When evaluating the performance of horizontal rotor mechanical surface aeration equipment in accordance with the ASCE Standard for Measurement of Oxygen Transfer in Clean Water, several factors should be considered with regard to their impact on the reported performance. These include basin geometry and testing volume, source water quality, dissolved oxygen measurement location, and external environmental factors including air temperature and humidity. Each of these factors may influence the reported performance of mechanical surface aeration equipment, specifically horizontal rotor aeration devices, resulting in an inaccurate estimation of the true equipment performance that should be expected in practical applications.