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| WATER LEVEL MONITORS AND AUTOMATIC BAROMETRIC PRESSURE COMPENSATION |
Advantages of Vented Water Level Loggers
- Eliminates errors of up to 10 inches which can be caused by barometric pressure changes
- Accurate readings in the field
- Sensor does not have to be moved to retrieve data
- Battery easily replaced without moving sensor or opening waterproof seals
- The mounting 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
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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 inches of water level error or an error
of over 2.5% for a 30 ft 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 air pressures during the same time period. We have interviewed
clients who faced this situation while monitoring a large lake and were using
absolute water level monitors (water level instrumentation 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 they purchased a second barometric pressure
compensation sensor for the other side of their lake and realized that the two sides of the lake had
significantly different pressure readings when 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 ft of elevation, or an error of 1 ft of water
level/1000ft).
What are absolute water level monitors and how do you take water level
measurements with them?
Unlike Global Water's WL400 model
differential water level monitors, there are absolute water level
sensors, like the DCX-22 model, 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 by several millibars and at sea
level it is common for pressure variations of around 80 mb or 31 inches 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 was carried
through into the design of 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 sensor page for more information on these products.
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