The Vens Filter Test Laboratory: Multipass, Integrity, Particle Counting and More

Vens Hydroluft tests filter elements in its own laboratory. A filter element is judged by a few numbers: the size of particle it stops, how much dirt it holds before it has to be changed, and the pressure drop it adds to the system. Those numbers come from tests, and this article describes the tests run in the Vens laboratory, what each one measures and how the results are used.

The tests at a glance

Test	What it measures	Applies to
Multipass	Filtration ratio (beta) at each particle size, dirt-holding capacity, pressure drop as the element loads	Hydraulic and lubricating oil elements
Integrity (bubble point)	The largest opening in the element: damaged media, a poor seam or a leaking end cap	Hydraulic elements and membrane cartridges
Particle counting	Number of particles per millilitre in each size band, and the cleanliness code	Test fluids and fluid samples
Flow versus pressure drop	Pressure drop across the element at different flows	Every element type
Filtration efficiency	Percentage of particles removed at a stated size	Cartridges, bags and elements
Visual and dimensional	Diameters, length, end caps, seals, pleats, marking and workmanship	Every element

Multipass test

The multipass test is the method set out in ISO 16889, the standard the hydraulic filter industry uses to rate elements. Test oil circulates in a closed loop through the element. A fine test dust (ISO medium test dust) is added upstream at a steady rate, and particle counters sample the oil before and after the element throughout the test. The test runs until the element reaches its end pressure drop.

It gives three results:

• Filtration ratio, or beta ratio. Particles upstream divided by particles downstream, at each size. A beta of 200 at 10 µm(c), written β10(c) = 200, means the element passes one particle of 10 µm or larger for every 200 that reach it: 99.5% removal. A beta of 1000 means 99.9%.

• Dirt-holding capacity. The grams of test dust the element holds before it reaches the end pressure drop. It tells you how long an element will last compared with another of the same rating.

• Pressure drop against time. How the pressure drop rises as the element loads, which shows how evenly the media fills.

This is why a rating stated as a beta ratio means more than a nominal micron figure. Two elements both called "10 micron" can differ widely in what they let through.

Integrity test

An integrity test finds the largest opening in an element. In the bubble point test for hydraulic elements (the method of ISO 2942), the element is wetted in a test liquid and air pressure inside it is raised slowly while it is turned. The pressure at which the first steady stream of bubbles appears shows the largest pore or defect. A damaged layer of media, a poorly bonded side seam or a leaking end cap shows up as a low bubble point.

Membrane cartridges are tested the same way, with the membrane wetted and the pressure raised until gas passes through the largest pores. The test does not damage the cartridge. See ASTM F316: bubble point and pore size of membrane filters.

Particle counting

An automatic particle counter passes the fluid through a light beam and counts and sizes each particle that blocks the light. Counters for hydraulic fluids are calibrated to ISO 11171, so sizes are written in µm(c). The counts at 4, 6 and 14 µm(c) give the three numbers of the ISO 4406 cleanliness code, for example 18/16/13, where each number stands for a range of particles per millilitre.

Particle counting is the measuring instrument of the multipass test, and it is also used to check the cleanliness of fluid samples. For the ASTM methods, see particle counting of hydraulic and lubricating oils (ASTM D7647).

Flow versus pressure drop

The element is fitted in a housing and clean fluid is pumped through it at a range of flows, with the pressure drop measured at each one (the method of ISO 3968). The result is the curve used to size a filter: at the design flow and the oil's viscosity, the clean pressure drop should be well below the setting of the bypass valve, so that the element has room to load before the bypass opens. Viscosity matters as much as flow: cold oil gives a much higher pressure drop than warm oil.

Filtration efficiency

For process filter cartridges and bags, efficiency is measured by challenging the filter with test particles at a set flow and counting the particles before and after it. The result is the percentage removed at a stated size. This is the test that separates a nominal rating, which has no fixed definition and differs from maker to maker, from an absolute or beta rating, which states the removal at a given size.

Visual and dimensional inspection

Every element is checked against its drawing or against the original element it replaces:

• outside and inside diameter, and length;

• end caps, and the size and material of O-rings and gaskets;

• pleat count, pleat spacing and the support layers;

• the side seam and the glue lines, the core and the outer wrap;

• marking and labels.

For a replacement element, fit comes first. An element with the right rating but the wrong seal or length will not seal in the housing, and dirt will go round it. This is the check behind every replacement found in FilterDex.

What this means when you buy elements

• State the rating your system needs as a beta ratio at a size, for example β10(c) ≥ 1000, not only as a micron figure. That is the figure the multipass test confirms.

• If your specification asks for test results, tell us which ones when you enquire.

• For a replacement, send the part number of the old element, or its dimensions if the number cannot be read.

More reading: ASTM standards for filters, types of filter media, classification of filters and how to choose a filter cartridge.

Find a replacement element in FilterDex, send your specification through the enquiry form, or message us on WhatsApp (+91 73974 98659).

Test results describe the element tested, under the stated test conditions. Performance in service also depends on the fluid, flow, temperature and contamination in your system. See our product information disclaimer.