Understanding laboratory test sieve calibration and ISO/BS, ASTM standards tolerance
B.S410/I.S.O.3310 and ASTM E 11 specify the technical requirements and corresponding test methods for test sieves. Understanding the standard tolerance and calibration parameters will help users optimise accurately， the performance of the test sieve.
1. Common Misconception
Many users have the common misconception that each aperture is the exact same size, and expect a sieve would cut a particle size distribution to that size. Even with a test sieve that is made from the best weaving technology, there is still a tolerance allowed on aperture sizes and deviation according to BS/ISO and ASTM standards.
2. Understanding the standards and the tolerance
It is important to understand the limitations and tolerance in order to conduct more accurate testing.
First we need to understand how each aperture size is measured.
The aperture sizes are measured on the centre-lines of the aperture separately in warp and weft directions as shown in the Figure 1. Aperture in fine sieves are examined by smart scope equipment.
The minimum number of apertures, for a 200mm diameter sieve, should be measured by the list of Table1. For other diameter, the number can be modified in proportion to the sieve area.
Then we need to understand the three parameters of the aperture in the standards:
(1) Maximum aperture size
In both ISO/BS and ASTM standards, aperture sizes are allowed to have variation, but the maximum permission (+X) is set up.
Taking 180micronmeters (No.80) test sieve as an example: the maximum variance is 47micronmeter (X=47microns) in ISO/BS and ASTM. For a 180micron (No.80) test sieve, any aperture size exceed 227micronmeter is not compliant with the standards.
(2) Average aperture size
The average aperture size cannot depart from the norminal size by ±Y.
For example, 180 micronmeter test sieve (No.80), minimum number of aperture size in both direction is 100. The average aperture size should be in the range of 187.61microns to 172.4microns (Y=7.6microns).
The average aperture size cannot depart from the principle size by ±Y.
(3) Maximum standard deviation
For maximum standard deviation, ISO/BS and ASTM have different requirements. ASTM standards categorize Compliance Sieves, Inspection Sieves and Calibration Sieves by different maximum standard deviation.
For example, 180 micronmeter test sieve (No.80), ISO/BS maximum standard deviation is 18microns. ASTM set up different standard deviation tolerance for Compliance Sieves, Inspection Sieve and Calibration Sieves. Inspection sieve is required to comply to the strictest tolerance, which is 14.24microns, then followed by Calibration Sieve 14.65microns, and Compliance Sieve 18.56microns.
As shown in Figure2, taking 180microns test sieve (NO.80) as an example, the ISO/BS standards allows the average aperture to vary from 172.4microns to 187.61microns, max standard deviation is 18microns, but no aperture is allowed to be bigger than 227microns. The blue columns in the Figure2 are actual calibration results of a Glenammer test sieve. Compared with the standard tolerance, Glenammer test sieve apertures are distributed much more evenly around the nominal size. Standard deviation is only 1.9 microns, whereas, ISO/BS compliance tolorance is 18 microns.
Considering the ISO/BS and ASTM standards allow apterture size variation, particles bigger than nominal size might come through. Therefore, if particles are required to cut to certain norminal size, we would recommend users choose a sieve that is slightly smaller than the nominal size to get a more accurate result.
1.BS410-1:2000 ISO 3310-1 International Standard Test sieves-Technical requirements and testing- Part1: test seives of metal wire cloth. (2000). Fourth edition 2000-07-15.
2.ASTM Standard E11-09 Specification for Woven Wire Test Sieve Clothes and Test Sieves. (2009). .
3.Rideal, G. (2006). Just how effective is your sieve. [Blog] Laboratory News. Available at: https://www.labnews.co.uk/features/just-how-effective-is-your-sieve-01-03-2006/ [Accessed 13 Sep. 2017].