IEC 60970 Insulating liquids - Methods for counting and sizing particles
7 Sampling
7.1 General remarks
The sample taken should only be used for the particle count determination. Further analysis may be done on the residual sample, but after the particle count determination.

The particle content of a sample is also dependant on the sampling point, time elapsed since the transformer was filled, the circulation rate and the time that the transformer has been left to stand prior to sampling.

With used liquids, oxidation products which are soluble at operating temperatures may precipitate when the sample is allowed to stand at room temperature for a prolonged period. This process, which is dependent upon the service age of the liquid, the time between sampling and analysis and the storage temperature, can affect the particle count.

For the above mentioned reasons, sampling is the main source of spreading of results.

7.2 Sampling vessels
The sampling vessels recommended in this standard are cylindrical, flat-bottomed, wide-necked, clear glass bottles fitted with a polypropylene threaded cap forming a seal with the bottle without the use of any inserts.

When an automatic particle size analyzer is used, the volume sampled shall be enough to allow a proper rinsing of the instrument's dead volume and measuring cell before the analysis.

For microscopy, the bottles shall have at least 100 cm3 capacity and be permanently marked to indicate 100 cm3 sample size.

An alternative method for insulating liquids in service using syringes as sampling vessels is given in Annex A.

7.3 Cleaning of sampling bottles
It is recommended that the bottles be cleaned to achieve a blank count of less than 200 particles above 5 µm per 100 cm3. The test should be performed on the filtered solvent used in the last stage of the cleaning process.

A cleaning method is given for guidance, but other methods can be used provided they achieve a similar or greater degree of cleanliness.

a) Wash with warm water containing a detergent
b) Rinse with warm water and drain
c) Rinse thoroughly with 0,45 µm membrane filtered acetone to remove water
d) Rinse with 0,45 µm membrane filtered petroleum ether 40 °C to 70 °C or with another suitable solvent. Leave 1 cm³ or 2 cm³ of solvent in the bottle and close the bottle.

If ultrasonic agitation is used before counting particles, the cleaning procedure must include ultrasonic treatment. Reject the procedure outlined in stage c) and instead place the sample bottle, filled with 0,45 µm membrane filtered acetone, in an ultrasonic bath for 1 min.

A residue of solvent in the bottle creates a positive pressure in the bottle helping to prevent contamination from the atmosphere when opening the bottle.

Warning: Attention is called to national regulations associated with the use of solvents.

The use of purchased sample bottles cleaned in accordance with ISO 5884 is allowed.

7.4 General directions for sampling
It is difficult to obtain representative samples from a drum. If sampling is found to be necessary, the procedure given in Annex A may be used or, alternatively, the procedures given in IEC 60475. In the case of sealed power transformers and instrument transformers or similar equipment with small liquid volume, the manufacturer's instructions on sampling procedure and quantity shall be followed. A sample from a transformer should preferably be taken during fluid circulation or immediately afterwards. The analysis obtained may depend on the sampling point selected. Confirmatory or follow-up samples should therefore always be taken from the same point.

Every precaution shall be taken when sampling not to contaminate the sample.

Outdoor sampling of insulating liquids in rain, fog, snowfall or high wind is only permitted if all precautions are taken to avoid contamination of the samples. In this special case the use of a cover is necessary.

Ensure that sampling is done by an experienced person.

7.5 Sampling procedure
Prior to taking the sample, the exterior of the sampling valve and the adjacent parts shall be thoroughly cleaned using lint-free wiping materials.
- Connect a length of plastic tubing, which is resistant to the liquid being sampled, to the sampling valve.
- If the sampling bottle contains filtered solvent, it may be necessary at low ambient temperature to warm the bottle with the hands, in order to create a positive pressure and thereby prevent the ingress of atmospheric particulate matter.
Warning: Petroleum ether, as well as many of its alternatives, is a highly flammable solvent, and appropriate safety measures must be observed when the bottle is opened.
- Thoroughly flush the valve and by draining a sufficient quantity of liquid into a waste container. The adequate draining volume depends on the total oil volume of the equipment (for big power units 5 l is recommended).
- Remove the cap from the bottle; do not empty out the solvent. Without interrupting the flow from the valve, substitute the bottle for the waste container and collect the required volume of sample as quickly as possible. Then remove the sample bottle and replace the waste container.
- Replace the bottle cap without over tightening.
- Close the sampling valve and replace any protection. Label the sample.

The sample shall be protected against light during transportation and storage.

7.6 Labelling of samples
7.6.1 Samples from tanks
Labels shall carry the following markings:
- tank identification;
- sampling point;
- type of insulating liquid;
- date of sampling.

7.6.2 Samples from electrical equipment
Labels shall carry the following markings:
- equipment identification;
- sampling point;
- equipment in factory/in service;
- liquid identification;
- temperature of liquid;
- date of sampling.

8 Preparation of the samples for analysis
Samples should be processed as soon as possible after sampling, because their long storage will generally lead to sedimentation of particles. Fine particles may also coalesce to form larger particles. For the above mentioned reasons, a shaking procedure will be necessary.

When automatic particle analysers are used, a detailed procedure for sample treatment is described in Clause 9 (Method A - Automatic particle size analyzer).

If high-energy ultrasonic treatment or higher shear mixing is used, there is a risk that the particle count determined will be enhanced owing to breakdown of sludge and other large particles and also that finely dispersed micro bubbles are formed in the liquid. If too high a vacuum is applied to air-saturated liquids, more micro bubbles may be released from the liquid.

NOTE It is advisable that the agitation and vacuum procedure developed be tested for effectiveness. This can be done by taking several samples simultaneously. One sample is tested immediately after sampling, with no agitation or vacuum treatment at all, another after treatment and a third after some storage time. The test should be applied to liquids with a varying degree of particle content, state of ageing and air content.

For optical microscopy systems shaking can be done manually, in a shaking machine, or by ultrasonic treatment (see Clause 10: Method B - Optical microscopy).