Ventilation filters are normally classified with the aid of standard laboratory tests. The most well known standards are the European standard EN779 (Anon. 1993) and the corresponding American standard ASHRAE 52.2 (Anon. 1999). Standard tests, however, produce information which may be insufficient for estimating the true filter performance. It has been observed that in many cases the filter loading with the atmospheric particles differ significantly from the one in the standard test. Thus, the true efficiency may be much lower than expected from the standard test. The disagreement may be especially strong in the case of electrostatically charged filters. The loss of electrostatic removal mechanism can lead to significant decrease in the efficiency in field conditions. The efficiency of the filter affects strongly on the quality of the supply air. Therefore, the correct measurement of the filter removal efficiency is of great importance.
The removal efficiency of a ventilation filter is a function of particle size. Thus, it would be desirable to determine how the filter performance depends on the particle size. This can be easily accomplished in the laboratory where ideal test conditions can be created and sophisticated particle measurement instruments can be used. In field conditions, however, the test conditions may be much more difficult and therefore the normal laboratory procedures cannot be directly applied. Besides fractional efficiency or penetration, the performance of a filter is sometimes characterized with more practical efficiency values. These values include the total mass efficiency or the mass efficiency for respirable particles or fine particles (i.e. dp < 2.5 ?m). This requires quite different test procedure and measurement techniques.
The most tempting alternative for filter measurements in field conditions is optical particle counter. This is mainly due to the high sensitivity and the easiness of use. The sensitivity of an optical particle counter is high enough to make it possible to measure filter efficiency or penetration with atmospheric particles. Another reason for the use of an optical particle counter is the fact that it has been successfully applied in filter efficiency measurements in laboratory.