Humidity measurement comparison tests: Dew point hygrometer versus wet bulb psychrometer

This report examines the various tests made to characterize and quantify causes of differences in results between the dew point hygrometer and the wet bulb psychrometer methods of measuring humidity. The Environmental Protection Agency Motor Vehicle Emissions Laboratory in Ann Arbor, Michigan is implementing the dew point hygrometer method in the Light Duty test cells. Tests were necessary in order to identify any potential impact on motor vehicle exhaust emissions and fuel economy test results.