Dew point temperature error from measuring dry bulb temperature and relative humidity

A component error analysis was performed to relate the accuracy of measured values of dry bulb temperature and relative humidity to errors in calculated dew point temperature. Dew point temperature errors were found to be greatest for dryer air (less than about 50% relative humidity) and to increase proportionately with dry bulb temperature. Error calculations for various combinations of dry bulb temperature, relative humidity, and measurement errors in each variable which are representative of current commercial sensors, are provided in tabular format. The magnitude of dew point errors from these calculations suggests that modern applications such as automated weather stations can have significant errors in computed dew point temperature, and even for very accurate sensors the dew point error can exceed twice the magnitude of the dry bulb sensor error for dryer conditions. Keywords. Temperature, Humidity y Measurement systems. Error analysis. A utomated weather stations often utilize relative humidity sensors. When combined with measured dry bulb temperature and atmospheric pressure, other psychrometric properties of the moist air can be computed. The dew point temperature is one useful property often obtained in this fashion, and is typically quoted to the same precision as dry bulb temperature. This practice assumes negligible error propogation from measured values of relative humidity and dry bulb temperature. However, as will be shown, this practice is incorrect. The purpose of this technical note is to quantify the error in computed dew point temperature, and to illustrate it for typical ranges in measured dry bulb temperature and relative humidity. Many automated weather stations and other data acquisition systems typically utilize some form of microprocessor. Measurement errors include those from sensor inaccuracy and also all errors associated with digital acquisition, including signal transmission error and analog to digital conversion errors (converter error, gain and zero errors, temperature effects on gain and zero, and converter linearity). These errors combine with each sensor's errors to affect overall accuracy of computed dew point temperature. A component error analysis on dew point temperature computed from measurement of dry bulb temperature and relative humidity was done following conventional techniques (Jordan et al., 1991):