The new Kestrel Handheld instrument range

Click for instructions:

Here for Kestrel 3000 models manufactured before August 2004
Here for Kestrel 3000 models manufactured after August 2004
Here for Kestrel 3500 models
Here for all versions of the Kestrel 4000

About Factory Calibration:
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. 

This manual explains the steps for calibrating the humidity measurement for the Kestrel.  This process will take approximately 3 hours, including setup and calibration.

Setting up the Humidity Chambers:

1. Open the sodium chloride container and move all of the contents into one of the large containers.  Use a scraper to make sure to collect ALL of the salt solution. 
2. Tightly screw the lid on the large container.
3. Open the magnesium chloride container and move all of the contents into the other large container.  Use the OTHER scraper to make sure to collect ALL of the salt solution.
4. Tightly screw the lid on the large container.
5. Let these containers sit closed, containing the salt solutions, for at least 1 hour.  Any less than 1 hour is insufficient for the chamber to settle and will yield poor calibration. If possible, let the container sit for 12 hours. 
6. You may want to remove the lanyard from the Kestrel in order to avoid getting salt solution on the Lanyard. The Kestrel can be rinsed after calibration to remove any salt solution.

RH Verification:

It is possible to test the Kestrel at humidity values other than 32.8% and 75.3%. To set up a test at another RH value, simply use a different saturated salt solution in the humidity chamber. Refer to the table on page 10 for various salts and their corresponding RH values.

To test the accuracy of the Kestrel at various RH values:

1. Set up a large container with a saturated salt solution using any one of the salts from the table shown below.
2. Place the open small container inside one of the sealed large containers and let sit for at least 12 hours.
3. Turn on the Kestrel and put it into relative humidity mode.
4. Place the Kestrel inside the sealed large container and let it sit for at least 30 minutes. After 30 minutes, the Kestrel should display an RH value within ± 3% of the chart listing.