In-Situ, Inc.

Water Level Measurements-Part 1


Courtesy of In-Situ, Inc.

Confused about which instrumentation to use in your water level monitoring system? In-Situ offers two categories of pressure sensors for water level measurements: absolute or non-vented sensors, and gauged or vented sensors. They look the same and both measure pressure and water level, but they have drastically different applications, outputs and accuracy. The absolute (or non-vented) water level sensor measures and records all of the pressure that is exerted on it, meaning both the water pressure AND air pressure. The gauged (or vented) water level sensor is manufactured and deployed to differentiate between water level and air pressure thus allowing it the ability to auto-compensate for barometric pressure and provide a true water pressure reading.

The unmodified data from an absolute water level sensor by itself is useful if you are really good at math on the fly; you will have to subtract out the atmospheric pressure from the pressure readings, multiply that difference by 2.307 to covert that data to feet of water, and then subtract the calculated feet from the depth reading. Sounds easy enough to do in your head? To make it easier, we have created a tool in our Win-Situ 5 Software to correct absolute water level data by integrating readings from a BaroTROLL data logger or local atmospheric data from someplace like an airport or weather station. A BaroTROLL water level data logger is deployed at the top of a well and collects detailed atmospheric data from the site. Other sources of atmospheric data (such as an airport) will be more general.

Accuracy for all of the sensors in the Level TROLL® line is 0.1% full scale of the pressure range of the sensor. That means to get the best accuracy that you possibly can, you should choose a water level sensor that is closest to the typical pressure range that you will be monitoring.

Because the absolute sensors require two inputs to give a water level reading, you will lose a bit of accuracy for that sensor. For example, a vented 15PSI level transducer with an accuracy of 0.1% FS will give you an accuracy plus or minus 0.42 inches. To achieve the same results with an absolute sensor you would use a 30PSIA water level sensor to measure the pressure in the well, and a BaroTROLL data logger to measure the atmospheric pressure. They both have an accuracy of 0.1%FS (0.83 inches) but when we add those together we get a total maximum error of 1.66 inches. I think this is better said with a picture…

Absolute vs Gauged Accuracy


So why would you ever use an absolute sensor? The drawbacks of one type of sensor are sometimes the benefits of another. Gauged sensors are more accurate, but they have to be open to the atmosphere and sometimes maintaining that is not achievable. If you have a site that is prone to flooding, a sealed well, or need a low cost solution where real-time data is not required, then we recommend an absolute sensor. But if you need the most accurate data, real-time data, or are using the water level sensor to control something, then we recommend a gauged sensor.

Whatever your needs, In-Situ offers a variety of water level data loggers within our Level TROLL line, so depending on your application, you should know when to use an absolute sensor and when to use a gauged sensor. Both of these sensors collect data in linear, linear average, or event logging modes, and are mostly used for long-term monitoring or slow-responding slug tests.

The Level TROLL 700 is the Cadillac of the Level TROLL line. It can be absolute or gauged, and has an expanded feature set to gather more accurate data. In addition to linear, linear average and event logging modes, it also gives fast linear (1 min or less), step linear, and true logarithmic. To accommodate those data collection modes, it also has an extended memory capability of 4MB. This water level data logger is typically used in aquifer characterization test, pump tests, slug test, and step drawdown tests. It can also be used in conjunction with a Virtual HERMIT to monitor several wells and graphically depict real-time data on a laptop.

I hope this has cleared up any questions you had about when use to use an absolute or gauged water level sensor. Our team of experts is ready to help you find the right solution for your project, and our support team is always standing by to offer 24/7 technical assistance. Whether you’re looking to rent or buy, we’re here to help!

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