Kestrel® RH Calibration Kit

Each Kestrel Pocket weather Meter is individually calibrated after assembly. In essence, each unit is "taught" what the correct relative humidity (RH) values should be for given signals from the RH sensor.

NK calibrates at RH=32.8% and RH=75.3%. Calibration is done in two sealed environmental chambers maintained at 25°C. The humidity is held constant with saturated aqueous salt baths: magnesium chloride for the RH 32.8% chamber and sodium chloride for the 75.3% chamber. Chamber temperatures are held constant at 25°C ± 0.5°C

The temperature and RH of each chamber is independently monitored using a precision chilled mirror dew point hygrometer (EdgeTech Model 2002) accurate to ± 0.1Cº and NIST (National Institute of Standards and Technology) traceable. A proprietary auto calibration routine controls the calibration process. After 60 minutes this routine stores a calibration value which forces the Kestrel to be correct at the RH value of the chamber.

The response of the humidity sensor is a highly reproducible, non-linear curve. This non-linear characteristic is stored in the microprocessor used to compute RH, and in this manner we are able to maintain an accuracy of ± 3% of scale over the relative range 5% - 95%.

Field Calibration

The Kestrel should be recalibrated if the RH sensor has been replaced or has shown drift in its RH measurement. Field calibration requires the use of the RH Calibration Kit. For instructions related to the field calibration procedure, please view the Field Calibration Kit Manual. This manual explains in detail all of the steps for recalibrating your Kestrel. This process will take approximately 13 hours, including setup and calibration.

Contents of the RH Calibration Kit

The RH calibration kit includes the following items:

• three clear, sealable containers
• one container with magnesium chloride solution
• one container with sodium chloride solution
• two wooden scrapers
• an insulated carrying case to hold all of the above and maintain a stable environment while performing calibration

The large container is used to create a stable environment. The small containers are used to hold the saturated salt solution. The saturated salt solutions generate well-defined humidities when they are confined to a contained environment.