The sample preparation issue is greatly simplified in the case of LAS-D, since powders are introduced to the measurement device in dry form with dispersion provided internally by the instrument. Aerosol dispersion methods for commercial LAS-D instruments are based on the use of either compressed air or an applied vacuum. In addition, each instrument company incorporates its own proprietary sample delivery and dispersion system, which might include, for instance, use of vibration or other mechanical devices. There were 13 participants who used LAS-D: 85% of these used systems based on compressed air, one used a vacuum based system, and one used a system incorporating both compressed air and vacuum, presumably in series. One should keep in mind that this information was reported by the participant, and is not necessarily an accurate and complete assessment of the instrument's actual specifications or capabilities. The duration of the measurements, another potentially significant measurement parameter, varied from 4−130 s. The median value was 15 s. So, on average LAS-D appears to be somewhat faster than the corresponding LAS-W measurement.
The pressure used during the measurement when compressed air was employed varied from 1−4 bar. As shown in Fig. 8, there is no clear correlation between the diameter of the finer fraction of the particles, represented by D10, and the reported air pressure. This lack of correlation may be due to the high level of scatter from other effects, including user and instrument bias, or these pressure levels may be above the minimum level necessary to fully disperse the material to the extent possible in a dry powder.
Like LAS-W, LAS-D requires the use of an appropriate optical model and, where appropriate, the selection of refractive index values. The majority of round robin participants reported using the Fraunhofer model (≈45%), 31% reported using Mie, 8% used both, and 15% were classified as "other." The category "other" includes those not reporting a specific model or providing information that could not be clearly identified with either Fraunhofer or Mie. The variation in the refractive index reported by LAS-D users is relatively small. This is not surprising, as most used the Fraunhofer model, which does not require knowledge of the optical constants.
In LAS-D, since the dispersing medium is air, the refractive index is needed only for the particle phase. All participants who reported a complex refractive index used 0.1 for the imaginary component. Most of the reported values for the real component were close to 1.7. One participant reported a value of 1.0, which is clearly too low for cement powder. Based on the typical composition of portland cement and the known refractive index values for the individual components ([Cyr et al., 2000]) a value near 1.7 seems appropriate. Again, a procedure for selecting or estimating the refractive index should be established as part of a LAS-based standard.
FIG.8−Relationship between D10 and the reported pressure used during LAS-D measurements.