The aim of this project is to be able to specify the design guidelines and considerations required to construct an aspirated psychrometer capable of measuring the wet-bulb temperature to within plus/minus 0.05°C. This is done by means of an analytical model used to predict the error in the measurement of the wet-bulb temperature over a range of conditions. Also to validate the model an aspirated psychrometer is built in accordance with the model and tested over a range of experimental test conditions. The analytical model and the experimental test apparatus are described in detail in this document.