Air change rate measurement: the log/linear decay of tracer gas concentration with time.

A great deal of the literature on general ventilation expresses the adequacy of the volumetric flow rate of air in terms of the number of room air changes per hour. Although the concept of air change rate has very little relevance to the control of contaminants as it relates to the size of the room and not to the scale of the problem, the overall amount of air entering and leaving a workplace is of fundamental importance in assessing the quality of the working environment. The measurement of air change rates is the traditional way of determining air movement and remains a very widely used and quoted concept, possibly because of the relative ease with which it can be measured and visualised. Air change rates can be measured by the use of tracers, usually in a gaseous form. In the UK, the Health and Safety Executive(!) gives a method for the on-site measurement of air change rates in factories and offices based on the decay of the concentration of tracer gas which has been mixed throughout the area. A log/linear analysis of concentration with time is used to deduce the air change rate. Using this analysis it has frequently been observed that a "dog leg" occurs in the log/linear plot of concentration/time which should yield a straight line. Possible causes of this and other non-linearities are investigated.