Laserfiche WebLink
<br />I <br />I <br />I <br /> <br />27 <br /> <br />center of the laser beam. This sampling system has a number of <br /> <br />advantages and avoids some the measurement problems associated with <br /> <br />these instruments when they sample the free air stream from aircraft <br /> <br />I <br />I <br />I <br />I <br />I <br /> <br />(Dye and Baumgardner, 1984; Baumgardner et al., 1985; Cooper, 1988). <br /> <br />The sample volume in our FSSP system is determined by the size of the <br /> <br />laser beam (constant -0. 2mm), the depth of field (DOF) , and the air <br /> <br />speed (constant). In aircraft FSSP systems, the DOF and the air speed <br /> <br />can vary: the DOF is determined with a combination of optics and <br /> <br />electronics and is typically 2 to 3mm. With an FSSP sampling in the <br /> <br />free stream, there are many droplets outside the DOF, and they can <br /> <br />produce significant coincidence errors in the droplet size measurements <br /> <br />I <br />I <br />I <br /> <br />(Cooper, 1988). However, in our system, the DOF is defined by the size <br /> <br />of the airstream (0. 65mm) carrying the droplets. This air stream is <br /> <br />centered in the optical DOF; there are no droplets outside the DOF. <br /> <br />The isokinetic sheath flow keeps the edges of the droplet stream <br /> <br />intact. <br /> <br />This was verified with smoke tests. The particle stream <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />velocity is higher than the minimum required, yet is well below the <br /> <br />velocity where size corrections become necessary for this factor <br /> <br />(Cerni, 1983). At the same time, the volume sample rate of <br /> <br />. 1 1 cm3 s-l <br />approX1mate y <br /> <br />-1 . . <br />s 1n most exper1ments, <br /> <br />results in particle transit rates of 1 to l03 <br /> <br /> <br />well below the 105 s -1 value for which <br /> <br />coincidence errors become important to measuring concentrations (Dye <br /> <br />and Baumgardner, 1984; Baumgardner et al., 1985; Cooper, 1988). There <br /> <br />are unavoidable inherent sizing uncertainties for all FSSP probes which <br /> <br />particularly occur in the 1 to 10~m range of particle sizes due to the <br /> <br />behavior of the Mie scattering function there (Pinnick and Auvermann, <br /> <br />I <br />I <br /> <br />1979). Multiple Mie peaks for droplets below 10pm can lead to <br />