Choosing a Hydraulic Filter: Beta Ratio and Change Point
The hydraulic filter keeps the oil clean enough for pumps and valves to survive. This guide explains how to choose the filter medium by beta ratio and the required ISO 4406 cleanliness class, and how to fix the right change point using differential pressure.
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What does the beta ratio tell you about a filter?
The beta ratio (βx) is defined by ISO 16889 as the ratio of particles of a given size upstream and downstream of the filter. β10 = 200 means that out of 200 particles of 10 µm or larger, only one passes through - an efficiency of 99.5 %.
The higher the beta ratio at the stated particle size, the finer the element filters. The index x (for example β7, β10, β12) names the particle size in microns for which the ratio applies. A high beta value at a small particle size therefore means fine, highly effective filtration.
Which cleanliness class does your system need?
Required oil cleanliness is stated to ISO 4406 with three code numbers, for example 18/16/13. They represent the particle count per millilitre larger than 4 µm, 6 µm and 14 µm. The lower the numbers, the cleaner the oil.
The target class is set by the most sensitive component in the circuit. Servo valves and axial piston pumps demand much cleaner oil than a simple gear pump. Only once the target class is fixed can you derive the matching beta ratio of the filter.
- Servo valves: ISO 15/13/10, fine filters β5 to β7 with a high beta value.
- Proportional valves and piston pumps: ISO 17/15/12, filter about β10 = 200.
- Directional valves and gear pumps: ISO 19/17/14, filter about β12 = 100.
- Low pressure and return lines: ISO 20/18/15, coarser filters are sufficient.
When is the right moment to change the filter?
The most reliable indicator is the differential pressure (Δp) across the element. As the medium clogs, the pressure drop rises. A clogging indicator (visual or electrical) signals when the preset limit is reached, usually between 2 and 5 bar depending on the filter type.
Do not wait until the bypass valve opens, because then unfiltered oil flows straight into the circuit. Check the indicator with the oil at operating temperature, since cold, viscous oil fakes an excessively high differential pressure.
- Read and record the clogging indicator regularly.
- Fit only original elements with the correct beta ratio.
- Renew seals and O-rings at every change.
- Have the oil analysed if you see sludge or discolouration.
Frequently asked questions
What exactly does β10 = 200 mean?
Out of 200 particles of 10 µm or larger, only one passes the filter. That is an efficiency of 99.5 % at this particle size and marks a fine element for demanding hydraulics.
Which cleanliness class does a servo valve need?
Servo valves typically require ISO 4406 15/13/10 or cleaner. Fine filters with a high beta value at 5 to 7 µm are suitable, protecting the sensitive control edges from wear.
How do I know the hydraulic filter is full?
By the rising differential pressure across the element. The clogging indicator trips at its limit (usually 2 to 5 bar). Check with the oil at operating temperature, because cold oil distorts the value.
How often must I change the filter?
Not by a fixed calendar, but by differential pressure. An initial change after the run-in of a new system is mandatory, after that the interval follows the clogging indicator or oil analysis.
Looking for the right hydraulic filter?
We supply suction, pressure and return-line filters with a defined beta ratio, matched to your required ISO 4406 cleanliness class.
Defined beta ratio
Filter elements tested to ISO 16889.
Cleanliness to ISO 4406
Matched to your system target class.
Measurably clean
Oil analysis and Δp are documentable.
Expert advice
Our specialists help you choose.