OCR Text |
Show 3. 2. 1. 3 Comparison Between Agl and ZnS Signals The output of the zinc sulfide aerosolizer in particles per second is c.onsict. erably less than the output of the silver iodide generators used. The zinc sulfide real time sampler is a fast response unit ; the limiting factor being the response of the pen recorder, with a time constant of milliseconds. Tn nucleus counter is a fairly slow response unit with its high volume cold cha ber. It has a time constant, required for charging the chamber, of approxi mately 30 seconds, a discharge time constant of 120 seconds and a pure delay (for crystal growth) time constant of 30 seconds. This delays the pe I broadens the plume and makes the measured peak concentrations approximately one half the actual peak concentrations. Figure 33 shows a typical comparison of signal tracks through a high concentration plume. The zinc sulfide unit shows the fine structure of the plume, while the lag in the nuclei count signal can be seen. In this case, the nucleus counter was overloaded and the actual peak concentration can only be estimated. The ordinate scale in the figure are relative and are not comparable in particles per unit volum The nucleus counter counts single particles and thus has a sensitivity of one particle per cubic meter, while the zinc sulfide unit threshold is realisticall of the order of 1, 000 particles per cubic meter. The high threshold, coupled with the lower release of particles per unit timi results in the zinc sulfide system being orders of magnitude less sensitive, An example of this is evident in the track presented in Figure 33. The peak zinc sulfide tracer signal is O. 5 m. v. , and from the calibration data this corresponds to 6. 4 x 1 o4 particles per cubic meter. Since the nuclei counte was overloaded, the exact nuclei count must be estimated from the shape of the curve. The peak count is estimated near 5, 000 per liter or 5 x 10 6 nucl per cubic meter. The outr,ut of the Agl generators is 10 14 nuclei per gram of Agl at -20°C, or 4 x 10 4 nuclei per minute. The ZnS tracer was released at 300 gm per minute or 4. 2 x 1012 airborne particles per minute. Comparison of the output of the two tracers yields: Output of Nuclei Generators Output of ZnS Aerosolizer = ---- 4. 2 X 10 = 12 (15) The ratio of measured tracer concentrations on the track gives: Measured Nuclei Concentration Measured ZnS Concentrations which is in reasonable agreement. 62 6. 4 X 10 4 (16) |