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All types of filter media are specified in a similar manner. Each has its own physical characteristics which are given on their own individual pages whilst their grading and its representation is common to all.

The specification of any filter media is based upon a sieve analysis. This is produced by placing a representative sample of about 100grams onto the top of a stack of test sieves of reducing aperture size. The stack is vibrated until the sample has divided into its various sizes. A table of results is recorded from which a grading curve may be drawn. An example best illustrates the process.

Example   Filtration Anthracite    Grade 2     1.18 - 2.50mm
                    Sample 132 grams

Sieve Retained Retained Retained Passing
mm grams % accum. % accum. %
2.50 4.10 3.10 3.10 96.90
2.00 26.90 20.40 23.50 76.50
1.70 39.10 29.60 53.10 46.90
1.40 39.70 30.10 83.20 16.80
1.18 18.20 13.80 97.00 3.00
Tray 4.00 3.00 100.00 0.00
132.00 100.00
The Results are plotted on a graph from which the media's specification can be obtained. Three types of graphical representation may be used which are discussed on the Graphs Page.
Specification   Example Comments
Top Size d95 2.50 mm maximum 5% over size
Bottom Size d5 1.18 mm maximum 5% under size
Effective Size d10 1.33 mm 10% passing
Uniformity Coefficient d60/d10 1.37 usual maximum 1.50
Hydraulic Size 1.69 mm see hydraulic page
Minimum Size d1 0.95 mm 1% passing
Over Size   3.10% maximum 5%
Under Size   3.00% maximum 5%
Size Ratio d95/d5 2.12 Usually about 2