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Technical Reference

Filter Efficiency Ratings The formula used to calculate the beta ratio is:

This information is provided as an aid to
understanding fluid filter efficiency terminology Beta ratio (x) = particle count in upstream oil
based on current ISO and SAE test standards. particle count in downstream oil
It is not proprietary and may be reproduced or (x) is a given particle size

distributed in any manner for educational purposes.
ß = 75
What is Beta Ratio? (5)
Beta ratio (symbolized by ß) is a formula used
to calculate the filtration efficiency of a particular fluid
filter using base data obtained from multi-pass testing.
Efficiency
In a multi-pass test, fluid is continuously injected with The beta ratio is commonly used to calculate the
a uniform amount of contaminant (i.e., ISO medium filtration efficiency of a filter and can be converted into an
test dust), then pumped through the filter unit being percentage of efficiency at a give particle size. The formula
tested. Filter efficiency is determined by monitoring oil below was used to calculate the performance of filters in
contamination levels upstream and downstream of the this catalog.
test filter at specific times. An automatic particle counter
is used to determine the contamination level. Through
this process an upstream to downstream particle count Efficiency = ß - 1 ß Efficiency
ratio is developed, known as the beta ratio. (x)
ß 2 50 %
5 80 %
(x) is a given particle size
1 particle > 5 microns 10 90 %
(downstream of filter) 20 95 %
75 98.7%
Fluid in
ß 75 is 98.7% 100 99 %
for particles 1000 99.9%
5µm and greater

ß = 75 is same as 98.7%@ 5µm ENGINE LIQUID FILTRATION - TECHNICAL REFERENCE
(5)

10 particles > 5 microns
(upstream of filter)

Micron Sizes of Familiar Particles
Grain of table salt 100 µm
Human hair 80 µm
Lower limit of visibility 40 µm 40 µm
White blood cell 25 µm 80 µm
Talcum powder 10 µm
Red blood cell 8 µm 25
µm
Bacteria 2 µm
Silt <5 µm 100 10 µm
µm 2 µm

www.donaldson.com Engine Liquid Filtration • 113