Sieves – Importance of Calibration & Certification

The determination of particle size distribution is an essential aspect of the characterisation of drug substances and pharmaceutical excipients. The field is somewhat confusing in the sense that a large number of methods exist for such determinations, and each has its own particular requirements for reduction and display of the results.

Particle size distribution studies on a given sample are performed using microscopy, sieving, or light scattering. Sieving remains one of the most popular methods for the classification of powders.

Particle size analysis using large diameter sieves had the reputation in the past of being irreproducible and inaccurate compared to modern methods. Since the accuracy of the results depended on the accuracy of the sieve apertures it was important to calibrate the sieves. Now that sieve calibration standards are available to measure mean sieve apertures, it is at last possible to accurately quantify sieving performance.

What is Calibration?

Calibration

(n.) The determination of the true value of the spaces in any graduated instrument.

The act or process of calibrating or the state of being calibrated.

What is aperture size?

Space between adjacent parallel wires, in mm.

(25.4 / n) – d where n is the number of wires in one linear inch and d is the diameter of the wire.

The various standards organisations specify that wire spacings and wire diameters should be measured in the X and Y directions. The trapezoidal apertures of a sieve however, have many dimensions depending on where the measurements are taken. No matter how many measurements are made, they are still uni-dimensional and often give little indication of the performance of the sieve on three-dimensional irregular particles.

A more accurate way of measuring aperture size is to measure the dimensions of individual apertures in both the X and Y direction and use the smallest value to give the equivalent spherical diameter of the openings.

Sieves as Calibrating Instuments?

With the increase in the importance of dispersed materials (powders, aerosols, emulsions etc.) to trade, there is an increasing awareness of the need to verify that instruments which measure particle properties, particularly size, are operating within defined limits of accuracy. As a minimum, this process requires some form of verification with reference to the international standards of mass and length (so-called traceability chain). In some cases, a formal calibration to establish instrument response in terms of size, shape or concentration may be required. Most emphasis is placed on particle size, as this is the variable that is generally of most importance to industry.

There is an increasing awareness that instruments used to measure the properties of dispersed systems (powders, aerosols, particles suspended in liquid etc.) cannot provide absolute values by themselves.

Quality systems, such as the ISO 9000 series require that the performance of instrumentation used in the measurement of properties deemed critical to the process for which the materials under investigation are being used, be verified on a regular basis as part of a method validation process, or standard operating procedure (SOP). The intention behind this requirement is to enable measurements made on a particular product at one location to be reproducible within well-defined limits anywhere else. Standard or reference materials (RMs) that are particle-based have an important part to play in this process. The main advantage of the sieving method however, is that it is a very inexpensive and therefore popular method of particle size analysis. Furthermore, when calibrated it becomes an accurate and traceable measuring device with micrometer precision.

Why calibrate a sieve?

Although wire weaving is extremely accurate in view of the technology used, modern day methods of particle size analysis are becoming much more accurate. The present tolerances on the weaving process are therefore considered too wide for some stringent applications such as pharmaceutical manufacturing.

For example, according to ISO 3310 - 1:2000, the mean aperture tolerance of a 250micron sieve is 240 -260microns, with a maximum standard deviation of 272microns. The maximum acceptable aperture is 308microns!

These variations are too great for some applications and the analyst needs to know the aperture size or cut point of the sieve precisely so that when it is replaced, the same results are obtained. The analyst also needs to be sure that the sieve has not been damaged or worn out prior to a critical measurement.

How are sieves calibrated at present?

Very few manufacturers in India know the meaning of calibration & fewer still have the means of calibration.

Pharma Spares has the capability of calibrating Sieves used for Industrial purposes using specialist and expensive Image Analysis methods. Individual apertures in the sieves are measured in the X and Y direction and the smaller dimension used, as it will be this dimension that retains a spherical particle. The aperture size is therefore the equivalent spherical diameter of the opening.

These openings are compared to the International specifications & a certificate of conformity is issued. If a sieve is suspected of being out of specification, after a certain period of use, it must be returned to the manufacturer for re-calibration.

What is the traceability of the standards?

The calibration method relates the measurements to an international unit of length such as NPL (a calibration graticule for microscopy from the National Physical Laboratories) so there is recorded traceability.

A set of sieves are calibrated against the National Physical Laboratory, Graticules to provide a direct link to the International unit of length.

How are the sieve standards certified?

Several standards have been designed in order to asses the quality of individual sieves. Thus, a sieve is suitable to be used as a measurement instrument only if it conforms to a certain standard. These standards basically set the tolerances for aperture and wire diameter sizes, as a function of the sieve nominal aperture.

In order to asses the compliance of a sieve with respect to a standard, a great number of apertures and wires throughout the sieving medium must be measured. Next, the maximum aperture size, the deviation of the mean aperture size from the nominal one, the number of apertures between the mean and the maximum aperture size, and finally the mean wire diameter, must be within the allowed tolerances tabulated by the standard.

Certified International standards are: ASTM: USA, JIS: Japan, BSI: United Kingdom, AFNOR: French, Tyler®: USA, DIN: German.

ISO: The International Organization for Standardization is a worldwide federation of national standards bodies (ISO member bodies).

ISO 9044 standard governs industrial wire cloth.

Pharma Spares Industrial Sieves Inspection System

It is an automatic digital Image Analysis inspection of sieves to assess its quality. It is able to perform the inspection of sieves with nominal aperture size ranging from several tens of microns to 1 cm. It is able to assess the compliance of a Particular sieve with respect to several certification standards.

Two main advantages are the dramatic increase of inspection speed and a higher accuracy. A much greater number of apertures are measured in less time and at a higher resolution than the manual methods currently employed. Our system is able to measure hundreds to thousands of apertures in a few minutes, depending on the nominal aperture which determines how many openings will be visible in a single image. Furthermore, measures have shown to be very accurate. Finally, it stores and prints an inspection report displaying the aperture size distribution, intended to be used as a quality certification document.

What Are Certified Sieves?

Certified sieves are standard Sieves, re-examined by an optical imaging system. Certified sieves are serialized and supplied with documentation detailing the dimensions of the openings and wire diameter of the mesh screen.

Each is supplied with a Certificate of Conformity and individually numbered to provide full traceability. 

Pharma Spares can supply a unique range of high quality traceable precision sieves for the pharmaceutical, food and fine processing industries.

The meshes for the new range of sieves have been pre-selected to give an actual aperture size, as close as possible to the normal size in both ISO and ASTM sieve series.

Features:

• All sieves are individually calibrated
• Actual size very close to nominal size
• True sizes, not wire spacing meaured
• Aperture sizes from 20micron - 10mm

Pharma Spares Certified sieves are engineered to the rigorous standards of the International Organi-zation for Standardization (ISO), Standard 9044-1999. These Standards detail a number of specifications to which any rated sieve will adhere. These ratings include permissible opening sizes, opening dimensions and number of allowable maximum openings in each sieve.