Use of a single particle intra-cavity laser particle spectrometer for measurements of HEPA filters and filter systems

Current tests of HEPA filters and/or filter installations using DOP aerosols and conventional forward light-scatter photometers are limited to measuring protection factors of 104 to 105. In addition, forward light-scattering photometers have markedly decreased sensitivity to less than 0.3 μm particles and basically measure only a scattering signal which is not uniquely related to any given concentration or size distribution of scatterers. These limitations require that high efficiency systems, such as multiple stage HEPA filters, be evaluated one stage at a time, a procedure which is quite often impractical for many existing air cleaning systems and which may be in error. In order to obviate these difficulties, a single particle intracavity laser particle spectrometer has been used to measure protection factors of up to 2.4 x 108 for multiple HEPA systems and individual double ply HEPA filters. Because of the instrumental size resolution, protection factor as a function of particle size can be determined from .06 μm to 2.9 μm. The lack of background enables single counts to be statistically significant. Since coincidence errors occur at particle concentration greater than 106/liter, a known dilution must be introduced to measure challenge concentrations greater than this. The dilution measurement may be accomplished with the aid of a forward light-scattering photometer