OCR Text |
Show the measurement volume. The real part of the refractive index of boiling sulfuric acid is known to within 3% uncertainty, while the imaginary part is known to be very small ("10" ) such that it has no effect on scattering. The uncertainty in the scattering cross section due to the 3% uncertainty in the real part has been calculated to be a factor of 2 for particle diameters greater than 20 um« The cross sections for smaller diameters are nearly equal, but quite "noisy". The resonant character of the cross section, which is particularly strong at low particle sizes, does not limit the absolute accuracy of the size determination but does limit the size resolution in the resonant region. The uncertainty in the size distribution due to statistical noise is equal to n^~ • where n^ is the number of counts in the ith particle size bin. Size bins having 10 counts, for example, will have an uncertainty in the number of counts of 1// 10 or 32%. Statistical noise, therefore, affects the relative distribution of particle sizes, but it does not in general affect the absolute accuracy of the size determination. The fourth factor affecting the size deconvolution is the uncertainty in the laser intensity distribution and the particle trajectory distribution in the measurement volume. These distributions determine the N x N deconvolution matrix which Is used to obtain the size distribution. The laser intensity distribution is determined by the optical alignment. For our measurements, the alignment was both very stable and repeatable, so the uncertainty in the laser intensity distribution was small. Calculations of the deconvolution matrix as a function of the particle trajectory distribution indicated that this distribution had an insignificant effect on the matrix values. Uncertainty in the deconvolution matrix would, like statistical noise, affect only the accuracy of the relative size distribution. Of the four main factors affecting the accuracy of the size deconvolution, three affect the relative size distribution while one, namely the uncertainty in the complex index of refraction, affects the absolute value of the particle sizes. Uncertainty in the signal strength distribution also affects the absolute value of the particle sizes. Since the uncertainty in the signal strengths is 20%, while the uncertainty in the differential scattering cross section is ~100% (due 24-19 |