Test Sieve Calibration Approaches and Uncertainty of Measurement
Sieving analysis is widely used in particle metrology in laboratories all over the world. It is the least expensive particle analysis method. However, one of the largest sources of variation in sieving analysis are the test sieves themselves due to the fact that internationally agreed standards for mesh manufacturing have a small tolerance in the standards (ISO 3310 and ASTM E11). Wire size, average aperture size and uniformity of aperture are specified with a certain set of tolerances in test sieve manufacturing.
The permissible variations for test sieve cloth differs from inspection test sieves, compliance test sieves, and calibration test sieves. The tolerance of inspection test sieves is the tightest, followed by calibration test sieves and then compliance test sieves. Glenammer only manufacture sieves to the standards required for inspection sieves. Therefore, guaranteeing the highest level of accuracy for particle analysis.
There are various approaches to measuring the variation.
1. High-powered Optical Method
Optical microscope is the most common calibration method at present. A large number of apertures and wires are selected to be measured in warp and weft directions. The distance between wires is measured as aperture size and wire width is measured as wire diameter.
The optical microscope calibration device uses light to illuminate the apertures size/wire width. Then it captures the image onto a computer to be analysed with a programme. A detailed report is automatically generated after the calibration procedure, in which every single aperture/wire inspected will be displayed on a numerical readout. The final calibration certificate will indicate the parameters requested in the standard.
Advantage: This methodology is measured with an accuracy of. So far, it is one of the most accurate calibration methods. Glenammer offer an in-house calibration service and UKAS calibration is also available.micrometer
The uncertainty is due to the relatively small sample of apertures.
For example, in ISO 3310 part 1, for a 200mm diameter 63 microns test sieve (No. 230), over 500 apertures are required to be measured in two directions. The mesh has 2.5 million apertures in total, but only 0.02% are inspected.
The accuracy of measurement will be reduced, because parameters of average aperture size, standard deviation and maximum aperture size (three main measurements used in the standard) calculated will vary from the actual mesh apertures.
2. Calibration Microspheres
Fine calibrated spherical glass beads are used to measure the sieve aperture size. To conduct the calibration, the beads need to be placed on the sieve first and shake until the beads stop passing through, then weigh the beads which have passed through, and calculate the percentage by weight. The effective aperture can then be read from the calibration curve.
Calibrating test sieves with microspheres has the advantage of being quickly done without needing special equipment.
Compared to the Optical microscope method, it is a less time-consuming method. More than one test sieve can be calibrated at the same time if they are nested and shaken together.
The aperture size is supposed to be measured by the beads diameter. The accuracy is highly dependent on the symmetry of the beads. This is because if the beads are not 100% spherical, the width of the beads won’t be exactly the same as the diameter.
There are other calibration approaches that are not as commonly used as professional calibration methods, but users can easily conduct these. For example, Material Testing Method and Etched Glass Slide Methods.
Material Testing Method is similar to conducting a normal particle size analysis. First, let the sample run through test sieves and weight the residue. Then compare the value with the “master sieve set”. However, the accuracy for this method cannot be guaranteed. It is possible that a test sieve can provide the same performance as the “master sieve set” but not meet the standards.
Glass Slid Method is basically counting the number of wires per inch of the mesh cloth to find out the aperture size. This method is limited to a certain range of aperture sizes in ISO/BS standard that can be measured. The accuracy is also questionable because the method is based on the assumption that every wire has the same diameter. In practice, wires cannot be guaranteed to have exactly the same diameter. Hence, this measurement can only provide a rough estimate for the aperture size.
In general, the High-powered Optical Method is the most accurate and reliable calibration method that have been widely used to measure the test sieves variation. Calibrating the test sieves regularly can minimize the analysis errors in the experiments. Glenammer can provide calibration services for brand new test sieves as well as used test sieves. Please contact our technicians if you require more information. +44 01292 261 444
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