Flow rate measurement in modern ambient air samplers – how accurate ?

New generation ambient air samplers operate at much lower sampling flow rates than do their “high volume” predecessors, namely 16.7 liters/minute. When these new air samplers emerged several years ago there were limited tools available with which to assess and calibrate their operating flow rate. In 1996 a flow meter specifically designed for these new air samplers came available, known as the StreamlineTM Flow Transfer Standard. With the establishment of a nation wide PM2.5 sampling network in 1999 which use exclusively 16.7 liter/minute samplers, Streamlines have come into wide usage. This paper addresses the “accuracy” of flow measurements produced by Streamline FTSs, and the subsequent accuracy of the flow rates of air samplers calibrated with Streamlines. A statistical uncertainty analysis was performed for the Streamline FTS used at a nominal flow rate of 16.5 liters/minute, yielding an uncertainty in its measured flow rate of ±0.67% which corresponds to ±0.11 liters/minute. A subsequent uncertainty analysis was performed for the flow rate of an ambient air sampler using a Streamline and a typical field instruments, yielding an uncertainty of ±0.98% (±0.16 liters/minute) at 16.7 liters/minute.