AUTOMATED TEMPERATURE AND RELATIVE-HUMIDITY CALIBRATIONS FOR THE OKLAHOMA MESONETWORK

A low-cost (under $12,000), fully automated, relative humidity calibration chamber capable of testing up to six probes simultaneously over a range of humidities from 5% to approximately 95% is designed, constructed, and tested for use by the Oklahoma mesonetwork. in addition, a low-cost (under $4,000) temperature calibration chamber is developed that can test seven probes over the entire meteorological range of interest (approximate to -30 degrees to +50 degrees C). These procedures were designed and developed for the Vaisala HMP 35C temperature and relative humidity probe but are applicable to any similar instrument. The initial implementation results are presented. The relative humidity calibration uses an optical condensation dewpoint hygrometer as the laboratory reference. Two precision resistance temperature detectors are used in the temperature calibration. It should be noted that commercial temperature and relative humidity chambers are available that would have been sufficiently accurate. However, the systems developed here lend themselves to automation and can calibrate multiple probes simultaneously. The facilities developed herein are capable of producing probes with an uncertainty of +/-3% for relative humidities from 5% to approximately 90%, and an error of +/-3.5% for humidities from 90% to 95%. An uncertainty of +0.2 degrees C can be attained using the temperature calibration.