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Activity

  • Evaluation by the Save our Water, Watershed Hydrologic Analysis Team, Marion Waggoner and Dave Yake.

    Background

    Broad Run drains a 10 km2 watershed straddling southeastern Pennsylvania and northwest […]

    • Thankyou so much for doing such detailed work. I’d been thinking of a boom situation for some measurements. However the atmospheric temperature will vary.

    • Great work guys. We are also interested in these sensors and have also been considering a boom-type setup.
      I think it might be worth getting one of these to see if it’s fit for our purpose.

    • Hey Dave,
      As part of my technical institute’s graduation requirements, I’m currently designing a project to test the accuracy of MaxBotix’s sensors (as well as a $75 vented pressure transducer) to determine their applicability for streamflow monitoring. I’m also a hydrometric tech so do put a lot of thought into collecting accurate data.

      With that said, I do have a couple of thoughts to consider in regards to how your test was designed.

      Here they are in point form for brevity’s sake:

      • The U20-001-04’s (13 ft range) accuracy is 3-6 mm, so that needs to be considered. It also needs to be compensated with barrometric pressure (not mentioned in the report) which adds additional inaccuracies.
      • When measuring water in a stilling well with a pressure transducer, and the surface of the water with an ultrasonic sensor, you can’t consider the two data sets as reporting the same values. In high-velocity situations, you can get positive or negative pressure, thus lowering or raising the water level, in your stilling well depending on how it is designed.
      • And most importantly, in your on-ground test, the image shows that the sensor is on a metal boom. This is problematic as long metal objects deform/bend with temperature. You metnioned that you also did this test with the sensor at the top of a stilling well, but I didn’t see those images.

      Anyhow, I don’t mean to be a downer. I basically just listed these points as I’ll be critiquing your work in a literature review as part of my project.

      Really though I think it’s great that you’ve gone to all this work to test the sensor. I learned a few ideas from reading this that I’ll be incorporating into my tests.

      Give me a shout at gallaugher.consulting@gmail.com if you are up to chatting about your tests and helping me to design mine.

      • Rory,

        I have copied the comments from our email so that others can view them… thanks again.

        dave

        The U20-001-04’s (13 ft range) accuracy is 3-6 mm, so that needs to be considered. It also needs to be compensated with barrometric pressure (not mentioned in the report) which adds additional inaccuracies.

        With respect to the barometric pressure: a. Pressure sensor for depth: Remember that we are compensating for barometric pressure changes in the Onset logger by having a parallel sensor following barometric pressure. Not sure he understood that.  b. Also, with respect to the ultrasonic sensor, the barometric pressure has a very minimal effect on the speed of sound in air. Marion remembers that it is 2 orders of magnitude below the effect of both temperature and relative humidity (which is much lower than the temp effect.) So, really the only important effect is the temperature effect on speed of sound. 

        </li><li>When measuring water in a stilling well with a pressure transducer, and the surface of the water with an ultrasonic sensor, you can’t consider the two data sets as reporting the same values. In high-velocity situations, you can get positive or negative pressure, thus lowering or raising the water level, in your stilling well depending on how it is designed.

        We discarded using the ultrasonic sensor on a stilling well due to the signal dropouts which were somewhat erratic as to when they occurred, but clearly were connected to temp changes from high to low which induced some kind of condensation on the walls (that caused deflection of the wave). Similarly, we actually dispensed with the stilling well on the Onset depth logger just for convenience. So, stilling well experience is not something where got comparison data. The Final result comparisons were not data from stilling wells..

        </li><li>And most importantly, in your on-ground test, the image shows that the sensor is on a metal boom. This is problematic as long metal objects deform/bend with temperature.

        We don’t believe your comments about the metal boom having a significant impact on correct. If you calculate the change in dimension for a uniform metal (ie..
        no bimetallic effect) that is heated on only one side (eg. sun) while the other side is cool (shady side) then you can see that the metal is much less than wood. For example, CTE’s for metal will run around 1 * 10**-5 while wood will be at least 10X higher. Still, neither will show much bending with temperature differentials that we see in outside testing. That said, however, it is a concern for planned experiments, and easy to manually measure the separate distance from the boom to the target over the temp range..

        We would be interested in your results if/when available…. In particular, it would be helpful to see what the temperature effect (water temp) has on a pressure sensor logger (such as the Onset or the CTD sensor used in the Stroud logger stations..