Concentration Measurement with Photometers

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.

The mathematical formula for the light scattered forward by the particle is not a very difficult, complex formula.  Basically, it says that the larger the particle, the more light scattered forward exponentially.  For example, an aerosol photometer will give you the same signal for ten one-micron particles as it would for one ten-micron particle. Since the aerosol photometer looks at a particulate sample as a group, it is a good instrument for indicating concentration. 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 t 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 is currently called the Internal Reference.

When an aerosol photometer is returned for recalibration it is thoroughly cleaned, optically realigned, 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 gain setting, a different polydisperse aerosol sample 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 is varies for different liquids and therefore photometric responses will differ from the gravimetric real-time concentration measurement.  To accurately measure concentration, a photometer must be calibrated to the specific aerosol that will be sampled.