Laboratory tests - Standard test methods

Particle size analysis

Particle size analysis is the process of splitting a sand into specific grain sizes and recording the percentage of the total sample mass retained on each sieve.

Certain conventions are current, amongst which is the calculation of AFS using BS mesh numbers. There will be a gradual shift to the use of micrometers for sizing of sieves and as this happens the old BS mesh numbers will become irrelevant.

However, it is recognised that AFS is currently used by many organisations in advertising literature for foundry and horticultural grades. Consequently the calculation of AFS is retained here.

Apparatus

Mechanical sieve shaker
Balance accurate to 0.1 g
Sieve brush with coarse and fine ends

The nest of sieves for granulometry, AFS and AGS calculation (with receiver and lid) are:

(Sieves to meet BS 41 O requirements)

Note:

Sieves with intermediate apertures may be added as required although these are not used in the AFS/AGS Calculations. Tests results involving non standard sieves must be adapted to the standard distribution above, prior to calculation.

Method

Weigh approximately 1OOg accurate to one decimal place of the representative sample of the material under test, and place into the top sieve. (This weight being recommended to avoid overloading of any particular sieve). Attach the lid and fit the nest on to the shaker. Shake for at least 15 minutes.

Remove the sieves and weigh the fraction retained on each sieve to one decimal place. Express each fraction as percentage w/w of the total sample.

Sieve Care & Replacement

Sieves must be kept in a clean and undamaged condition. Regular visual inspections for tears, holes etc. must be made and sieves should be cleaned regularly to prevent blinding of the apertures. An ultrasonic bath (preferably with sweeping frequency controller) is strongly recommended for this purpose.

Sieves are subject to wear during use. As a result of this there is a tendency for apertures to change size. It is essential to employ some sort of comparison technique to monitor the wear, and to derive sensible wear factors which determine when the sieve should be replaced.

One way to do this is to compare the test results with a standard sieve set which is used only for this purpose. A standard sand may be used for this purpose but it is important to recognise that sieve analysis is not a high precision technique and that a standard sand may give surprising variations from test to test and, particularly, from operator to operator. It is essential to know the limits of scatter generated by the system before decisions can be taken concerning rejection of sieves due to wear.

Note 1

Different types of sieve shaker have different sieving actions which may lead to variations in results on a similar sample over a 15 minute period.

Note 2

When removing sand grains from the sieves the coarse end of the brush is used on the reverse

side of the sieves down to 250 μm only. Sieves finer than this must be brushed with the fine end to preventdamagetothesieves. Consistentbrushing techniques covering the whole sieve area should be used with no attempt to remove individual grains deliberately.

Note 3 Sieve Weighing Computer System

This technique lends itself to a semi automated procedure where the tared and loaded sieve weights are directed to a Personal Computer via the output port on the balance. The sieve analysis, AGS and AFS calculations are performed by the computer and may be presented in any convenient reporting form. This data may also be included in a Laboratory database and may be especially useful where computers are networked on a site or connected by a L/MS (Laboratory Information Management System).

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