0018933 back ofthe filter screen except during the sampling periods, 2.2.3 Discussion of Possible Errors in Sampling. Polydisperse aerosols cmntain an aggregate of particles whose sizes are arranged in accordance with a characteristic frequency distribution, When the aerosol is sampled under ideal conditions, the ratios of the numbers of particles in the various size ranges will be preserved unchanged in the collector, However, a departure from the initial size distribution may be encounteredif thecollecting device has a dimensional bias (non-isokinetic condition}, or if some of the particles are broken up during the sampling operation. Isokinetic sampling conditions will be achieved with a filtering device moving through the aerosol at subsonic speeds if the air velocity into the intake of the filter is identical with the flow rate past the outside, As used in Project 2.8, both the wing tank and coincident samplers were close to isokinetic since the velocity ratios were respectively 0,8 (or greater) and 0.7-0.9. However, in a few cases, the velocity ratios for the coincident units were much less,due to leaks in the line to the compressor pump (see Appendix B). The E-1 sampler used on the WB-50's was poor isokiretically, but this was considered to be immaterial for height line sampling where the particlesin a given region should be fairly uniform in size. Samplers, such as the project rockets, which move at supersonic speed with respect to the aerosol are expected from aerodynamic theory to be unbiased. a - . < C SAN BRUNO FR — 449