Design specifications for wet-bulb aspirator apparatus

Aspirated psychrometers are commonly used for the measurement of the wet-bulb temperature of a moist air stream. By knowing the wet-bulb temperature, dry-bulb temperature, and pressure, it is possible to determine the thermodynamic state of a moist air stream. ASHRAE Standards 41.6-1994 (ANSI/ASHRAE 2006) and 41.1 (ANSI/ASHRAE 1991) provide detailed design guidelines and considerations required to construct an aspirated psychrometer capable of measuring the wet-bulb temperature to within ±0.10°C (±0.18°F). This article documents the results of ASHRAE research project RP-1460 (2010), which had the objective of establishing design guidelines and considerations required to construct an aspirated psychrometer capable of measuring the wet-bulb temperature to within ±0.05°C (±0.09°F). First, an analytical model is presented and used to optimize an aspirated psychrometer and to predict the error in the measurement of the wet-bulb temperature over a range of conditions. Next, the details of an experimental apparatus are presented. The apparatus is used to verify the results of the analytical model and to establish the accuracy of the optimized psychrometer. Finally, conclusions are drawn as to the practicality of measuring the wet-bulb temperature to within ±0.05°C (±0.09°F).