The official newsletter of Nanozen Industries

It is well known that the response from such light scattering-based monitors depends on particulate properties such as shape and refractive index. Thus, different calibration factors are required depending on the properties of the sampled dust. Moreover, the photometric response of photometers/nephelometers is known to be strongly dependent on particle size. Photometers typically underestimate mass concentration of coarse particles (larger than 2.5μm) and overestimate mass contributions from finer particles (smaller than 0.2μm). These biases could result in large particle concentration errors due to the high variability of aerosol concentrations and size distributions typically present in workplaces, especially as the monitor changes distance,
orientation, or interference to the source of particles which causes the profile of particle sizes to change. Multiple calibration factors would be required for
different size fractions (e.g. PM10, PM2.5, PM1, inhalable, thoracic, respirable, etc.) of the same aerosol type. Figure 1 above shows the particle size distribution of various mine dust samples. Given the variation in particle size distribution, it is almost impossible to calibrate photometers accurately for every mining activity and site.

Evidently the selection of reference dust used for the calibration of real-time optical monitors has a significant impact on the accuracy of mass concentration readings. If the optical and physical properties of the dust of interest differ significantly from the calibration particle material, dust-specific calibrations against gravimetric references are required to meet desired measurement accuracy levels. Such calibration is usually conducted with direct comparison of the dust monitor to the Federal Reference Method (FRM) using gravimetric filter samplers (U.S. EPA 1997b). Once an OPC has been calibrated to the dust type, it can be used confidently with varying particle sizes and combinations of sizes for that dust. This is not true of photometers/nephelometers for reasons discussed above. Photometers must rely on a specific profile of dust sizes to be accurate and must be recalibrated whenever this profile changes.

Please refer to Figure 2. In a case study, the performance of Nanozen’s DustCount monitor was evaluated for coal mine dust. Coal dust was chosen as a light-absorbing aerosol with substantially different optical properties from the standard Arizona Road Dust used as the default calibration for DustCount monitors. When the OPC-based DustCount was only calibrated with ARD, the percentage error was 87.49% when monitoring coal dust with varying percentages of respirable crystalline silica.

DustCount monitors were then calibrated with a specific coal dust sample against gravimetric filter measurements. Subsequently, a series of runs over several hours were carried out at various coal dust concentration levels and size distributions. DustCount monitor readings were simultaneously compared against a TEOM monitor (Series 1400a, Thermo Scientific). The TEOM, an EPA designated FRM equivalent instrument, was considered as the real-time mass concentration reference along with gravimetric analysis in these tests.