Two figures often appear side by side on an equipment data sheet: filtration accuracy ≤1 μm and cleanliness NAS 1638 class 5. Both look as if they describe how well the machine filters, but they do not measure the same thing at all: the first is a size, the second is a count. Mixing the two is one of the most common mismatches in site acceptance and in day-to-day judgement.
One measures particle size, the other measures how many are left
Filtration accuracy describes the capability of the filter medium itself: how large a particle can be held back. The filtration accuracy of the ZYD-100 is ≤1 μm, corresponding to the fine filtration stage built from five 150-500-5 fine filter elements in parallel. That figure is tied to the material of the medium, its fibre structure and its filtration area, and it is a capability verified on the machine under factory conditions.
Cleanliness describes the state of the oil right now: how many particles are still present per unit volume of oil, graded by NAS 1638, with class 5 the level a machine of this type should reach after treatment. It is the result of a sampled and tested sample, and it depends on the machine as well as on whatever happened on the way to that sample.
Four steps sit in between
Holding the particles back in the filter element is only the first link in the chain. Between the oil inlet and the oil outlet there are at least four places where the result can be altered: the sealing between the filter element and the housing, the particles already present in the housing and the pipework, the impurities carried in during an element change, and the contamination of the sampling step itself. If any one of them goes wrong, the result will be either accuracy that passes with cleanliness that fails, or the other way round.
- With poor sealing, the oil bypasses the filter medium and enters the next stage directly. The oil that comes out carries the particle load it started with, cleanliness is poor, and the differential pressure gauge can still look perfectly normal.
- Residual particles in the pipework and the housing are washed out together once a new element is fitted, so the cleanliness of the first few batches is visibly off.
- If the sealing face on the end cap is not cleaned when the element is changed, particles in the old sealing compound contaminate the downstream side again.
- If the sample bottle, the container and the sampling pipework are not clean, the test result reflects the sampling process rather than the machine.
The two opposite deviations have to be separated
| Direction of the deviation | The more likely cause | The next action |
|---|---|---|
| Cleanliness is out of limit and a new element does not improve it | Poor sealing lets oil bypass, or the upstream side is carrying particles in | Stop the machine and check how the end caps are seated, then review the sampling step |
| Cleanliness is within limit but the differential pressure sits low for a long time | The flow resistance has not been established and the bypass flow is high | Review how the differential pressure builds up and the load on the pump |
| The first few batches are poor and then the result settles | Residual particles in the housing and the pipework are being washed out | Sample and retain the first batches separately instead of merging them into a qualified batch |
| Two test results differ widely, on two different machines | There is a systematic difference in the sampling and the containers | Unify the sampling method before comparing the figures |
Write the two requirements separately at acceptance
When an acceptance clause is drafted, these two belong in separate requirements with their own basis of judgement: filtration accuracy is a commitment about the capability of the machine, cleanliness is the result for one batch of oil at one moment in time. The first is judged from the machine structure and the filter element configuration, the second from sampling and testing. With the same machine and the same batch of oil, merging the two into one sentence means that once the two sides disagree it is no longer clear which requirement was not met.
Filtration accuracy is the upper limit promised by the filter medium, cleanliness is the actual state of one batch of oil, and neither statement can answer for the other.
