Demystifying Water Level Loggers

Whether open-channel or full-pipe application, a vital aspect of flow management in piping and distribution systems is water level logging.

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Whether open-channel or full-pipe application, a vital aspect of flow management in piping and distribution systems is water level logging. With ever-increasing level logger product choices today, though, it’s more difficult for users to evaluate, with a solid grasp, the various accuracy specifications suppliers present. Still, understanding exactly what type of accuracy a logger will provide can be critical.

For the selection process, develop an educated eye when it comes to accuracy specifications, and identify claims that may be misleading or inaccurate. Here are some useful questions to ask suppliers for a better understanding of how a water level logger will perform in your application:

1. Does the accuracy specification apply across the full-calibrated measurement range of the logger? The accuracy a water level logger can achieve at the high or low end of a given range may be far different from that at the middle. So, it’s important to find out if the logger’s accuracy specification refers to a single point or the entire measurement range. Knowing the full-range accuracy will assure you it will meet your accuracy requirements.

2. Do temperature variations cause additional error outside of the accuracy specification? Some water level loggers aren’t able to effectively compensate for temperature changes, which cause incorrect pressure readings. As such, it’s important to find out if error that results from temperature changes is included in the accuracy specification, or if there’s a separate error term that must be added. Accurate temperature compensation is especially important in surface water monitoring applications such as stream and coastal studies, where water temperatures can vary widely from sun exposure.

3. Does the accuracy specified relate to only the logger’s sensor, or to the entire logger? A water level logger’s sensor and analog-to-digital converter (ADC) both contribute to accuracy error. Error from the ADC can be just as significant as sensor error. For this reason, you’ll want to confirm with the manufacturer that the specified accuracy refers to the entire instrument rather than just the sensor. Also don’t be confused by impressively small resolutions, such as a resolution of 0.001% based on a logger with 0.1% accuracy. Resolutions this small usually indicate “noise,” or data that’s below the threshold of meaningful measurements.

4. Is drift important? The pressure sensors in water level loggers will drift over time. Whether you need to be concerned about drift depends on your application. It’s important in cases when absolute pressure values are needed, or if there are no recent reference level or depth measurements available. This may be the case if a water level logger is deployed for over a year with no reference level readings taken during that time. Otherwise, drift isn’t a significant factor since it’ll be offset by regular (i.e., monthly) manual reference level readings. Regardless of whether drift will impact your data, it’s advisable to ask the manufacturer for drift specifications.

5. How do I know if each and every logger meets the specified accuracy? Some water level logger products experience significant variations from unit to unit and production batch to production batch. So, it’s a good idea to ask the manufacturer if the logger’s accuracy has been verified or measured against NIST-traceable standards. Some companies stand behind accuracy specifications by providing a calibration certificate of accuracy with each logger.

Conclusion

In general, when evaluating water level loggers, don’t be afraid to ask hard questions. Suppliers should be able to provide you with hard numbers, not just marketing buzzwords or range qualifiers. For a hands-on approach to demystifying logger accuracy specifications, consider testing level loggers from different suppliers yourself. A simple accuracy test may involve placing two different loggers in the environment you wish to monitor for a week, taking a reference reading at the beginning and end of the week and comparing those readings to your logged data to compare accuracy.

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About the Author: Nick Lowell is director of hardware engineering at Onset Computer Corp., of Bourne, MA. Contact: 800-564-4377 or www.onsetcomp.com

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