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BIZOMAT MAKEOS · Maintenance

Hydraulic Filter Selection (Beta Ratios, Cleanliness Code, Pressure Drop)

Will this filter bring the oil to my target cleanliness code, and is its pressure drop acceptable at my flow and oil viscosity?

Selection check of a hydraulic filter: particle counts per millilitre at 4, 6 and 14 µm upstream, the counts after a filter with your beta ratios at each size and its capture efficiencies (as the filter beta ratio tool), the ISO 4406 codes before and after (as the ISO cleanliness code tool) against the target, and the element pressure drop scaled from the maker’s rated value for your flow and viscosity against the allowable drop.

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How this tool works

Inputs, units and where to find them

InputUnitTypical source
Upstream counts (per mL)
≥ 4 µmper mLParticles of 4 µm(c) and larger per mL upstream (from an oil sample).
≥ 6 µmper mLParticles of 6 µm(c) and larger.
≥ 14 µmper mLParticles of 14 µm(c) and larger.
Filter beta ratios
β at 4 µmFilter beta ratio at 4 µm(c) from the maker’s ISO 16889 data.
β at 6 µmBeta ratio at 6 µm(c).
β at 14 µmBeta ratio at 14 µm(c).
Target code
Target at 4 µmTarget ISO 4406 range number at 4 µm (from the component maker’s requirement).
Target at 6 µmTarget at 6 µm.
Target at 14 µmTarget at 14 µm.
Pressure drop
Element pressure drop at rated flow and viscositybarClean element pressure drop the maker gives at the rated flow and viscosity.
Rated flowL/minFlow at which that drop is given.
Rated viscositycStViscosity at which that drop is given.
Actual flowL/minFlow through the filter in service.
Actual viscosity (coldest running)cStOil viscosity at the coldest running condition.
Allowable clean element dropbarLargest clean element drop you accept (well below the bypass setting).

What each input means

Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.

Upstream counts (per mL)

What this means: ≥ 4 µm
Illustration: ≥ 4 µmA row of identical items being counted; ≥ 4 µm is how many there are.nExample: 15000 per mL≥ 4 µm
What it is
The number of particles 4 µm(c) and larger in each millilitre of oil before the filter, from a laboratory or online particle count.
Units and format
Enter a number in per mL: per mL. The form accepts 0 per mL or more. Required: the tool does not calculate until every input is valid.
Example
The form opens with 15000 per mL as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Get it right
A cumulative count that includes all larger particles, so it must not be smaller than the 6 µm count.
What this means: ≥ 6 µm
Illustration: ≥ 6 µmA row of identical items being counted; ≥ 6 µm is how many there are.nExample: 3000 per mL≥ 6 µm
What it is
Particles 6 µm(c) and larger per millilitre upstream of the filter; cumulative, so it includes the 14 µm and larger particles.
Units and format
Enter a number in per mL: per mL. The form accepts 0 per mL or more. Required: the tool does not calculate until every input is valid.
Example
The form opens with 3000 per mL as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Get it right
Must lie between the 4 µm and 14 µm counts.
What this means: ≥ 14 µm
Illustration: ≥ 14 µmA row of identical items being counted; ≥ 14 µm is how many there are.nExample: 1000 per mL≥ 14 µm
What it is
Particles 14 µm(c) and larger per millilitre upstream, the largest size range in the three-number ISO 4406 code.
Units and format
Enter a number in per mL: per mL. The form accepts 0 per mL or more. Required: the tool does not calculate until every input is valid.
Example
The form opens with 1000 per mL as a worked example. It is an illustration, not a recommended value: replace it with your own figure.

Filter beta ratios

What this means: β at 4 µm
Illustration: β at 4 µmTwo wheels of different sizes linked together; β at 4 µm compares one with the other.β₄Example: 10β at 4 µm
What it is
The element’s beta ratio at 4 µm(c): upstream count divided by downstream count in the maker’s multi-pass test.
Units and format
Enter a plain number with no unit. The form accepts 1 to 100000. Required: the tool does not calculate until every input is valid.
Example
The form opens with 10 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
Downstream count is upstream ÷ β, so doubling β halves the count after the filter.
What this means: β at 6 µm
Illustration: β at 6 µmTwo wheels of different sizes linked together; β at 6 µm compares one with the other.β₆Example: 75β at 6 µm
What it is
The beta ratio at 6 µm(c) for the chosen element, from the same multi-pass test data.
Units and format
Enter a plain number with no unit. The form accepts 1 to 100000. Required: the tool does not calculate until every input is valid.
Example
The form opens with 75 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: β at 14 µm
Illustration: β at 14 µmTwo wheels of different sizes linked together; β at 14 µm compares one with the other.β₁₄Example: 200β at 14 µm
What it is
The beta ratio at 14 µm(c) for the chosen element, from the maker’s data sheet.
Units and format
Enter a plain number with no unit. The form accepts 1 to 100000. Required: the tool does not calculate until every input is valid.
Example
The form opens with 200 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.

Target code

What this means: Target at 4 µm
Illustration: Target at 4 µmA number line with a marker at the value of Target at 4 µm, between its lowest and highest accepted values.030Example: 18Target at 4 µm
What it is
The first number of the ISO 4406 cleanliness code you need after the filter, covering particles 4 µm(c) and larger.
Units and format
Enter a plain number with no unit. The form accepts 0 to 30. Use a whole number. Required: the tool does not calculate until every input is valid.
Example
The form opens with 18 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Get it right
A code range number, not a particle count; each step up roughly doubles the allowed count.
What this means: Target at 6 µm
Illustration: Target at 6 µmA number line with a marker at the value of Target at 6 µm, between its lowest and highest accepted values.030Example: 16Target at 6 µm
What it is
The second number of the target cleanliness code, covering particles 6 µm(c) and larger.
Units and format
Enter a plain number with no unit. The form accepts 0 to 30. Use a whole number. Required: the tool does not calculate until every input is valid.
Example
The form opens with 16 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Target at 14 µm
Illustration: Target at 14 µmA number line with a marker at the value of Target at 14 µm, between its lowest and highest accepted values.030Example: 13Target at 14 µm
What it is
The third number of the target cleanliness code, covering particles 14 µm(c) and larger.
Units and format
Enter a plain number with no unit. The form accepts 0 to 30. Use a whole number. Required: the tool does not calculate until every input is valid.
Example
The form opens with 13 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.

Pressure drop

What this means: Element pressure drop at rated flow and viscosi…
Illustration: Element pressure drop at rated flow and viscosityA pressure gauge whose needle moves with Element pressure drop at rated flow and viscosity; the needle follows the value you type.pExample: 0.2 barElement pressure drop at rated flow and visco…
What it is
The pressure drop across a clean element read from the filter maker’s curve at the rated flow and viscosity below.
Units and format
Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0 bar to 50 bar. Required: the tool does not calculate until every input is valid.
Example
The form opens with 0.2 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Rated flow
Illustration: Rated flowA pipe with liquid or gas moving through it; Rated flow is the volume that passes per unit of time.QExample: 100 L/minRated flow
What it is
The flow at which the maker quotes the clean element pressure drop.
Units and format
Enter a number in L/min: litres per minute. The form accepts 0.000001 L/min to 100000 L/min. Required: the tool does not calculate until every input is valid.
Example
The form opens with 100 L/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Rated viscosity
Illustration: Rated viscosityFluid layers between a fixed and a moving plate, sliding past each other; Rated viscosity is the fluid’s resistance to that flow.νExample: 32 cStRated viscosity
What it is
The oil viscosity at which the maker quotes the clean element pressure drop.
Units and format
Enter a number in cSt: centistokes (kinematic viscosity, 1 cSt = 1 mm²/s). The form accepts 0.000001 cSt to 5000 cSt. Required: the tool does not calculate until every input is valid.
Example
The form opens with 32 cSt as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Actual flow
Illustration: Actual flowA pipe with liquid or gas moving through it; Actual flow is the volume that passes per unit of time.QExample: 80 L/minActual flow
What it is
The flow through the filter in service, for example pump flow for a pressure filter or return flow for a return-line filter.
Units and format
Enter a number in L/min: litres per minute. The form accepts 0.000001 L/min to 100000 L/min. Required: the tool does not calculate until every input is valid.
Example
The form opens with 80 L/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
The element drop rises in direct proportion.
Get it right
Return filters can see more than pump flow when cylinders with a large area ratio retract.
What this means: Actual viscosity (coldest running)
Illustration: Actual viscosity (coldest running)Fluid layers between a fixed and a moving plate, sliding past each other; Actual viscosity (coldest running) is the fluid’s resistance to that flow.νExample: 46 cStActual viscosity (coldest running)
What it is
The kinematic viscosity of the oil at the coldest temperature the system runs, when the element drop is highest.
Units and format
Enter a number in cSt: centistokes (kinematic viscosity, 1 cSt = 1 mm²/s). The form accepts 0.000001 cSt to 5000 cSt. Required: the tool does not calculate until every input is valid.
Example
The form opens with 46 cSt as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
The element drop rises in direct proportion.
Get it right
Kinematic viscosity in cSt (1 cSt = 1 mm²/s), not dynamic viscosity.
What this means: Allowable clean element drop
Illustration: Allowable clean element dropA pressure gauge whose needle moves with Allowable clean element drop; the needle follows the value you type.pExample: 1 barAllowable clean element drop
What it is
The highest clean element pressure drop you accept, chosen to leave margin below the bypass valve setting for the dirt the element will collect.
Units and format
Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.000001 bar to 50 bar. Required: the tool does not calculate until every input is valid.
Example
The form opens with 1 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.

Method and formulas

  1. Downstream count = upstream ÷ β; efficiency = 1 − 1 ÷ β (as the filter beta ratio tool).
  2. ISO 4406 range number = ⌈log₂(count per mL ÷ 0.01)⌉ (as the ISO cleanliness code tool).
  3. Element drop ≈ rated drop × (Q ÷ Q_rated) × (ν ÷ ν_rated) for laminar flow through the media.
  4. Example: the oil goes from 21/19/17 to 18/12/9 against 18/16/13; the element drops 0.23 bar.

Limitations

  • Single pass through the filter; a real system reaches a balance between ingression and filtration.
  • Housing and bypass valve drops are not included.
  • Example values are illustrations. Replace them with values for your actual task.

Typical uses

  • Choose a filter rating for a target oil cleanliness
  • Check a filter’s pressure drop with cold oil
  • Explain an oil sample that misses its target code

Who it is for

  • Maintenance engineers
  • Hydraulic engineers
  • Reliability engineers

What you get

  • Downstream counts and efficiencies
  • ISO codes before and after against the target
  • Element pressure drop

Interpreting the result

  • Code numbers fall by one for every halving of the count: a β of 200 drops the code by about seven.

References

  • Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2439–2442. Topic reference; no source tables or recommended settings are reproduced.
  • ISO 4406 Hydraulic fluid power — Fluids — Method for coding the level of contamination by solid particles

BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.

Open the Hydraulic Filter Selection (Beta Ratios, Cleanliness Code, Pressure Drop) calculator

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