A total of 57 tests were conducted during the evaluation of HFC 125 and SF6 as Halon 1301 simulants. Figures 2, 3, and 4 illustrate the gaseous agents' concentration histories at different leakage rates and compartment sizes. The decay rates and the maximum concentration relative differences were calculated for these curves. Table 1 shows the overall test results which include the average data scatter bandwidths, decay rates, relative differences in rates, and relative difference in concentration histories (normalized by the peak concentration).
Figures 2, 3, and 4 provide a qualitative comparison between the evaluated agents. For the first five minutes, these charts show that HFC 125 mimics very well the concentrations of Halon 1301, but after this period, the difference between these two agents increased significantly. These results provide additional evidence that HFC 125 is an excellent simulant for engine nacelle fire suppression systems certification testing which is of very short duration (0.5 second). As shown in figure 2, HFC 125 achieved concentrations that resulted in differences that were on the unsafe side (i.e., indicative of a higher halon concentration) when compared to the baseline agent; this difference increased with time and as the leak rate of the compartment decreased. One of the reasons for such a difference is the lower specific vapor density, 4.2, when compared to halon, 5.1. For these reasons HFC 125 was not evaluated in the metering system. SF6 did not match halon's initial concentration as well as HFC 125 during the total flood discharge, but as time passed, the concentration difference was much better than that of HFC 125. According to Grosshandler et al. (1995), sulfur hexafluoride reached significantly higher pressures in the pipeline and vaporizes too quickly from the observable spray plumes. On a positive note, the difference that exists between the concentrations of SF6 and Halon 1301 are on the safe side. This conservative difference indicates that the simulant concentration in the compartment may be lower than what the halon concentration would be during the certification tests, providing a margin of safety.
Table 1, specifically under the average data scatter bandwidth column, shows that the Halon 1301 decaying concentration scatter, from all of the tests conducted, is between 2% and 4% while HFC 125 and SF6 are between 3% to 4%, and 2% to 9%, respectively. The fifth column of table 1, decay rate, shows that sulfur hexafluoride decay rates are similar to those of halon. With the exception of the decay rate calculated at 2 CFM, SF6 had the lowest maximum relative difference when compared to the decay rate values obtained with HFC 125. SF6 decay history at 2-CFM leak rate did not follow a typical exponential decay causing a higher difference. The maximum decay rate differences between SF6 and Halon 1301 at different leak rates were 11.60% at a leak rate of 11 CFM, 1.60% at 21 CFM (with the gas probe at 22.5" from the floor), 0.40% at 21 CFM (with the gas probe at 45" from the floor), and 1.74% at 50 CFM. The maximum decay rate differences for HFC 125 were 41.50% at a leak rate of 2 CFM, 15.60% at 11 CFM, 12.00% at 21 CFM (with probe 22.5" from the floor), and 26.60% at 50 CFM. These decay rates were calculated using the Microsoft Excel exponential smoothing analysis tool. In the seventh column of this table, the maximum normalized concentration relative differences between the identified simulants and halon are presented. The period selected to calculate these differences was between the time at peak concentration and the time prior to the activation of the metering system or the time when the volumetric concentration reached 3%. Once again, SF6 had the smallest difference between its concentration and that of Halon 1301. In this case, the resulting concentration maximum differences were 3.70% at a leak rate of 2 CFM, 2.14% at 11 CFM, 5.84% at 21 CFM (Probe at 22.5"), 9.02% at 21 CFM (Probe at 45"), and 20.23% at 50 CFM. The maximum values for HFC 125 were 14.96% at 2 CFM, 10.14% at 11 CFM, 16.56% at 21 CFM, and 19.52% at 50 CFM. The majority of these maximum values were found at the end of the normalized concentration history curves. When considering the test data scatter, some of these relative difference values would be negligible.
Figure 5 illustrates the concentration history of SF6 and Halon 1301 after the metering system was activated. As mentioned, the discharge flow and pressure were arbitrarily selected; the main purpose of this setup was to compare the concentrations of the agents rather than to optimize the system. The system used had a 0.035" diameter nozzle and a discharge pressure of 40 psig. Qualitatively, the graph shows that SF6 mimics very well the injected concentration levels of Halon 1301 using this specific metering system. Table 1 displays the maximum average concentration relative differences between these two agents at two distinct leak rates; they are 8.18% at 11 CFM and 7.38% at 21 CFM. These differences fall within the data scatter.