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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
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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