Standardizing the measurement of minimum extinguishing concentrations of gaseous agents

The cup-burner test method has had a long history as a tool for measuring the minimum flame extinguishing concentrations (MEC) of gaseous fire-extinguishing agents for flammable and combustible liquid fuels. The pioneering work by Hirst and Booth (1977) set out the basic principles of the cup-burner and its use. Saso et al. (1993) reported on scale effects on MEC measurement. Preece et al. (2003) reported on parametric effects on MEC measurement. A number of investigators have provided determined values of MEC for a number of inert gas and fluorocarbon agents. See Dlugogorski et al. (1996); Saito et al. (1996); Moore et al. (1998); Sheinson et al. (1998). Takahashi et al. have reported on several investigations of flame extinguishment in the cup-burner including conducting a detailed computational fluid dynamic modeling including reaction kinetics and mass and heat transfer (2003); physical and chemical aspects of flame extinguishment (May 2005); and evaluation of flame extinguishment in low gravity (January 2005). Application of the cup-burner method has, in some instances, produced inconsistent results. MEC values reported by several investigators for the same inert gas agents have exhibited differences that are significant with respect to the way in which they are used (Senecal 2005). Thermochemical analysis and data obtained in a consistent manner suggested that there is a reasonable basis to expect highly consistent, and even predictable, MEC values for inert gas agents. It appeared that the likely source of measurement variation was due to procedural and mechanical differences in the conduct of the cup-burner test among various laboratories. A Task Group, appointed by the NFPA GFE-AAA technical committee on gaseous fire-extinguishing systems, revised the cup-burner test procedure including the specification of a standardized test apparatus. This article describes an inter-laboratory study conducted in 2006 to (1) determine consensus value benchmark minimum extinguishing concentrations (MEC) of an inert gas agent and a halocarbon agent; (2) evaluate the reproducibility of the revised test method; and (3) evaluate the day-to-day consistency (repeatability) of the test method. The results of an inter-laboratory (round robin) study have established benchmark MEC values for nitrogen and HFC-227ea with excellent reproducibility. The results show that MEC values reported by most laboratories were in very good agreement, i.e., exhibited good reproducibility based on averages of three replicate determinations on different days. Analysis of the replicate data revealed a variable degree of repeatability, suggesting that a laboratory will need to review key procedural details in some cases.