An aerosol photometer reacts to particulate that is drawn through the viewing area, (optical convergence point) and scatters light forward. The amount of scattered light is directly proportional to the mass concentration of suspended particles within the sampled air. The scattered light is optically focused on a photomultiplier tube and converted to an electrical current which is amplified, processed, and displayed. Therefore an aerosol photometer is an excellent, instantaneous concentration indicator, though to obtain accurate results, the photometer must be calibrated to the aerosolized material whose concentration is to be measured.
An example of this occurred in the early 1960’s. The Naval Research Laboratory (NRL) started calibrating ATI photometer response against the monodispersed aerosol generated in the Q-127 Aerosol Penetrometer to increase accuracy. David W. Crosby, of ATI, designed and patented an adjustable Light Leak that could be set at any point and used to adjust the photometer sensitivity to any particulate concentration level for a specific aerosol. In modern ATI photometers this became the Internal Reference.
When an aerosol photometer is returned for re-calibration it is thoroughly cleaned, optically aligned, electronically calibrated, performance checked, and then it is calibrated to a specific aerosol(s), a polydisperse aerosol having a known NIST traceable concentration. After the photometer has been thoroughly warmed up, an aerosol sample is taken and the photometer sensitivity (gain) is adjusted for a reading of 100%. Using that same 100% gain setting, an aerosol sample with the same particle size distribution and of the same base material, producing a reading of 50% would indicate a concentration of 50 micrograms per liter.
It is easy to assume that the same response would be obtained if another aerosol were measured that was generated by the same nozzle and a different, though similar, liquid. Unfortunately this is not true. Some information on this subject is available in a white paper presented by David W Crosby at the 21st International Department of Energy/Nuclear Air Cleaning Conference in 1990 and also published in the “1993 Proceedings of the Institute of Environmental Sciences” on page 559. In summary, even if the aerosol size is similar, the refractive index varies for different liquids and therefore photometric responses will differ from the gravimetrically determined real-time concentration measurement. To accurately measure concentration, a photometer must be calibrated to the specific aerosol, substance and distribution, that will be sampled.