New pore size measurement for air filters

Whitehouse Scientific, a leader in the field of particle size standards, sieve calibration and filter testing, has developed a new device for measuring the retention properties of filter elements in compressed air lines.

The principle of the technique is to challenge the air filter with a dilute suspension of glass microspheres, drawn through a vibrating filter under vacuum. The particle size of the microspheres passing, as measured by microscopy and image analysis, provides an unambiguous measure of the cut point of the filter. This is especially important in critical applications such as pharmaceutical processing or air and oil filters in military vehicles. Hitherto, the performance of such filter elements could be measured only in sheet form, as when fabricated into three-dimensional forms the retention properties could be seriously compromised. This is exemplified by tests performed in Whitehouse's laboratories, where a 5µm filter tested as a conical cylinder allowed the passage of microspheres of up to 130µm in diameter. The Whitehouse method uses narrow particle size distribution standards, which give extremely accurate results that are traceable to NIST. Maximum pore sizes from more than 1mm down to 3µm can be measured. Whitehouse Scientific is the highest ranking European certification laboratory for primary methods of particle size analysis. It has filled in excess of one million bottles using a unique 100 stage spinning riffler system.