One of the techniques to be included in the upcoming revision of ASTM Standard E 741 is the so-called constant flow technique. This technique is effected by injecting tracer into a room or structure at a constant rate and measuring the resulting tracer concentration. For steady-state conditions, this resulting concentration can be interpreted in terms of an equivalent air leakage rate, assuming the source injection rate is known. An increasingly popular variant of this technique entails the use of passive injectors and samplers to obtain an estimate of long-term average air leakage rate.
We have undertaken an examination of numerical solutions to the first-order differential equation governing the concentration. To simplify our considerations, all calculations are based on a single well-mixed zone. Instantaneous and time average concentration histories are generated using measured air exchange data. These histories are then examined for their utility in predicting actual air leakage rates.
The passive long-term average technique appears to underpredict the actual air leakage rate for the limited data considered. Values of air leakage inferred from instantaneous measurements are also provided for comparison with actual air leakage rates.