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How Big is a Micron

The Beta Rating System

A beta ratio (symbolized by the Greek letter β) is a formula that is used to calculate the filtration efficiency of a particular fluid filter by using base data obtained from multi-pass testing. The purpose of a multi-pass test is to have a lab based procedure that will give one reproducible data that can be used to evaluate the efficiency of hydraulic filters. In a multi-pass test, fluid is continuously injected with a uniform amount of contaminant then pumped through the filter unit being tested. Filter efficiency is determined by monitoring the fluid contamination levels upstream and the contamination levels downstream of the 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 ratio is developed, known as the beta ratio.

The formula used to calculate the beta ratio is: Beta ratio(x)= particle count in the upstream fluid
Where (x) is a given particle size

For example: β10(c)=1000 represents that there are 1000 times more particles upstream than particles downstream that are 10µm or larger. The (c) indicates that the testing was done with APC’s calibrated with NIST fluid.

Beta stability is the measurement of how well a filter is able to hold up its measured beta ratio when the pressure drops below normal standards. Ex: beta 200 stability=210 psid means that the beta ratio will not fall below 200 until it reaches 210 psid.

How large is a micron?

It is difficult to put the measurements and data one gets from the Beta rating system in perspective, particularly when the most of the particles are smaller than a grain of salt. The unit of measurement for these tiny objects, white blood cells, bacteria, and silt being just a few of these things, is often a micron. One micron is .0001 centimeter, or .001 millimeter. In fact, 40µm is the lower limit of visibility, any object that is smaller a person can’t see without the help of a microscope. Keeping all this in mind, it is easy to see how most people don’t really understand how miniscule objects measured in microns really are.

A Chart of the Micron Sizes of Familiar Particles

Screen Shot 2016-08-08 at 2.26.23 PM

*Image courtesy of the Donaldson Hydraulic Filtration Product Guide
A Grain of table salt: 100µm
A Strand of Human hair: 80µm
A Pin Point: 75µm
The Lower limit of visibility: 40µm
A White blood cell: 25µm
Talcum powder: 10µm
A Red blood cell: 8µm
Can be felt with a fingertip: 5µm
A Grain of Silt: <5µm
The Smallest Bacteria: 2µm


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