Manual Barometric Correction - Waterra-In-Situ

Manual Barometric Correction
Correcting for Barometric Pressure Effects Using a
Spreadsheet
June 2008
Background
Certain monitoring applications may require the use of
instruments with absolute (non-vented) level sensors.
Barometric pressure fluctuations may cause errors in
level data. This error can vary from 3 to 8 cm daily, or
even exceed 20 cm as strong weather systems move
through the area during long-term studies. Therefore,
to correct for these fluctuations, barometric data that
correspond to the level data must be collected.
Sources of barometric data
Barometric data from weather stations often do not
represent the true ambient barometric pressure.
Instead, weather data typically include barometric
pressure values that are normalized to sea level.
Generally, air pressure decreases as elevation above
sea level increases because there is less atmosphere. To
make weather reporting consistent, weather stations
commonly report what the barometric pressure
would be if the location was at sea level (corrected for
elevation).
For example, the average true barometric pressure in
the mountains of Colorado is about 22 inHg (about
740 mbar). However, a broadcast weather report
would report a barometric pressure of around 30 inHg
(1,015 mbar) because a correction has been made
for elevation.
The true air pressure is the factor that affects water
level readings from an absolute pressure instrument.
Data from absolute instruments must be corrected
with barometric pressure data measuring the true air
pressure and not values that have been corrected for
altitude. The In-Situ BaroTROLL® instrument records
true ambient barometric pressure.
Finally, a user-supplied reference value should not be
used when recording barometric pressure. Only record
the raw pressure values (e.g. PSI, mbar).
Data format
Without proper forethought, the most time consuming
step when manually correcting level data will be
editing the spreadsheet file. To simplify this arduous
task, synchronize the sampling schedules of the two
instruments before actually collecting data, so that
level and barometric readings are taken simultaneously.
To further simplify matters, the In-Situ Baro Merge
software can automatically correct the time stamps
from the barometric and level data.
If the sampling schedules are not synchronized and
you are not using Baro Merge software, you will have
to make barometric readings in one column align with
corresponding level readings in another column, or
interpolate the barometric data time stamps to match
the level data time stamps.
For the following instructions, we will assume that time
stamps of barometric data match the time stamps of
the level data in your spreadsheet file.
Correcting level data in a spreadsheet
The following steps explain how to manually correct
level data for barometric fluctuations.
1. Export the absolute pressure level data to a
spreadsheet application such as Excel®. (See the
Win-Situ® 5 online help for assistance.)
2. Place the barometric data in an adjacent column.
3. Create a column that has the barometric data from
step 2 converted to the same units as the water
level data (for example, meters of water).

4. If the water level data were collected using a
reference (either level / surface elevation or level /
depth-to-water mode), then you will need to
create another column that reports the change in
barometric pressure since the time the reference
was set in the instrument. The barometric pressure
change should equal zero at the time the level
reference was set.
Hint: For example, if the instrument was set to apply the level
reference at the start of the log/test (first data point), then
in the new column, you should subtract the barometric
pressure at the time of the first level reading from all
subsequent barometric pressure values. If a level reference
was set prior to the start of a log/test, then you should
subtract the barometric pressure at that time from all
barometric readings.
Making corrections
Add or subtract the adjusted barometric pressure from
the water level data, depending on which recording
mode you used for the level data.
Pressure mode—This is the easiest case. Assuming
that the barometric pressure data are in the same units
as the level readings, subtract the barometric readings
from corresponding level readings.
Depth mode—Convert barometric pressure data to the
same units as the water level measurement (e.g., meters
of water, centimeters of water). Subtract the converted
barometric readings from the water level readings.
Level / surface elevation mode—Convert the
barometric pressure data to the same units as the water
level measurement (e.g., meters of water, centimeters of
water). Create a new column that shows the change in
barometric pressure since the water level reference was
set in the submerged instrument. Subtract the values
in the barometric change column from the water level
readings.
Level / depth-to-water mode—Convert barometric
pressure data to the same units as the water level
measurement (e.g., meters of water, centimeters of
water). Create a new column that shows the change in
barometric pressure since the water level reference was
set in the submerged instrument. Add the values in the
barometric change column to the water level readings.
Conclusions
For general use, In-Situ® recommends using vented
instruments and cables that automatically correct
for barometric pressure changes. However, specific
circumstances of an installation may require the use of
non-vented instruments such as the Level TROLL® 100
and Level TROLL® 300.
Level data collected using absolute pressure
instruments do not always require correction (shortterm
studies such as slug tests or vertical profiling
typically do not). However, when barometric correction
is required, a BaroTROLL® instrument used in
conjunction with Baro Merge software can simplify
your data analysis. In the absence of these tools, data
can be adjusted using the procedure outlined above
with any barometric data logger and a spreadsheet.
For more information contact In-Situ Inc.
221 East Lincoln Avenue
Fort Collins, CO 80524
1-800-446-7488 (toll-free in U.S. & Canada
1-970-498-1500 (international & domestic)
Internet: www.in-situ.com
In-Situ, the In-Situ logo, ® and are trademarks or registered
trademarks of In-Situ Inc. © 2008, In-Situ Inc., Fort Collins, CO
USA. Excel is a registered trademark of Microsoft Corporation.
Correcting for Barometric Pressure Effects Using a Spreadsheet Page 2