Rory G

Hi David,

Just to follow up on your comments, and then provide the report that I wrapped up last month where I tested the Maxbotix sensor.

Just a quick reply to each comment here:
1) yep, definitely understand that barometric pressure is only relevant to the pressure transducer and that the ultrasonic sensor is mostly affected by air temp.
2) stilling well placement is finicky, and so thought I would just comment on that. It’s not where I would have placed the well but to each their own.
3) Interesting to learn that wood would bend more than metal. I had not done the calculations but was told by Water Survey Canada that they accidentally collected errored water level data because they had an ultrasonic sensor on a metal boom that fluctuated with temp.

I should mention too that my comments were in preparation for a literature review that I was preparing for my report. There isn’t much material available on the accuracy of Maxbotix sensors, and so was overly critical of what I did find to show my professors that I have my own thoughts on the current research. In reality, though I really enjoyed your work and found it quite helpful.

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.