To determine BC concentrations, the ObservAir measures the optical absorption of particulate matter collected on the disposable filter tab. As the sensor draws in polluted air, light absorbing BC accumulates on the filter, and the intensity of light transmitted through the filter dims predictably over time. For example, if the ObservAir is drawing 100 ccm of air and the light signal dims by 10% over 1-minute, this correlates to an average BC concentration of 5 ug/m3 (not actual figures). However, if the light intensity dims too much, this mathematical relationship deteriorates, and the BC signal degrades (BC concentration measurements are lower than they should be). Therefore, the filter must be replaced periodically to maintain BC measurement performance.
The filter should be replaced when the light intensity has dimmed to ~40% of its original value with a clean filter (optical attenuation value of ~80). Given this threshold, the filter replacement interval is a function of the average BC concentration and the ObservAir's sample flow rate. The higher the BC concentration and flow rate, the shorter the filter life. The table below shows recommended filter replacement intervals at various BC levels and flow rate settings. For example, at a concentration of 1 ug/m3 and 100 ccm flow rate, the filter will last about 3 days before requiring replacement. Setting the sample flow rate is a balance between maximizing filter life and maintaining BC measurement resolution - please see the ObservAir manual for detailed instructions/guidance.