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Barometric Pressure and Altitude Adjustment
The Kestrel Meter measures“station”pressure — the actual air pressure in the measurement location —
and uses this value to calculate barometric pressure and altitude. Station pressure changes in response to
two things — changes in altitude and changes in the atmosphere. Because the Kestrel Meter is constantly
changing location and altitude, it is important to enter adjustments or “references”when accurate pressure
and altitude readings are needed.
Barometric pressure is station pressure corrected to sea level. In order to make the correction, the Kestrel
Meter needs an accurate reference altitude. Altitude is the height above sea level. In order to correctly
calculate altitude, the unit needs an accurate barometric pressure reference, also known as an“altimeter
setting”. Fortunately, you only need to know ONE of these values (current barometric pressure or current
altitude) in order to set your Kestrel Meter up to show accurate readings.
Starting with the known barometric pressure for your location
You can obtain your current barometric pressure by checking an internet weather site
for a nearby location, or contacting a local airport. Set this value as your reference
pressure on the ALTITUDE screen to determine your correct altitude: Press the button
to enter the reference setting mode. Press the button to increase the reference
pressure or the button to decrease the reference pressure. You will notice that the
altitude will change with changes in the reference pressure. Press the button to exit the adjustment
mode. Set your Kestrel Meter down on a table and allow the altitude reading to stabilize. (Note: very small
changes in pressure generate noticeable changes in altitude. In order to provide meaningful readings for
activities where altitude changes quickly, the Kestrel Meter features rapid altitude response. This is why
the altitude readings tend to uctuate by a few feet.) After obtaining a current altitude from the ALTITUDE
screen, move to the BARO screen and enter this value as your reference altitude by following the same
procedure. Both readings will now be accurate.
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Backlight
Press the button to activate the backlight. The light will remain activated for one minute. Press the
button within one minute to deactivate the light manually.
Airow
The Kestrel Air Flow Tracker will display the volume airow, based on the instantaneous air velocity
and cross-sectional area of the opening through which the air is passing. If your particular industry
uses a standard of practice for air ow measurements, be sure to adhere to those standards when using
this product.
To set the dimensions of the duct or opening, press the button while viewing the AIR FLOW screen. The
DIMENSIONAL SETTING screen will appear with the word SHAPE highlighted.
The shape of the opening can be selected by pressing the or button. The options are round or
rectangular openings. Press the v button to highlight the dimensional settings. For round openings, simply
set the correct diameter of the opening. Use the and buttons to increase and decrease the value.
Holding these buttons will increase and decrease the value quickly. For rectangular openings, set both the
length and the width of the opening.
When the dimensional settings are correct, press the button to exit the DIMENSIONAL SETTING
screen and return to AIR FLOW screen. The displayed airow value will be the result of multiplying the
instantaneous air velocity by the area of the programmed opening.
Note that you may change the units for setting opening dimensions under the Main Setup Menu (see the
Main Menu setup section instructions). Choices are inches, feet, centimeters and meters.
The Kestrel Air Flow Tracker also features an averaging function that allows you to traverse a duct and the
unit automatically averages the CFM. This method may yield more accurate results, especially in ducts
with variable ow or with registers on them. To do so, from the current Air Flow screen, simply press the
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Kestrel®4200 Pocket Air Flow®Tracker
Starting with a known altitude for your location
You can obtain your altitude from a topographical map or local landmark. Google Earth
is an excellent free program that provides the exact altitude for any given address:
www.earth.google.com/. Set this value as your reference altitude on the BARO screen
to determine your barometric pressure: Press the button to enter the reference
setting mode. Press the button to increase the reference altitude or the button to
decrease the reference altitude. You will notice that the barometric pressure will change with changes in
the reference altitude. Press the button to exit the adjustment mode. Again, allow the Kestrel Meter to
stabilize, then enter the value from the BARO screen as your reference pressure on the ALTITUDE screen by
following the same procedure. Both readings are now accurate.
When reviewing stored data, remember that changes in pressure AND changes in location/altitude will
aect the stored values. When tracking pressure changes relative to weather, set the reference altitude on
the BARO screen and keep the Kestrel Meter in one location. Your graph history will now show trends in
barometric pressure. Your altitude as shown on the ALTITUDE screen will change as the weather changes,
but you can ignore this screen for this purpose.
If you are planning a day hike would like to track your altitude, you’ll need to enter the correct reference
pressure on the ALTITUDE screen as described above in “starting with the known barometric pressure.”
You can now track the altitude changes as you hike. In this instance, you should ignore the values on the
BARO screen, since the pressure changes will be due to changes in elevation far more than to changes in
the weather.
In general, changes in barometric pressure associated with weather changes are small over the course of
one day, but they will aect the accuracy of the altimeter over time. This is why aircraft reset their altimeters
at every aireld by entering the eld’s“altimeter setting”or reference pressure. Accordingly, if accurate
altitude readings are your primary interest, you should reset the reference pressure on your Kestrel Meter
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Kestrel®4200 Pocket Air Flow®Tracker
button to enter the Max/Average screen. Press the button to begin averaging, and traverse the
duct. When nished, press the button again to stop the averaging. Press to clear the data. See the
“Applications Examples”section for more information.
Relative Humidity
The Kestrel Meter is capable of measuring RH to a high accuracy: +/- 3% RH between 5 and 95%. To ensure
operation within these specications, please follow these recommendations:
• Avoid taking measurements in direct sunlight, which will heat the air inside the humidity sensor
enclosure and cause inaccurate readings.
• If your circumstances force you to expose the Kestrel Meter to a large temperature swing prior to taking
a relative humidity reading (such as when taking a Kestrel Meter stored inside at 70° F outside to a
temperature of 40° F), you will need to take additional steps to ensure that the Kestrel Meter’s external
temperature sensor is in thermal equilibrium.
- Ideally, provide an airow of at least 1 M/S (2.2 MPH), over the temperature sensor — point the
Kestrel Meter into the airow. If there is no airow, simply wave the unit back and forth so air
passes over the sensors. With airow over the temperature sensors and humidity chambers,
readings within specications will be provided within two to three minutes, even after a large
temperature shift.
- If no airow can be provided, you must allow sucient time for the RH value to stabilize. This can
take as long as 20 minutes — the greater the temperature change, the greater the time. You can
use the logging capability of the Kestrel Meter to conrm that the unit has stabilized to
a correct reading: Set the memory options to a relatively short logging interval (20 seconds
works well), select the graphical display of RH, and you can see when the value is no longer
changing signicantly. At that point, the RH value is stable and can be relied upon to be within the
accuracy specications.
