The Age of Air

C’est De Dificultés Qui Naissent Les Miracles

The Main Concept

ISO 16000-8

According to ISO 16000-8 it is important to properly estimate The Mean Age of Air in a measured zone, or each of the building zones. This important characteristic indicates the average time the air has spent  in a zone of the building directly correlated to accumulating contaminants levels. Other words the time it takes to exchange the air in a zone. The Age of Air is directly correlated with the air concentration of contaminants released from continuous types of sources. For the Air Quality studies there are many ventilation measurement techniques that can be applied.

Methods for Measure the Age of Air

in Ambient Air Zone

Decay Method

The principle of the decay method is to mark the air in the ventilated system with tracer gas and determine the rate at which the marked air is replaced with unmarked air. The concentration history is recorded as a function of time. The local mean age of air is obtained from the quotient of the integral of the concentration versus time and the initial concentration. The decay method can generally be used without problems up to air change rate n = 10 h.

Active Homogeneous Emission Method

In the active homogeneous emission method, tracer gas is fed at measured constant rates into the zones by a suitable adjustable flow injection device. The injection rates shall be proportional to the zone volumes.The steady state tracer gas concentration of room air is measured using a suitable gas analyser. The local mean age of air is obtained from the quotient of the steady state concentration and the injection rate per unit volume.

Passive Homogeneous Emission Method

In the passive homogeneous emission method, tracer gas is emitted at known constant rates into the zones using diffusion sources. The emission rates shall be proportional to the zone volumes. The steady state tracer gas concentration of room air is measured by collecting an integrating sample in a sorbent tube (actively using an air sampling pump or passively using diffusion sampling) and analysing this sample afterwards in an especially equipped laboratory. The local mean age of air is obtained from the quotient of the steady state concentration and the emission rate per unit volume.

The Identification Strategy

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For measuring time-varying conditions in simple buildings the recommendation is to use the active homogeneous emission method, and for complex buildings the recommendation is to use the passive homogeneous emission with active sampling. Please see ISO 16000-8 for details.

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