Isokinetic Sampling In Unidirectional Flow

By Mark Hallworth

Maintaining environmental control in pharmaceutical manufacturing relies heavily on unidirectional airflow to minimize contamination risks. To obtain accurate particle data in these zones, the sampling velocity must match the surrounding air velocity—a principle known as isokinetic sampling. When velocities are mismatched, either sub- or super-isokinetic conditions can arise, potentially distorting particle counts. While EU GMP Annex 1 specifies an airflow range of 0.36–0.54 m/s, it is vital to understand how standard sampling equipment performs within these limits. Industry consensus accepts a 5% margin for sampling error, which remains achievable with standard probe designs across the regulatory airflow range. This introductory guide examines the relationship between probe velocity and sampling accuracy to ensure compliance and reliable environmental monitoring. Review the full application note for detailed technical analysis.