In-Situ, Inc.

The importance of Dynamic density compensation in Water level monitoring

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Courtesy of In-Situ, Inc.

OBTAINING SUPERIOR WATER LEVEL ACCURACY IN COASTAL ENVIRONMENTS
In-Situ Inc. water level instruments measure the sum of all pressures (atmospheric and hydrostatic) exerted on a pressure transducer and use that data to calculate water levels. Water density contributes to the total hydrostatic pressure. Saltwater has a higher specific gravity than fresh water. A standard column of saltwater exerts more pressure per square inch (PSI) on a pressure transducer than the same column of fresh water. Higher PSI levels are typically interpreted as increasing water levels, but many times are simply due to increasing salinity levels.

In environmental monitoring applications, typical water level sensors can not measure water density variations (due to salinity changes) over the course the monitoring period. The monitoring instruments report all pressure variations as changing water levels. More sophisticated water level sensors can compensate for different water density via input of a fixed, or static, specific gravity value. This compensation method, however, is only effective if the salinity levels do not change during the monitoring period. If not compensated for, changing salinity levels can impact water level accuracy by up to 2%.

The In-Situ Aqua TROLL 200 instrument (see Figure 1) monitors and records water level, temperature, plus conductivity and salinity. The Aqua TROLL 200 instrument automatically and continuously corrects its depth and level parameters for changes in water density due to changes in salinity. This can dramatically improve the accuracy of depth and level measurements in estuaries and coastal waters where tides and rainfall continuously affect the local salinity.

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