The measurement of air supply volumes and velocities in cleanrooms

Air supply volumes and velocities in cleanrooms are monitored by airflow measuring hoods and anemometers but these measuring methods can be inaccurate if used incorrectly. It is demonstrated in this article that measuring hoods are accurate if the air supply passes evenly out of the hood, as occurs when the air volume is measured from a four-way diffuser or no air supply diffuser. However, when a swirl diffuser was investigated, the measuring hood gave readings more than 50% greater than the true volume. The reasons for the inaccuracy, and methods to correct it were established. Vane anemometers give inaccurate readings at the face of high-efficiency air supply filters, and it was found that the most accurate reading was found about 15 cm from the filter face. The number of readings required across the filter face to obtain an accurate average velocity was investigated, as was a scanning method using overlapping passes.