Understanding filtration › 3. Filter media

CHAPTER 3.3

Permeability and filtration fineness

Almost every cloth data sheet states an air permeability. It is a useful figure, but often misread: it describes a new cloth with air flowing through it, not a cloth in service under a filter cake. Knowing what it says, and what it does not, avoids many misleading comparisons.

Nicolas Ract · 8 min read · October 2026

What is measured

The air permeability of a fabric is the air flow passing through it, per unit area, for a given pressure difference between its two faces. The reference method in Europe is ISO 9237 (adopted as NF EN ISO 9237). According to a testing laboratory that applies it, the standard test area is 20 cm², and the pressure difference is in principle 100 Pa for apparel fabrics and 200 Pa for industrial textiles, with other values (50 or 500 Pa, other areas) possible by agreement; the result is expressed as an air velocity, in mm/s [1].

The test is carried out on new, dry, clean cloth. That is what makes it a good manufacturing check: one cloth manufacturer describes it as an essential production parameter for guaranteeing a consistent product [2]. It is also what limits how much it can predict about behaviour in service.

Units, and how to convert between them

The same measurement appears in several forms depending on where the data sheet comes from. Area and time conversions are exact; pressure conversions are approximate.

For pressure, a textile laboratory states that, as a first approximation, a linear relationship can be assumed: multiply by 1.6 to go from a value at 125 Pa to a value at 200 Pa, and by 0.625 for the reverse [3]. It is a convenient approximation, not an equivalence: two cloths of very different structure do not necessarily respond in the same way to a change in pressure.

What permeability tells you

• The relative openness of the new cloth: for comparable fibre, yarn and weave, a more permeable cloth lets liquid through more easily at the start of the cycle.
• Manufacturing consistency: from one batch to another of the same reference, permeability should remain within the tolerance stated by the manufacturer [2].
• The condition of a used cloth, by comparison with a new cloth of the same reference: a sharp drop signals blinding, an increase signals thinning or opening of the fabric (see chapter 4.1).

What it does not tell you

A study presented in 2022 by a cloth manufacturer, which varied a single weaving parameter at a time, concludes that there is no point in using air permeability to select a medium on the basis of particle retention; at best it can help improve filtration time, "without complete certainty" [2]. There are several reasons for this:

• Retention depends on structure, not just on average openness: pore shape and size, yarn type, weave, finish. The same study notes that pore size is a good retention criterion only for comparing media of similar design [2].
• In service, the cake does the filtering: in cake filtration, particles stop at the surface of the medium and it is then the cake that does most of the filtration [4]; its resistance then often exceeds that of the cloth (see chapter 1.3).
• Behaviour with respect to blinding does not show up: a very open cloth can let fines penetrate and blind faster than a tighter cloth with a smooth surface (see chapter 3.2).
• Air is not the slurry: viscosity, fibre wetting and the presence of solids change the flow.

Fineness and retention: no "micron" rating for a cloth

People often ask "how many microns?" for a filter press cloth. One filter press manufacturer answers clearly: cloths have no micron rating, they are characterised by their air permeability [5]. When a rating does exist for a medium, you need to know what it refers to:

• a nominal rating expresses an approximate retention capability, highly dependent on the test protocol; it is a comparison indicator, not a guaranteed threshold [4];
• an absolute rating describes the ability to retain completely particles of a defined size, under standardised test conditions [4].

For a filter press cloth, the actual fineness therefore depends mainly on the product: its particle size distribution, its tendency to bridge, the filling rate.

Filtrate clarity and blinding: the start of the cycle

At the very start of a cycle, no filter cake has yet formed. The filtrate may then come out cloudy during an initial phase, sometimes called "bleeding" [4]. Then the particles form bridges over the pores, even when they are almost as large as the pores; this layer becomes the real filter. If poorly controlled, this bridging can develop into pore blockage [4].

Hence the central trade-off in choosing a cloth, between flow and retention. Field experience, as a general trend:

• More open cloth: faster cycles at the start, but longer initial cloudiness and penetration of fines into the fabric, hence a risk of blinding.
• Tighter cloth: clear filtrate sooner, fines retained at the surface, but lower flow and sometimes a cake that sticks.

Blinding itself shows up as a loss of open area: at constant flow the pressure drop rises, at constant pressure the flow falls [4]. It is dealt with in chapter 4.4.

Field experience: a filtrate that stays cloudy well beyond the start of the cycle points less to a cloth that is "too open" than to a hole, a defective seam or neck, or a leak between plates (see chapter 4.3).

Comparing two data sheets

Before comparing figures, check that they are comparable:

Two remarks. The same study points out that some weaving parameters that strongly influence retention and cake moisture do not always appear on data sheets [2]. And it warns against the idea that a heavier cloth with similar characteristics will last longer: the failure mode must be analysed first [2].

Field experience: two data sheets may look almost identical and give different results on the same filter. The data sheet is used for pre-selection; the decision is made through a comparative trial on the actual product (see chapter 6.2) and judged on cost per tonne (chapter 6.3).