6. PREPARATION OF THE SAMPLE
6.1-
Collection of the sample
It is very important to obtain results that adjust to the reality that you strictly carry out the
measures as it is mentioned next, otherwise, the obtained results could be erroneous.
According to the IEC-156 standard it will be necessary to adopt the following indications.
The oil sample will be taken in the place of the transformer, circuit breaker, etc. where the oil is
more contaminated, generally in the lowest zone of the containers that contains it.
The amount of sample will be approximately three times superior to the capacity of the test cell.
A topaz coloured glass bottle is the most appropriate container. The transparent glass bottles can
be used, but they must be protected of the direct action of the light until the test is made. The
plastic containers don’t have to be attacked by the oil to test and if they are used, they cannot be
reused.
In order to close the bottles, use a spiral cap, preferably of polyolefines or polytetrafluorethylene.
The containers will be filled almost until the top, leaving approximately a 3% of the volume of the
container as free air space.
In case the container is reused, it must be cleaned (don’t forget the cap) by means of washing with
a suitable dissolvent to eliminate the rests of the last sample. The containers will be washed finally
with acetone, and the rests of this dissolvent will be eliminated by means of hot air blown.
After the cleaning, the containers must be closed immediately and stay closed until the moment of
their use.
6.2-
Preparation of the sample to test
According to the IEC-156 standard it will be necessary to adopt the following indications.
The test will be done, if another thing is not specified, with the sample as it’s received, without
drying or extracting the gas.
The operation of sample preparation will we done in a dry and free of dust place.
At the moment of the test, the temperature of the test liquid and the room temperature will not
differ more than 5ºC and for the arbitration tests the temperature of the oil will be between 20ºC ±
5ºC.
The container that contains the oil sample to test must be shacked softly and turned upside down
several times to assure the most homogenous distribution of the impurities contained in the oil,
trying to avoid the formation of air bubbles. Avoid the unnecessary exposing of the sample to the
air of the atmosphere.
Immediately before its use the cell will be cleaned according to 8
th
point.
The oil is now spilled slowly in the test cell, avoiding the formation of air bubbles.
The temperature of the liquid is measured and written down.
It is very important for the correct development of the test that the electrodes are totally covered
by the oil, so that the distance from the surface of the oil to the axis of the electrodes is not less
than 40 mm.
