PFT measurements in ventilation ducts.

The passive perfluorocarbon method (PFT-method) has been successfully applied in ventilation measurements in rooms. The method is, in principle, also applicable to air flow measurements in ventilation ducts. There are, however, several problems in applying a passive sampling technique in a duct. First, the concentration of the tracer may not be uniform through the cross-section of a duct. Second, the velocities in a duct are normally an order of magnitude higher than in a room. Third, the orientation of the sampler in respect to the flow may affect the uptake rate, This study concentrated on the solution of these three problems. The velocity of the air in a duct had significant effect on the uptake rate of the sampler. The magnitude of the effect was in the order of an 11-16 % increase for each 1 m/s increace in nominal velocity. This indicated that the passive PFT-technique should not be used without correction for uptake rate in duct measurements. More studies are needed in order to establish this correction. The deviations between samplers were found to be higher in a duct than in a test room. This means that the location of the samplers should be chosen carefully. A good first estimate would be the locations proposed by several standards for air velocity measurements. The orientation of the sampler had only a small effect on the uptake rate. It was found that the samplers should not be placed with their open end against the flow.