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.

Contents of the Humidity Calibration Kit:
The RH calibration kit includes the following items:

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.

Kestrel 3000 Calibration                                                             
 

Setting up the Kestrel 3000:

Before recalibrating the Kestrel, the expected humidity values must be programmed into the unit:

1. Turn on the Kestrel. Press the MODE button until the unit is in dew point mode.
2. Hold the ON and MODE buttons simultaneously until the display reads “P1” (approximately 6 seconds).
3. Press the ON button. The display will show a value with the last digit blinking. This value represents the expected humidity of the sodium chloride chamber. If the value reads 75.3, simply press the ON button twice and skip to step 4. Otherwise, the value must be changed to 75.3 as described in the following 2 steps:
   1. Press the MODE button to change the value of the rightmost digit. When the desired value is displayed, press the ON button to move to the middle digit.
   2. Press the MODE button to change the value of the middle digit. When the desired value is displayed, press the ON button to exit setting.
4. The display will read “P 2”. Press the ON button.
5. The display will again show a value with the last digit blinking. This value represents the expected humidity of the magnesium chloride chamber. If the value reads 32.8, simply press the ON button twice and skip to step 6. Otherwise, the value must be changed to 32.8 as described in the following 2 steps.
   1. Press the MODE button to change the value of the rightmost digit. When the desired value is displayed, press the ON button to move to the middle digit.
   2. Press the MODE button to change the value of the middle digit. When the desired value is displayed, press the ON button to exit setting.
6. The display will return to the normal dew point mode. Please note that the RH measurement will NOT be any different than before setting the calibration values. The unit is now ready for calibration.

Running the Calibration Routine:

Once the expected RH values have been programmed, the unit may be recalibrated:

1. Press the MODE button until the unit is in relative humidity mode.
2. Hold the ON and MODE buttons simultaneously until the display reads
   “C 1” (approximately 6 seconds).
3. Press the MODE button. The display will alternate between “C 1” and a value, this will continue for 60 minutes. Place the unit into the sodium chloride chamber and immediately close the lid tightly. After 60 minutes, the unit will display “C 2”, indicating that the first humidity chamber routine is complete, and the unit is ready for the second chamber.
4. Remove the unit from the sodium chloride chamber and press the MODE button. The display will alternate between “C 2” and a value, this will continue for 60 minutes. Wipe sodium chloride from the unit. Place the unit into the magnesium chloride chamber and seal it immediately. After 60 minutes, the unit will display “End”, indicating that the second humidity chamber routine is complete, and the unit has been calibrated.
5. Remove the unit from the magnesium chloride chamber and press the MODE button. The Kestrel® weather meter is now fully calibrated and ready to take accurate humidity measurements.

If the ON button is pressed at any time during the above routine, the unit will shut off and the re-calibration will be aborted.

Kestrel 4000 Calibration                                                             
 

Setting up the Kestrel 4000:

Before recalibrating the Kestrel, the expected humidity values and calibration time must be programmed into the unit:

1. Turn on the Kestrel. Press the ( ) button to enter the main menu. Select the Humidity Cal option from either the Main Menu or the System Menu (depending on which version Kestrel).
2. With the menu option Period highlighted, adjust the length of time that the Kestrel will remain in the each humidity chamber by pressing the left and right arrow buttons. It is recommended to use 60 minutes.
3. Press the down arrow to highlight the RH Point 1 option. Adjust the humidity value to the expected humidity reading in the magnesium chloride chamber by pressing the left and right arrow buttons.
4. Press the down arrow to highlight the RH Point 2 option. Adjust the humidity value to the expected humidity reading in the sodium chloride chamber by pressing the left and right arrow buttons.

Running the Calibration Routine for the Kestrel 4000

1. With the menu option Start 1 highlighted, press the left or right arrow button to begin the calibration period.
2. Place the unit into the magnesium chloride chamber and immediately close the lid tightly. After the preset length of time (factory setting is 60 minutes), remove the unit from the chamber.
3. With the menu option Start 2 highlighted, press the left or right arrow button to begin the calibration period.
4. Place the unit into the sodium chloride chamber and immediately close the lid tightly. After the preset length of time (factory setting is 60 minutes), remove the unit from the chamber.

Your Kestrel 4000 is now recalibrated and ready to take accurate measurements.

If the unit is turned off at any time during the above routine, the re-calibration will be aborted.
 

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.