ASTM D7647: Particle Counting of Hydraulic and Lubricating Oils

Hard particles in hydraulic and lubricating oil wear pumps, valves and bearings, and they speed up the breakdown of the oil itself. ASTM D7647 is a test method for counting them. It is used to judge whether a filtration system is doing its job, whether extra off-line filtration is needed, and whether the oil should be changed.

What the standard covers

D7647 measures the number and size of particles in new and in-service lubricating and hydraulic oils, using automatic particle counters that work on the light extinction principle and are calibrated to ISO 11171. It covers particles from 4 µm(c) up to 200 µm(c), the upper limit depending on the counter, and oils up to 1000 mm²/s at 40 °C. Mineral oils and synthetic oils such as polyalphaolefin, polyalkylene glycol and phosphate ester can be tested.

Why the oil is diluted

A light extinction counter counts anything that blocks the beam. Two things that are not hard dirt can therefore inflate the count:

• Free water. Small water droplets are counted as particles.

• Soft particles. Some additives and their by-products, such as silicone antifoam and oxidation products, scatter light. They do not cause wear, but they raise the count.

D7647 dilutes the sample with a solvent that disperses the water into droplets too small to count and dissolves the soft particles, so that the count reflects the hard particles that matter. Dilution also lowers the viscosity of heavy oils, which would otherwise upset the counter.

From counts to a cleanliness code

The counts are usually reported as an ISO 4406 code. The three numbers stand for the particles per millilitre at 4, 6 and 14 µm(c). Each step up in a number roughly doubles the particle count, so 19/17/14 is about twice as dirty as 18/16/13.

How this guides the choice of element

1. Set the cleanliness code the system needs, from the equipment maker or the most sensitive component.

2. Choose an element whose beta ratio at the right size can reach it, for example β10(c) ≥ 1000. The beta ratio comes from the multipass test described in our filter test laboratory article.

3. Check the result with particle counts on oil samples taken from a live line, in a clean bottle.

Vens elements and D7647

Vens hydraulic and lubricating oil filter elements can meet cleanliness requirements measured by ASTM D7647, depending on the element and the rating chosen. Replacements for more than 65,000 element part numbers from 35 brands, with their micron ratings, media and dimensions, are listed in FilterDex, for example HYDAC, Pall and Parker.

A related method, ISO 11500, counts particles in hydraulic fluid by light extinction without the dilution step.

More: ASTM standards for filters and classification of filters.

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ASTM is a trademark of ASTM International. Vens Hydroluft is not affiliated with ASTM International. The standards are described in our own words from the scopes ASTM publishes; work to the edition named in your specification. Whether a filter meets a requirement depends on the product, its rating and the conditions of use, so we confirm it for each enquiry. See also our product information disclaimer.