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, I 1 l f j I , J . I I j I I , I I. I to obtain a view of the vertical extent at that downwind distance. Once a level was reached at which the plume could no longer be detected the plane would step outward and conduct another series of transects. The continuous SF6 analyzer mounted inside ~he aircraft had a response time of approximately .4 second. This extremely fast response time allowed real-time measurement of the tracer gas concentration field from the plane. A pre- and post-flight calibration of the analyzer was performed for each SF6 release. Six calibration gases of known concentrations (10.2, 95, 485, 972, 2902 and 4870 parts per trillion from Scott Marrin, Riverside, California) were injected into the inlet port of the analyzer and the analyzer output was recorded. The output was nearly linear over the range of concentrations which were measured during the project. An example of a pre- and post-flight calibration is shown in Figure 4.1. Pre-flight calibrations were done as close to departure time as possible and were nearly always slightly less responsive than post flight calibra- tions. For this reason the two calibrations were incorporated together for data analysis work. This resulted in a better estimate of the response over the entire flight. If most of the plume sampling was done later in the flight then more weight was placed on the post flight calibrations. It should be noted that in a past field project the analyzer output was found to drop by 4/10 of 1% per 100 meter climb in altitude in a pressurized cabin. This occurrence would differ from aircraft to aircraft depending upon a number of factors. Response with altitude was not checked on board the King Air but, 1n general, working altitudes were fairly low so this .factor should not have significantly influenced the output. 4-3