.

If it is necessary to preheat the sample, this shall be done whenever possible in the original sample container after removal of sufficient samples for other tests. Sufficient space should be left for the expansion of the liquid to avoid breakage of the container.

Where samples have to be transferred to other containers these shall be covered glass beakers or glass-stoppered Erlenmeyer flasks cleaned at least to the standard of Clause 7.

If the test has to be carried out at ambient temperature, the original container shall be allowed to stand in the room in which the test is to be made until the sample attains ambient temperature. Where tests are to be carried out at an elevated temperature and the sample cannot be heated in the test cell, the sample container or the auxiliary receptacles shall be stoppered, ensuring there is an appropriate volume in the containers to allow for expansion of the liquid. The containers are then heated in an oven to a temperature 5 °C to 10 °C above the required test temperature.

With liquids that are susceptible to oxidation, the heating period shall not exceed 1 h.

If the liquid has to be heated in a separate oven, it may be found desirable to restrict the use of an oven to a particular class of liquid to prevent contamination effects.

In order that representative specimens may be obtained, the sample is homogenized by tilting the container and gently swirling the liquid several times immediately prior to taking a sample for test.

Wipe the mouth of the container with a clean lint-free cloth and pour off a small portion of the liquid to wash the external surface.

10 Conditioning and filling the test cell
10.1 Cell conditioning
After the electrodes have been cleaned and dried, take care not to touch their surfaces with bare hands. Pay attention also to the cleanliness of surfaces on which cell components are placed. Do not expose them to contamination from vapours or dust.

To eliminate any influence of the cell cleaning procedures on subsequent tests, it is essential to pre-condition a dry cleaned cell with two cell fillings of the next liquid to be tested. High viscosity liquids may need a longer pre-conditioning time.

10.2 Filling the cell
Rinse the cell three times with a portion of the oil sample, drain and discard the liquid. If the inner electrode(s) need to be removed to perform this, avoid resting them on any surface and avoid the collection of dust particles on wetted surfaces of the cell.

Refill the cell, taking care to avoid entrapment of air bubbles. Bring the cell and contents to the required test temperature. The time required to reach the test temperature will depend on the heating method and may typically vary from 10 min to 60 min. Measurement must be started within 10 min of reaching a temperature within +/-1 °C of the required test temperature.

Great care should be taken to avoid contact of the liquid or the cell parts with any source of contamination.

It may be found desirable to restrict the use of a cell to a particular class of liquid. Contaminants that are benign in one liquid could be active in another with the possibility of transfer of the contaminants.

Ensure the environmental atmosphere is, as far as possible, free of any vapours or gases liable to affect the quality of the liquid.

11 Test temperature
These test methods are suitable for testing insulating liquids over a wide range of temperatures. Tests should be carried out at 90 °C unless otherwise specified by the appropriate specification for the particular liquid application.

Temperature measurement shall be capable of resolving to within 0.25 °C.