A comparison of different methods of calculating interzonal airflows by multiple tracer gas decay tests.

Measurement methods based upon multiple tracer gas techniques have become an established branch of the study of air infiltration and interzonal air movements. Three general groups of techniques have emerged, namely constant concentration, constant emission, and decay. Of the decay type group of techniques, several methods of deriving airflows from measured concentration/time curves have been suggested. Broadly speaking, these techniques can be classified into three types: numerical methods involving the use of concentration gradients; numerical methods involving the use of integration of concentration/time data; and thirdly, techniques based upon analytical solutions for the fundamental tracer gas equations. The favoured method of analysis at UMIST has been that of a simplified analytical solution in which the effects of tracer gas re-circulation are only taken into account if the degree of connection between zones if high. This method analysis has been successfully validated for the cases of two and three interconnected cells under controlled conditions in environmental chambers. However, up until now, no direct comparison with the results generated by other methods using the same raw concentration/time data has been made. This paper describes an exercise in which site data for two and three zone regimes is analyzed by several different methods, and the results obtained by each method compared. It is demonstrated that, in particular, concentration gradient methods appear to be particularly ill-suited to dealing with site data which exhibits irregularities in concentration-time profiles caused by fluctuations in windspeed and wind direction. Integration techniques only appear to be marginally better.