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OTS80PB and OTS60PB OTS100AF, OTS80AF and OTS60AF
16
Preparations for use
Immediately prior to testing, vessels stored full of oil
must be drained and all internal surfaces, including the
electrodes, rinsed with fluid taken from the sample to
be tested.
The vessel should then be drained again. Swiftly and
carefully fill with the test sample, taking particular care
to avoid the formation of bubbles. If the vessel was
stored empty, or if it is to be used for a different type of
fluid from that with which it was filled during storage,
it should be cleaned with an appropriate solvent before
rinsing and filling.
1 2
3 4
Fig 3: Preparing the vessel
ASTM D1816 specifies the use of a dry hydrocarbon solvent such as kerosene, which meets the requirements of D235.
Solvents commonly used include acetone and, in the USA, toluene. Toluene is banned in Europe.
Solvents with a low boiling point should not be used as these evaporate rapidly, cooling the vessel and giving rise to the
risk of condensation.
Use lint-free clean-room wipes to clean the vessel. Do not use paper towels as they may introduce particles that hold
moisture, causing breakdown values to be dramatically reduced.
Touching the electrodes or the inside of the vessel should be avoided and during cleaning, the electrodes should be
checked for pitting or scratches that may cause breakdown voltage values to be decreased. Remember that the rules of
cleaning apply to all parts that will come in contact with the oil sample during testing.
Setting the electrode gap
Setting the electrode gap accurately is
very important; the results obtained are
only valid if the gap is correct. A big
problem is movement of the electrodes
after the gap has been set and for
this reason, many users of oil test sets
check the electrode gap frequently –
sometimes before every test. A better
solution is to use test sets where the
electrodes can be locked in position,
such as the instruments in Megger’s
latest OTS range.
✓
✖
Fig 4: Megger electrode gap gauge
Megger recommends the use of flat, smooth gap gauges. The latest Megger gauges have a black anodized coating,
which not only provides a smooth surface but also shows when the gauge is worn, as the shiny aluminium starts to
show through the coating.
When setting the electrode gap using the Megger gauge - See Figure 4
Ensure the gap is set in an empty vessel and before pouring in the sample to be tested to avoid contaminating the
sample with the gap setting process.
Hold the gap gauge with the key chain and place between the electrodes. This enables better view of the gap
gauge and prevents the gauge being forced into the existing electrode gap.
Carefully adjust the electrode gap without over tightening. The gap gauge should just be able to slide between the
electrodes with little force required to move it, while it is held from the chain. If force is required over tightening has
resulted and the electrodes must be adjusted to release this and rechecked ensuring that only a light force is needed
to move the gauge.
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