OCR Text |
Show calculated using Mie scattering theory for a refractive index of 1.35 - lxl0~8i (for boiling sulfuric acid), a HeCd laser wavelength of 4416A, a lens which subtended scattering angles from 168 to 174°, and for an unpolarized laser beam. Figure 3 shows that, while the scattering cross section is, on average, a monotonically increasing function of particle size, it is not monotonic over very small ranges of particle size. The "noisy" behavior of the cross section for the conditions of our measurement effectively places a lower limit on the resolution of the measurement. Other factors which must be considered in the measurement of particle sizes are the transmission of the optics, the steadiness of the laser power, and the attenuation of the incident and scattered beams by the particle-laden gas stream. In industrial environments in particular, optical access windows will become dirty with time, resulting in the need to measure the transmittance of the windows and account for changes in this factor. Also, the laser power is frequently not stable, resulting in the need to measure the laser power several times during a test. Changes in the laser power of 10 to 50%, usually negative, over a time span of 30 minutes are possible. The problem of attenuation of the incident and scattered beams may be the most difficult factor to account for because it can fluctuate much more rapidly than the previous two factors. If the average attenuation is known, fluctuations in the attenuation will tend to statistically cancel out if the attenuation is independent of the size of the particle being measured. If the uncertainty in the attenuation is significant, the accuracy of the particle measurements will be diminished. Since the particle density for the measurements presented in this paper was low, this factor had a negligible effect on the results. The optical design for the laser backscatter particle size measurements which are reported in this paper is shown in Figure 4. A 40 mW HeCd laser was used. The laser emits vertically polarized light, but the beam was effectively depolarized by the use of a quarter wave plate to circularly polarize the light. This was done because the scattering cross section is less "noisy" for unpolarized light than for polarized light. A 14.5 cm diameter plano-convex lens was used to collect the scattered light. An adjustable slit was used as an aperture 24-9 |