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Tom, I checked out that new sensor, the I2C interface from Maxbotix is a new feature, it might be handy, but I think the serial output is just as easy to use. My main concern is that the 7040 model is kind of a general purpose rangefinder used for object detection, usually indoors. In the datasheet, it warns that different target sizes will result in different range measurements, even if they are at the same distance. Basically, there is no filtering or target size compensation. The MB7389 model that I use is specifically designed to measure water level, so it has a special filter that returns the largest signal (the water surface), so all other sources of noise and reflections are ignored.
And most importantly, the MB7389 has internal temperature compensation that is applied at each power-up. Without this, the varying temperature of an outdoor environment will make a non-compensated sensor rather inaccurate. And my recent tests have shown that we need to also use the maxbotix external temperature compensation sensor (only $5) in order to eliminate all of the effects of temperature variations.
I’ve had many communications with the technicians at Maxbotix in the past year, and they assured me that the MB7389 is the best model for water level measurements with TTL-level serial output, so I would recommend that you try that one.
The rest of your post on your blog sounds interesting. That’s the kind of content we’d like to have here on this site as well, so you should post it here on a blog post so users here can comment easily on it.