Pharmaceutical Sieves.
Sieves are the simplest and sieving is the most frequently used method for size separation.
Construction:
Sieves for pharmaceutical testing are constructed from wire cloth with square meshes, woven from wire ,of brass, bronze, 'stainless'' steel or any suitable materials.
Sieves should not be coated or plated.
There must be no reaction between the material of the sieve and the substance to be sieved.
Types of Sieves:
Woven wire sieves.
Plain weave
Twilled weave
Bolting cloth sieves.
Bar screens.
Punched plates (Perforated Screens)
Standards of Sieves:
Common standards used for sieves are:
(a) Tyler standard sieve series (in USA)
(b) US standard. sieve series (in USA)
(c) British standard sieve series (in UK)
(d) German DIN (Deutsche lndustrienormein) (in Germany"and Europe)
(e) IP standard sieve series (in India)
(f) International test sieve series (ISO) (World wide).
It is required that wire-mesh sieves will be made from wire of uniform, circular cross-section and for each sieve the following particulars are stated.
Sieve Number:
This is the number of meshes in a length of 25.4mm (i.e. 1 inch), in each direction.
Nominal size aperture:
This is the distance between the wires, so that it represents the length of the side of the square aperture.
Nominal diameter of the wire:
The wire diameter is selected to give a suitable aperture size.
It is also required to give the necessary strength to avoid distortion.
The diameter of the wire is represented by Standard Wire Gauge.
Approximate screen area:
This standard expresses the area of the meshes as a percentage of the total area of the sieve.
It is governed by the diameter of the wire.
It is generally kept within 35 to 45% of the total area of the sieve.
This represents the useful area of a sieve. Greater screen area is preferred.
Aperture tolerance average:
Some variation in the aperture size is unavoidable and this variation is expressed as a percentage, known as aperture tolerance average.
It is the maximum limit within which the dimension of meshes can be allowed to vary and still be acceptable for sieving.
Finer wires are likely to be subject to a greater proportional variation in diameter than coarse mesh.
Commonly Asked Questions.
Define,
Aperture tolerance average.
Approximate screen area.
Nominal diameter of the wire.
Sieve Number.
Nominal size aperture.
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