ISO 4407 specifies how to determine the particulate contamination of a hydraulic fluid by capturing the particles on a membrane filter and counting them under an optical microscope. It is the manual, microscope-based counting method in the hydraulic-cleanliness family. In a hydraulic system the fluid is both the power-transmitting medium and a lubricant, and the level of solid contamination directly affects component wear and system reliability — so quantifying it accurately is a condition-monitoring fundamental.
What it covers
The method separates the particles from a measured volume of fluid by vacuum filtration onto an analytical membrane, then sizes and counts them on the filter surface. It addresses the full chain that controls the accuracy of the result: cleaning of glassware, calibration of the microscope, determination of the effective filtration area, preparation and blank analysis of the membrane, evaluation of whether the loaded filter is suitable for counting, and the statistical counting procedure used to keep the workload manageable while preserving precision. It supports counting by transmitted or incident light, manually or with image-analysis assistance, and it standardises how the counts are converted into a contamination result and how that result is reported. The procedure produces particle counts in defined size classes, which is what allows the result to be expressed as a standard cleanliness code.
Why it matters in practice
The microscope method is the direct, visual reference for hydraulic cleanliness. Because the operator sees the particles, it distinguishes genuine solid contamination from artefacts such as water droplets, fibres, or gel that can mislead an instrument, and it remains the fallback when a fluid is dark, hazy, or otherwise unsuitable for the automatic light-extinction method. The trade-off is that it is labour-intensive and operator-dependent — which is exactly why the standard is so prescriptive about glassware cleanliness, blanks, calibration, and a disciplined counting protocol. Its main limitation in routine use is throughput, so it tends to be reserved for reference checks, dispute resolution, and samples the automatic method cannot handle.
How we use it
We treat ISO 4407 and the automatic light-extinction method (ISO 11500) as complementary: both feed the same ISO 4406 cleanliness coding, so a microscope count and an automatic count can be read on a common scale. We lean on the automatic counter for routine throughput and reach for the microscope method when a sample is unsuitable for automatic counting, when an automatic result looks questionable, or when a count needs an independent visual confirmation.