The time-tested psychrometric method is employed in state-of-the-art fashion with the FNA-Series psychrometric humidity sensors. A pair of matched precision thermistors, one for air temp. and one for evaporative temp., provide highly-accurate measurement of relative humidity, dry bulb temperature, wet bulb temperature, dew point, mixture ratio, enthalpy, and partial vapor pressure. The instruments employ a cotton stocking that wicks water from a reservoir to the surface of the wet bulb temperature sensor. An internal fan blows air across the wet bulb sensor, causing the moisture to evaporate, cooling the sensor. The extent of evaporation and, therefore, the degree to which the wet bulb sensor is cooled, is inversely proportional to the relative humidity of the air. The dry bulb sensor acts as a reference to the wet bulb.
The precision thermistors used in the FNA-series psychrometers are tested and matched at the factory to ensure the highest accuracy. A barometric pressure may be programmed into Almemo devices to further improve accuracy. When used in conjunction with barometric pressure sensor #FDA612-MA, Almemo devices may be programmed to automatically correct for barometric pressure.