Pick a hydraulic filter by three numbers. The micron rating must suit the tightest clearance in the circuit. The beta ratio, on glass-fibre elements, tells you how well the filter removes that size. And the bypass must open only when the element is loaded. Glass (A) for sensitive valves, cellulose (P) for standard return duty, mesh (M) for suction.
Most filter mistakes are not about brand loyalty. They are about buying on a micron number alone and ignoring the two numbers that decide whether that micron figure means anything. This article explains all three, shows the media codes that tell them apart, and walks through a few selection examples with MP Filtri filters.
Why a micron number on its own is a half-answer
A hydraulic filter is sold on its micron rating, for example 10 µm. But 10 µm tells you almost nothing until you know two things: whether that rating is nominal or absolute, and at what efficiency it was measured.
Nominal ratings come from the maker’s own test method. They are useful as a rough size guide, and no more. Absolute ratings come from a standardised test, ISO 16889, which pushes a controlled contaminant through the element and counts what comes out. That test produces the second number you need, the beta ratio.
On MP Filtri elements the media letter in the code tells you which kind of rating you are buying:
- A is glass fibre with an absolute rating. Typical codes read A03, A06 or A10, where the number is the rated micron.
- P is impregnated cellulose with a nominal rating, in codes such as P10 or P25.
- M is wire mesh with a coarse nominal rating, in codes such as M60 or M90.
A full code, for example CS050P10, is the element family (CS spin-on for MPS heads) followed by size, media letter and micron number. If you need help decoding one, we covered it in detail in our MP Filtri element cross-reference guide.
Beta ratio: the number that makes microns honest
The beta ratio is the ratio of particles at or above a given size upstream of the element to the same particles downstream. In plain terms, if the filter sees 200 particles at or above 10 µm upstream and one downstream, the element has a beta ratio of 200 at 10 µm.
Remove the arithmetic and it becomes a removal efficiency. A beta of 75 removes about 98.7 percent of those particles in one pass. At beta 200 that figure reaches 99.5 percent, and at beta 1000 it reaches 99.9 percent. The gap between 99.5 and 99.9 sounds small, but in a circuit that circulates oil continuously, the extra 0.4 percent of particles per pass is the difference between a valve that wears normally and one that sticks early.
So demand a beta ratio whenever the application is sensitive. A glass-fibre element with an absolute rating should have a datasheet that states the beta point and the test standard. If a supplier quotes only a micron number and cannot say how it was tested, treat the element as nominal and assume the filter is coarser than the number suggests.
For most industrial circuits, a beta of 200 at the rated micron is the workhorse choice. For servo valves, proportional valves and variable pumps with tight clearances, look for beta 1000 or a lower micron rating. The pump or valve manufacturer usually publishes a target ISO 4406 cleanliness code, for example 19/17/14. The three digits count particles larger than 4, 6 and 14 µm per millilitre of oil. Work backwards from that code to the filter rating instead of picking a micron out of habit.
Bypass valve: why it opens and what it costs you
The bypass valve is a pressure-relief path around the element. When the pressure drop across the element reaches the cracking pressure, the valve opens and oil flows around the element instead of through it.
The design intent is to keep the machine running. A clogged element, a cold start with thick oil, or a pressure surge can push the pressure drop high enough to collapse the element or starve the pump. The bypass sacrifices filtration to prevent that damage.
That trade-off deserves respect. I have traced more than one breakdown to a bypass valve doing exactly what it was built to do, quietly routing unfiltered oil past an element that should have been changed weeks earlier. The bypass is a safety device, not a licence to ignore the indicator.
Selection rules for the bypass:
- The cracking pressure sits between normal operating pressure drop and the limit of the element and housing. The housing datasheet states both numbers, so match the bypass to the housing, not to preference.
- On return-line and suction filters, a bypass is normal and desirable. The machine keeps running and the indicator tells maintenance when to act.
- On pressure filters protecting servo or proportional valves, run an electric clogging indicator, MP Filtri calls these VSP, and wire it so the machine shuts down or alarms instead of running on bypassed oil. A visual indicator only helps if someone is standing in front of it.
- If a circuit absolutely cannot tolerate bypassed oil, choose a filter without a bypass, but understand that a blocked element then has no escape route and can collapse. That design choice belongs on critical loops with electric monitoring, not on general return lines.
A, P or M: how the media letter changes your choice
| A media, glass fibre | P media, cellulose | M media, wire mesh | |
|---|---|---|---|
| Rating type | Absolute, beta-tested | Nominal | Nominal, coarse |
| Typical codes | A03, A06, A10 | P10, P25 | M60, M90 |
| What the number claims | Removal at a stated beta under ISO 16889 | A rough size cut-off | Catches coarse particles only |
| Washable | No | No | Yes, cleanable |
| Usual home | Pressure and return loops protecting pumps and valves | Return-line and low-pressure duty | Suction strainers |
| Skip it when | Budget duty with no sensitivity spec | A servo or proportional valve is downstream | It is the only filter protecting a pump |
Selection examples: what goes where
Three worked examples cover most of the machines we see, from injection moulding to machine tools to mobile hydraulics.
Servo or proportional valve circuit. The valve clearances are the tightest point in the machine, so protect them at the source. Put a high-pressure line filter, MP Filtri FHP or FMP series, on the pump outlet with an A06 glass element, or A03 where the manufacturer demands it. Fit an electric VSP indicator so the control system reacts to a blocked element. On the return side, hold the target cleanliness with an A10 or P10 element in a return housing.
Standard return line with a gear or vane pump. These pumps tolerate more contamination than servo valves, so P10 or P25 cellulose in a return-line filter, MPF or MPFX series, is normally enough and costs less than glass. Keep the bypass so a cold start does not burst the element, and use a visual VST indicator with a set maintenance routine. If the OEM specifies a tight ISO 4406 target, step up to an A-rated element instead of assuming P will get there.
Suction line. This is where people overspend. Fine filtration at the suction inlet restricts flow and can starve the pump, which destroys it faster than dirt would. Use a coarse M60 or M90 mesh strainer with generous bypass, and do the fine filtration on the pressure or return side where the oil can be pushed through it.
One habit saves more filters than any table: note the ISO 4406 target, the most sensitive component, and the housing series before you call a supplier. With those three facts a filter can be specified in one pass.
How to turn this into an order
Write down the circuit position, the pump and valve types, and the cleanliness target if you have one. If you have an old element, read the code and the media letter. Then we can name the housing series, element media and micron rating, and confirm the beta ratio from the datasheet before anything is ordered.
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