ASHRAE 129-1997 (RA 2002) PDF

This standard defines a method of measuring air-change effectiveness in mechanically ventilated buildings or spaces. The method involves an age-of-air approach to air-change effectiveness and employs tracer gas procedures to measure the age of air. The age of the air at a given location is the average amount of time that has elapsed since the air molecules at that location entered the building. The definition of air-change effectiveness is based on a comparison of the age of air in the occupied portions of the building to the age of air that would exist under conditions of perfect mixing of the ventilation air.