There are two numbering systems for cleanliness, NAS 1638 and ISO 4406. Their numbering runs in exactly opposite directions: in one of them a higher number means cleaner oil, in the other a higher number means dirtier oil. Reading the two systems together is where the simplest mistakes in a discussion of cleanliness are made.
The opposite directions are the easiest trap
In NAS 1638 a higher number means cleaner oil. The cleanliness figure of the ZYD-100 is NAS 1638 class 5, which means that the total particle count in the oil sample falls inside the range that this class corresponds to.
ISO 4406 runs the other way: a higher number means dirtier oil. It uses three codes to describe the particle counts in different size channels, and each code is the logarithm of the particle count, so a complete cleanliness designation consists of three numbers.
Because the directions are opposite, when the two systems are put side by side you first have to confirm whether you are looking at a scale where a lower number is cleaner or one where a higher number is dirtier. The accident that occurs most often on site is to treat a code such as 18/15/12 from ISO 4406 as a class number and to conclude that 18 is worse than 12.
The size channels are divided differently
The two systems count different size ranges. The codes commonly used in ISO 4406 correspond to different size channels, for example the group of 4 μm, 14 μm and 38 μm, and each code corresponds to the particle concentration in one size channel; NAS 1638 combines the particle counts of two size channels into a single class.
Different channels mean that the two numbers are not describing the same thing at all. Of the three codes in ISO 4406 it is often the particle count in the middle channel that relates most closely to the actual work, because particles of that size are the ones most easily intercepted by a filter element and the ones least easily separated by settling.
Why a conversion table can only be treated as an approximation
The conversion tables in circulation are not exact conversion tables. The reason is that the test conditions are not consistent: the sample volume differs, the cleaning method differs, the shape and the composition of the particles differ, and the settings of the counting instrument and of the counting threshold differ as well. Take the same sample on the same machine and change the set of test conditions and the number moves.
There are version differences in the standards themselves, the combination of channels used changes from one version to another, and if the versions referred to on the two sides of a conversion table do not match, the result can be far off.
How to quote both on the same machine
If a technical document has to give both designations, the correct approach is not to look up a table, but to take the particle counting data of the same batch of oil samples and issue the reports against the two standards separately, so that both numbers come from the same set of raw data.
At the same time the following items have to be written out: the size channels used, the unit of the particle count, the name and the version of the standard the test is based on, the sampling point and the sample volume, the condition and the temperature of the oil sample, and the thresholds used for the judgement. A cleanliness figure that is missing any of this information means nothing outside its context.
| Comparison | NAS 1638 | ISO 4406 |
|---|---|---|
| Direction of the numbering | The higher the number, the cleaner | The higher the number, the dirtier |
| Form of the designation | A single class | A combination of three codes |
| Size channels | Two channels combined into a class | Several channels given a code each |
| Form of expression | A class number | A logarithmic expression of the particle count in each channel |
| Example | The cleanliness figure of the ZYD-100 is NAS 1638 class 5 | Three codes have to be given together |
- When a number is quoted, give the full name and the version of the standard; do not quote the number alone.
- Check the direction of the numbering, in particular when the two systems are being compared.
- The two designations have to be based on the same set of raw counting data; do not look up a separate conversion table for each.
- Agree only one system in the contract and treat the other as a reference, so that the criteria do not differ at acceptance.
- Put the sampling point, the sample volume and the condition of the sample into the report; these cause disagreement more often than the number itself.
A conversion table can tell you roughly which band you are in; it cannot decide for you how clean that batch of oil actually is.