Development of field test method for ventilation filters.

Ventilation filters are classified according to the results from standard laboratory tests. These tests, however, produce information which may be insufficient for estimating the true filter performance. The properties of a ventilation filter in the ambient air conditions may be quite different compared to filter behavior in 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 a significant decrease in the efficiency. The efficiency of the filter affects strongly on the quality of the supply air. Therefore, the measurement of the filter removal efficiency is of great importance. The guidelines for the in situ determination of the fractional efficiency of ventilation filters have been defined in Eurovent method 4/10 (1). This method includes detailed instructions for the efficiency measurement procedure. It also includes instructions about the proper use of an optical particle counter (i.e. zero check, avoiding of excessive particle concentrations and extremely unfavorable test conditions). The test conditions in the field tests can vary strongly depending on time and the location of the test site. Weather conditions together with the nearby particle sources may affect the concentration, size distribution and the optical properties of the ambient air particles. Thus, it is possible that the properties of ambient air particles vary strongly and rapidly which complicates the efficiency measurement. It is also possible that the concentration of ambient air particles is either too high or too low for a certain measurement instrument.