A low-cost psychrometer for field measurements of atmospheric humidity

A design for an aspirated wet and dry bulb psychrometer, suitable for continuous field measurement of atmospheric humidity, is described. The psychrometer is simple and cheap to make, with a material cost of UK £40 per unit. It is mainly constructed from ready-made plastic plumbing parts, thus avoiding the need for expensive workshop facilities. The wet and dry bulb temperature sensors are suitable for direct connection to a solid-state data-logger. During extensive field use, it has proved reliable and easy to maintain. The accuracy of the device was assessed by intercomparison with an Assman psychrometer, a standard instrument for measuring atmospheric humidity. The test was conducted under high ambient saturation deficits (4.08 ± 1.22 kPa) and shortwave radiation fluxes in Niger, West Africa. Reasonable agreement between the two instruments was observed, with the low-cost psychrometer giving a mean saturation deficit 0.12 ± 0.25 kPa higher than the Assman.