![]() ![]() The indication of the purity class according to ISO is carried out in 3 classes, for example with "20/15/12", where each number stands for one of the 3 counting classes. The old ISO 4406 cumulatively counts particles (>2), >5 and >15μm.The new ISO 4406 cumulatively counts particles >4, >6 and >14μm.However, the two methods NAS and ISO differ considerably in the evaluation of the measured particle numbers: While the specification of cleanliness classes according to the ISO 4406 standard is predominant in Europe, in the USA the NAS 1638 standard is common. With the number of particles determined for certain particle sizes, such as 5μ, assigned cleanliness classes can be specified. A particle count is performed to assess the purity of an oil. Many machine failures are due to contamination of the oil.
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