Relative Humidity
Calibration Kit
For Calibrating All RH
Measuring Kestrel® Meters
Model Numbers
3000, 3500, 4000, 4200,
4250, 4300, 4400, 4500
Revised 6.8.12
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3
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.
While the calibration process is very simple, it is important that you follow
the steps carefully! A careless calibration could enter incorrect values into
the unit and degrade its performance.
● ● ● ● ●
©2011 Nielsen-Kellerman Co.
Kestrel, the Kestrel bird, NK, Nielsen-Kellerman and the NK logo are registered
trademarks of the Nielsen-Kellerman company.
Please refer to Standard Practice for Maintaining Constant Relative Humidity by Means
of Aqueous Solutions ASTM Designation: E104-85 (reapproved 1996) for
additional information.
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NK factory calibrates at RH = 32.8% and RH = 75.3%. Calibration is performed in two sealed environmental
chambers maintained at 25ºC. The humidity is held constant with saturated aqueous salt baths: magnesium
chloride for the 32.8% chamber and sodium chloride for the 75.3% chamber. Chamber temperatures are
held constant at 25ºC ±0.5Cº.
The temperature and RH of each chamber are independently monitored using a precision chilled mirror dew
point hygrometer (Edge Tech 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 that ensures the Kestrel is 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%.
NK’s factory calibration is traceable to the National Institute of Standards and Technology, ensuring the
reliability you need. Our primary Calibration Standards are sent for calibration in accordance with NIST
requirements and based on a regular schedule. Only approved laboratories and NIST itself are used for these
calibration services. Calibration certicates and stickers may be requested (at an additional charge) when
purchasing your Kestrel or returning it for Factory Recalibration (see next section).
4 5
Factory Recalibration / Certication
The Kestrel should be recalibrated if the RH sensor has shown drift in its RH measurement. The Kestrel RH
sensor is specied to develop less than 2% drift over 24 months. You may return your Kestrel to NK for
Factory Recalibration of the relative humidity and related measurements. Recalibration is available with and
without certication of NIST traceability and calibration stickers. Note that NK can recalibrate and certify any
of the values measured by your Kestrel– wind speed / air ow; temperature; barometric pressure and
humidity. Please contact NK for more information about Factory Recalibration and certication.
Field Recalibration
As an alternative to Factory Recalibration, this kit allows you to perform dozens of recalibrations in the
eld to ensure that your Kestrel is always performing within specications. 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 Humidity Calibration Kit
● two jars with color-coded salt chamber labels, plastic grids and sealing lids
● one jar with no grid (for water)
● one vial or sealed bag with magnesium chloride (MgCl) salt and blue color-coded label
● one vial or sealed bag with sodium chloride salt (NaCl)and red color-coded label
● one RH Calibration Kit insulated bag/carry case
● one color-coded labeled measuring cup
Please read ALL of these instructions before
beginning the recalibration of your Kestrel!
● ● ● ● ●
While the calibration process is very simple, it is important that you follow the
steps carefully! A careless calibration could enter incorrect values into the unit
and degrade its performance.
● ● ● ● ●
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The RH Calibration Kit insulated bag and jar of water are used to create a stable environment. The jar of
water will stabilize the temperature inside the Cal Kit bag. The saturated salt solutions generate well-dened
humidities when they are conned to the contained environment of the two gridded jars inside
the insulated, stabilized Cal Kit.
6 7
Preparing the Calibration Salt Solutions
For your rst use only, you will need to add water to the provided salts to prepare the saturated salt
solutions for the calibration chambers. Note that it is important not to cross-contaminate either solution
with the other salt. For this reason, every item is color-coded. Make sure to keep red items with red, and
blue with blue, throughout the mixing and calibrating process.
You will be mixing the salts with water. We recommend distilled water if available, but clean uncontaminated tap water will not measurably aect the accuracy of your results. For both solutions, your aim is to
produce a “slurry” in which you can still see visible grains of salt as well as liquid water. Look at the
following pictures for reference:
5) Place the lid with the same label on the jar and tighten securely.
6) Repeat for the Magnesium Chloride 32.80% solution, lling the measuring cup to the BLUE line marked
MgCl (5 ml.) and using the BLUE color-coded supplies. You may need to use a stirring stick to scrape the
salt out of the vial or bag. Make sure not to add more than 5 ml. of water! It is better to start with
slightly less water, and add it a drop at a time, as this solution tends to become too wet.
Note that the salt solutions will tend to change over time: the NaCL solution will tend to dry out, and
the MgCl solution will tend to pull humidity from the air and get too wet. For this reason, keep the
lids securely closed on the calibration chambers except when opening them to load a unit for calibration. You may add drops of water to the NaCl solution to return it to the proper “slurry” consistency if
required. You may also place the MgCl solution in a clean pan in a slow oven to dry it if it becomes too
wet, or you may simply purchase replacement salt bags and remix the solutions from scratch.
Just Right Too Wet Too Dry
1) Open the two labeled jars. Remove the two spacer grids from the jars.
2) Open the sealed vial or bag labeled “Sodium Chloride 75.30%” and dump the salt into the bottom of the
jar with the same label.
3) Fill the small measuring cup to the RED line marked “NaCl” (13 ml.) with distilled or clean tap water.
Add to jar.
4) Using a stirring stick, stir the solution for at least a minute. Observe the consistency.
If necessary, add a few drops of water to achieve the desired slurry consistency, and stir some more.
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Stabilizing the Calibration Kit Temperature
1) Choose a Calibration Location: It is very important that the temperature of the calibration chambers
be kept as constant as possible during the calibration process. Ideally, the Cal Kit should be used inside
a temperature-controlled structure (house or oce) with a temperature variation of ±2°F or less.
2) Prepare the Stabilizing Jar: Fill the third jar with room temperature tap water. Put both salt chambers
and the temperature stabilizing water jar inside the Cal Kit case. Zip the case closed.
3) For even more accurate calibration results, or where a stable environment is not available, the entire Cal
Kit may be placed inside a large insulated cooler together with additional containers of water (gallon jugs
work ne).
8 9
4) Stabilize the Temperature: Place the Kestrel unit to be calibrated on top of the Cal Kit. Place both in the
location where you will perform the calibration and leave them there undisturbed for at least 10 hours.
(If using the additional cooler, place inside the cooler with the full jugs of water.) This stabilization period
will ensure that the salt chambers, temperature-stabilizing water jar(s) and Kestrel are all at the tempera-
ture of the calibration environment.
Two-Button Kestrel 3000:
1) Turn on the Kestrel 3000. Press the [MODE] button until the unit is in dew point mode ( ).
2) Hold the [ON] and [MODE] buttons simultaneously until the display reads “P 1” (approximately
six seconds).
Your Cal Kit, stabilizing water chamber and Kestrel are now at the
same temperature as their environment and each other. You are
ready to proceed to performing the calibration.
Calibrating Your Kestrel
NOTE: Throughout the calibration process, keep both salt chambers and the jar of water inside the
Cal Kit except when turning the unit on, moving the unit between chambers or starting the
calibration cycle.
Before recalibrating your Kestrel, the expected humidity calibration chamber values must be programmed
into the unit. The salts provided will create chamber humidities of 75.3 (sodium chloride) and 32.8 (magnesium chloride). These are the default calibration values factory programmed into your unit. The following
directions cover the steps required to ensure that your Kestrel is programmed for the correct values, set the
calibration time period and run the calibration routine.
IMPORTANT! Identify your Kestrel model: The calibration instructions dier depending upon
which Kestrel model you have. You must identify your model and follow the correct directions. The
three procedures are:
For the Two-Button Kestrel 3000, the 75.3% sodium chloride chamber is used rst, and for the
Three-Button 3000/3500 and 4000/4200/4250/4400/4500, the 32.8% magnesium chloride chamber
is used rst.
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3) Press the [ON] button to display the rst calibration value, which will have the last digit blinking. To
keep the displayed value (75.3 for the sodium chloride chamber), simply press the [ON] button twice to
skip to step 4. If it is necessary to change the value, press the [MODE] button to change the ashing
digit. When the desired number is displayed, press the [ON] button to move to the middle digit. Again,
press the [MODE] button to change the ashing digit. When the desired number is displayed, press the
[ON] button to continue setting the second calibration value.
4) The display will now read “P 2.” Press the [ON] button to display the second calibration value, which will
have the last digit blinking. To keep the displayed value (32.8 for the magnesium chloride chamber),
simply press the [ON] button twice to exit setting. If it is necessary to change the value, press the [MODE]
button to change the the ashing digit. When the desired number is displayed, press the [ON] button to
move to the middle digit. Again, press the [MODE] button to change the ashing digit. When the desired
number is displayed, press the [ON] button to exit, setting the second calibration value.
5) The display will return to the normal dew point mode. (Note that the RH measurement will NOT be any
dierent than before setting the calibration values.) The unit is now ready for calibration.
6) Press the [MODE] button until the unit is in relative humidity mode( %).
10 11
7) Hold the [ON] and [MODE] buttons simultaneously until the display reads “C 1” (approximately 6 seconds).
Three-Button Kestrel 3000/3500:
8) Press the [MODE] button. The display will alternate between “C 1” and the programmed rst chamber
value. This will continue for 60 minutes to indicate that the rst calibration point is being taken.
9) Place the unit into the sodium chloride chamber, close the jar tightly, place it inside the Cal Kit and zip
the bag closed. If you are using an additional cooler, place the zippered bag inside. Leave the Cal Kit
undisturbed for 60 minutes. After that time, remove the Kestrel from the rst chamber. The display
should read “C 2,” indicating that the rst calibration point is nished and the unit is ready for the
second point.
10) Thoroughly wipe o any sodium chloride clinging to the Kestrel or lanyard. Press the [MODE] button
to begin the calibration count-down for the second point. The display will toggle between “C 2” and the
programmed second chamber value. This will continue for 60 minutes.
11) Place the unit into the magnesium chloride chamber, close the jar tightly, place it inside the Cal Kit and
zip the bag closed. If you are using an additional cooler, place the zippered bag inside. Leave the Cal
Kit undisturbed for 60 minutes. After that time, remove the Kestrel from the second chamber. The display
should read “End,” indicating that the second calibration point is nished.
12) Press the [MODE] button to return to the normal mode of operation. Wipe or rinse the unit clean. Your
Kestrel is now recalibrated and ready to take accurate measurements.
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1) Begin with the Kestrel 3000/3500 turned o. Hold the left button while pressing the center button. The
display will read “P 1.”
2) Press the center button to display the rst calibration value. To keep the displayed value (32.8 for the
magnesium chloride chamber), simply press the center button to skip to step 3. If it is necessary to
change the value, use the left arrow button to adjust the value down, and the right arrow button to
adjust it up. When the desired number is displayed, press the center button to continue to setting the
second calibration value. The display will read “P 2.”
3) Press the center button to display the second calibration value. To keep the displayed value (75.3 for the
sodium chloride chamber), simply press the center button to exit value setting. If it is necessary to
change the value, use the left arrow button to adjust the value down, and the right arrow button to
adjust it up. When the desired number is displayed, press the center button to exit value setting. The
display will read “C 1.”
4) Press the center button to begin the calibration count-down. The display will toggle between “C 1” and
the number of minutes remaining in the calibration.
5) Place the unit into the magnesium chloride chamber, close the jar tightly, place it inside the Cal Kit and
zip the bag closed. If you are using an additional cooler, place the zippered bag inside. Leave the Cal
Kit undisturbed for 60 minutes. After that time, remove the Kestrel from the rst chamber. The display
should read “C 2,” indicating that the rst calibration point is nished and the unit is ready for the
second point.
12 13
6) Thoroughly wipe o any magnesium chloride clinging to the Kestrel or lanyard. Press the center button
4) Press the button to highlight “RH Point 2.” Use the and arrow buttons to adjust and set the
humidity value for the second calibration chamber (75.3 for the sodium chloride chamber).
to begin the calibration count-down for the second point. The display will toggle between “C 2” and the
number of minutes remaining in the calibration.
5) Press the button to highlight “Start 1.” Press the or button to begin the rst calibration period. A
timer will appear next to “Start 1” and will immediately begin counting down the calibration period in
7) Place the unit into the sodium chloride chamber, close the jar tightly, place it inside the Cal Kit and zip
seconds (i.e., beginning with 3600 for 60 minutes).
the bag closed. If you are using a secondary chamber, place it inside the second chamber. Leave the Cal
Kit undisturbed for 60 minutes. After that time, remove the Kestrel from the second chamber. The display
should read “End,” indicating that the second calibration point is nished.
6) Place the unit into the magnesium chloride chamber, close the jar tightly, place it inside the Cal Kit and
zip the bag closed. If you are using an additional cooler, place the zippered bag inside. Leave the Cal Kit
undisturbed for 60 minutes (or other time period selected in step 2). After that time, remove the Kestrel
8) Press the center button to return to the normal mode of operation. Wipe or rinse the unit clean. Your
Kestrel is now recalibrated and ready to take accurate measurements.
from the rst chamber. The display should be on “Start 1” with the counter no longer visible, indicating
that the timer has counted down to “0” and the rst calibration point is nished. The unit is now ready for
the second point. Note: the plastic spacers should not be used for the calibration of the Kestrel 4400. When placing the Kestrel 4400 in
the jar, inserting it upside down (shown in picture) is recommended. The black globe is more easily cleaned than the bottom of the Kestrel.
Kestrel 4000/4200/4250/4300/4400/4500:
1) Turn on the Kestrel Meter. Press the button to enter the main setup menu. Press the button to high-
light the “Humidity Cal” option, which will be found either on the main menu or under the “System”
sub-menu, depending on your software version. Press the or button to enter the humidity cal
setup screen.
2) Press the button to highlight “Period.” Use the and arrow buttons to adjust the setting for the
length of time that the Kestrel will remain in each humidity chamber. The default and recommended
setting is 60 minutes.
3) Press the button to highlight “RH Point 1.” Use the and buttons to adjust and set the humidity
value for the rst calibration chamber (32.8 for the magnesium chloride chamber).
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7) Thoroughly wipe o any magnesium chloride clinging to the Kestrel, lanyard or
black globe (if 4400 model). Use the down arrow key to highlight “Start 2.” Press the or
button to begin the second calibration period. A timer will appear next to “Start 2”
and will begin counting down the second period in seconds.
8) Place the unit into the sodium chloride chamber, close the jar tightly, place it inside the Cal Kit and zip
the bag closed. If you are using an additional cooler, place the zippered bag inside. Leave the Cal Kit
undisturbed for 60 minutes. After that time, remove the Kestrel from the second chamber. The display
should again show “Start 2” highlighted with the timer no longer visible, indicating that the second
calibration point is nished.
9) Press the button twice to exit the calibration and main menus and return to the normal mode
of operation. Your Kestrel is now recalibrated and ready to take accurate measurements.
14 15
Using the Calibration Kit to Check Your
Unit’s Calibration Accuracy
Kestrel 4000/4200/4250/4300/4400/4500: Press the button to enter the main setup menu. Press the
down arrow button to highlight the “Auto O” option. Use the left/right arrow buttons to disable the autoo feature. Press the button to exit the main setup menu. Place the unit in the desired salt chamber and
leave it undisturbed in the chamber inside the Cal Kit for one to two hours. Remove the chamber from the
Cal Kit and view the %RH through the jar. The reading should be within 3 percentage points of the expected
value for that chamber’s salt solution. For further conrmation of the calibration accuracy, you may set the
unit to log the humidity data at 2-5 min intervals. Review the graph to be certain that the %RH had stabilized
and that the temperature was not changing at the point where you took the conrming reading.
Kestrel 3000 (Two-Button and Three-Button) and 3500: It is not possible to disable the auto-o feature
on these models. With the unit OFF, place it in the desired salt chamber inside the RH Cal Kit for an hour or
more to allow the temperature and humidity to stabilize. After the stabilization period, open the jar quickly
and turn on your unit. Using the [MODE] or / buttons (depending on model), select the relative humidity mode. Return the unit to the jar and leave it undisturbed in the jar inside the RH Cal Kit for 20 minutes.
Remove the jar from the Cal Kit and view the %RH through the jar. The reading should be within 3 percentage points of the expected value for that chamber’s salt solution. Be sure to take your conrmation reading
within 30 minutes before auto shut-o.
Verication of Other RH Values
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 dierent saturated salt solution in the humidity chamber. Refer to the table on the
following page for various salts and their corresponding RH values.
1) Set up a large, clear sealed container with a saturated salt solution using any one of the salts from the
table on page 15 and tap water to make a slurry similar to the slurry provided with the Cal Kit.
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2) Follow the value checking instructions above for your unit. The Kestrel should display an RH value within
± 3 percentage points of the chart listing for the salt solution chosen.
NOTE: Please observe all Material Safety precautions applicable to the salt you have chosen to use.
SALT BATH PUBLISHED RH AT 25 ° C
LITHIUM BROMIDE 6.37%
LITHIUM CHLORIDE 11.30%
POTASSIUM ACETATE 22.51%
MAGNESIUM CHLORIDE 32.80%
POTASSIUM CARBONATE 43.16%
MAGNESIUM NITRATE 52.89%
SODIUM BROMIDE 57.57%
POTASSIUM IODIDE 68.86%
SODIUM CHLORIDE 75.30%
POTASSIUM CHLORIDE 84.34%
POTASSIUM SULFATE 97.30%
* Based on: Humidity Fixed Points of Binary Saturated Aqueous Solutions, Greenspan, Lewis; Journal of Research of the
National Bureau of Standards-A. Physics and Chemistry, Vol. 81A, No. 1, p. 89 January-February 1977
NEED HELP?
Our NK Knowledge Center has answers to many common questions, along with tips and tricks
for using NK products. It’s available 24-7 at www.nkhome.com/knowledge center/.
For email assistance with the installation or operation of your NK product, write
techsupport@nkhome.com.
For help with an apparent malfunction, or to arrange or inquire about a repair, write
repairs@nkhome.com.
Or call 800.784.4221 (610.447.1555 outside of the USA), Monday to Friday, 9 to 5.
Nielsen-Kellerman
toll free: 800.784.4221
NKhome.com
info@NKhome
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