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WATER LEVEL MONITORS AND AUTOMATIC BAROMETRIC PRESSURE COMPENSATION
Advantages of Vented Water Level Loggers
- Eliminates errors of up to 10” which can be caused by barometric pressure changes
- Accurate readings in the field
- Sensor does not have to be moved to take data
- Battery easily replaced without moving sensor or opening waterproof seals
- When a non-vented logger is taken out of water, errors may be caused by the extreme thermal and pressure stress
- The cable often kinks or bends, which makes it difficult to return the sensor to the same location in the water, which causes additional errors in the data
Why do water level monitors need to have barometric pressure compensation?
Storms and elevation cause changes in barometric
pressure. Stormy weather can produce typical barometric pressure differences in
the range of 25 mb during a single day, while the storm passes over your water level
monitoring site. Since one millibar (mb) equals one centimeter of water this equates
to almost 10” of water level error or an error of over 2.5% for a 30’ range sensor. Remember
also that these barometric pressure differences are spread across the storm as well. This
is significant if you are monitoring a large area and have multiple water level monitors located
around the site, as areas of the site may be under different pressures during the same time
period. We have talked with clients who have faced this situation while
monitoring a large lake and were using absolute water level monitors (water level intrumentation
that does not have automatic barometric compensation). After they had gathered several
years of data they were forced to view all of it with suspicion when the purchased a second
barometric pressure compensation sensor for the other side of their lake and realized that
the two barometric pressure monitoring sites had significant diferences as storms passed over.
Elevation also plays an important part in
barometric pressure compensation requirements. When an absolute transducer is
moved to a different elevation than where it was manufactured additional corrections must
be made because of the significant changes in barometric pressure (about 35 mb per
1000’ of elevation, or an error of 1’ of water level/1000’).
What are absolute water level monitors and how do you take water level measurements with them?
Unlike Global Water’s differential water level monitors,
there are absolute water level sensors, like the DCX-22, that do not use
vented cables to automatically compensate for barometric pressure changes. These transducers have
one side of the sensing element exposed to the water while the other side is sealed. To correctly interpret
the data from these types of water level monitors the purchase of an external
barometric pressure monitor is required at each site and the elevation for
each monitoring site within a system must be determined. You will need to purchase one of these
barometric pressure monitors to record the air pressure for each site typically within 100 mile radius (given terrain
conditions, see above) of your water level monitoring site. Purchasing this barometric
compensation sensor should not be viewed as optional since even areas with stable pressure throughout the year
vary a several millibar and at sea level it is common for pressure variations of around 80 mb or 31” of water level
change throughout the year. In addition to having to purchase the extra barometric compensation sensor
you will also have to determine the elevation of each of your water level monitors. To actually calculate
your site’s water level reading you will be required to process all of this information, including each barometric
pressure reading, through some type of interpretation software that uses built in formulas to give you the true water
level reading at your monitoring site. Finally choosing to use absolute water level monitors to monitor your
site will decrease the overall accuracy of the measurements, because the error of both the water level
monitor and barometric pressure sensor must be considered.
Is there an alternative to absolute water level monitors?
We are proud to say emphatically, YES, they are called differential water level monitors. Global
Water made a decision early on to provide its clients with easy to use water monitoring
equipment. This philosophy has carried into our water level monitors and data
loggers. Each water level monitor uses a differential pressure transducer with automatic
barometric pressure compensation. This type of water level monitor, when under water,
measures the water level only since changes in the barometric pressure caused by storms or
elevation changes are the same on both sides of the sensor, automatically canceling each other
out. They do not require any external hardware and do not require you to import data from two
separate sensors to get correct data. As an added bonus you can read your site's true current
water level depth while interfacing directly with the instrument. We strongly recommend that
you use differential water level monitors (vented pressure transducers) for their accuracy, simplicity
and reliability. Please visit our water level data logger or
water level sensorpage for more information on these products.
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