Calibration of sieves

It is not widely realized that analyses of the same sample of material, by different sieves of the same nominal aperture size, are subject to discrepancies that may be considerable. These discrepancies may be due to non-representative samples, differences in the time the material is sieved, operator errors, humidity, different sieving actions and differences in the sieves themselves which may be due to wear. 

One way of standardizing a single set of sieves is to analyze the products of comminution. It is known that the products are usually log-normally distributed hence, if the distribution is plotted on a log-probability paper, a straight line should result. The experimental data are fitted to the best straight line by converting the nominal sieve aperture to an effective sieve aperture. 

The traditional way of determining the median and spread of aperture sizes for a woven wire sieve is to size a randomly selected set of apertures using a microscope. Due to the method of manufacture, the measurements for the warp and weft will tend to differ. The limiting size may also be determined by using spherical particles. These are fed on to the sieve which is then shaken and the excess removed. Many spheres will have 

Standard polystyrene spheres are also available for calibration purposes from Gilson and Duke Standards. Stork Screens Inc. measure apertures and screen wire dimensions using wax impressions and the impressions are examined with an automatic image analyzer. 

Sieves may also be calibrated by a counting and weighing technique applied to the fraction of particles passing the sieve immediately prior to the end of an analysis. These will have a very narrow size range and the average particle size may be taken as the cut size of the sieve. A minimum number ( ) of particles need to be weighed to obtain accurate volume diameters let this weight be m and the particle density be / then  

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The above equations apply only when the weight-in-size distribution is known to follow the log-probability distribution. Their application to a complete sieve-analysis also depends upon calibration of each sieve of the series used as explained in subsequent paragraphs,... 

Table 21—Hatch s (1933) Calibration of Four Sieves (Tyler Series) Using...

Material retained on (mesh) Size of sieve opening (/i) Calibration size (m)... 

Complete instructions and procedures on the use and calibration of testing sieves are contained in ASTM STP447B [7]. Contents include analytical methods, information relating to wire mesh, perforated plate and micromesh sieves, dry and wet testing and other methods. This publication also contains a list of all published ASTM standards on sieve analysis... 

All aperture widths shown in Figure 4.7 are for nominal 50 pm sieves but comparative tests on the same material using these sieves could yield enormous differences in the residual weight. In order to obtain agreement between different sets of sieves it is therefore necessary to calibrate them and thenceforth to monitor them to detect signs of wear. 

The results may be expressed in terms of the nominal size, although it is preferable to use calibrated sieves. A reference set of sieves should be used after every fiftieth analysis for comparison purposes in order to detect wear. In essence, the smaller the sieve loading, the more rapid is the sieving operation. The low weights however lead to errors in weighing and intolerable percentage losses. 

The Community Bureau of Reference has five quartz samples prepared as reference materials for the calibration of apparatus, these are described in chapter 11. These powders were analyzed by gravity sedimentation except for BCR 68 which was analyzed by sieving. Analyses were carried out at five laboratories and the results compared to give a measure of the quality of the standards. 

Fig. 5 When calibrating a sieve mesh with trapped irregularly shaped grains, a subset of powder grains is obtained that can be used to generate a shape description of the powder grains. (From Ref. l) (A) Profiles of typical sand grain trapped in a sieve mesh (B) length and width distributions of sand grains trapped in a sieve mesh and (C) elongation ratio (shape) distribution for sand grains trapped in a sieve mesh.

The material of the sieves can be important as well. One should make sure the sample analyzed is not reacting with the material of the sieve to avoid any unforeseen reaction that may change the particle size or shape during the analysis. Other than the sieve size and sieve materials, wear on the sieve can also result in data inaccuracy. Thus, it is important that one examines the sieve for wear and tear prior to the sieve analysis. Calibration of the sieves should also be done routinely to ensure data accuracy. 

Fig. 16 Calibration curves of the p(2-VPy) standards for two different PEG types and the mixture of PEGs with different contour lengths. The plus symbol represents the theoretical curve for the mixture of sieving PEGs calculated from measurements on the single-PEG systems...

Cas samples were periodically removed through a valve-controlled port at the top of the reaction vessel. A portion of each sample was injected into a Varian Aerograph Model 920 gas chromatograph with either a 5A molecular sieve column for measuring H2 and CO or a silica column for measuring CO2 and CO. Known H2/CO mixtures were used for calibration of the molecular sieve column. 

The sampler sucks sample of gas at a constant rate throngh sieves of different pore sizes to collect particles of size below 3 pm, between 3-6 pm and above 6 pm. The particles of different size collected in 8 h (or longer time as per instractions) are weighed. Since gas volrrme sncked in these 8 h is also known dne to calibration of snction blower, the SnspendedParticrrlate Matter (SPM)/nm can be calcrrlated. The sieve pore sizes shorrld be selected as per need to analyse the gas sample. 

The amount of specks (i.e., black specks in maize grits, bran specks in flours) can negatively affect the appearance of processed snack foods. Specks are present in dry-milled fractions due to broken sieves or the improper calibration of purifiers. 

The particle size distribution of AP was determined and Compared using the Ro-Tap, the Alpine Air Jet Sieve and the Micromerograph. For this purpose six different samples of ground AP were chosen and separated using sieves calibrated with National Bureau of Standards spherical glass beads. The results were plotted... 

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