Outdoor air (OA) and return air (RA) humidity measurements are essential to implement combined differential enthalpy/temperature airside economizers at air handling units (AHUs). However, due to the lack of reliable air humidity sensors, economizers are more often disabled or replaced by less-efficient temperature economizers. On the other hand, the OA humidity ratio may be obtained from weather station data and the RA humidity ratio may be correlated with the supply air (SA) temperature under humid OA conditions when enthalpy economizers are applied. With these two hypotheses, the economizers can be readily achieved through an engineering approach without direct humidity measurements at AHUs. The purpose of this paper is to experimentally validate these two hypotheses. The engineering approach is explained first and then experiments are conducted on selected AHUs in three cities, Laramie, WY, Norman, OK and Miami, FL. The experiment results reveal that 1) the OA humidity ratio can be obtained from the weather station data with the standard deviation of 0.0010 kgv/kga (or lbmv/lbma) for dry and moderate humid climates and with the standard deviation of 0.0026 kgv/kga (or lbmv/lbma) for humid climates; and 2) the RA humidity ratio can be correlated to the saturated humidity ratio at the SA temperature with the standard deviation of 0.0005 kgv/kga (or lbmv/lbma) under humid OA conditions.