The original float/pendulum idea

Once I had seen the old fashioned air speed indicators, it did not take me long to find references to an even older idea: the hydrometric pendulum. (mentioned in books as old as 1884) So a simple pendulum (or float!) would indeed work if I was able to measure the angle of deflection.  I scribbled a few doodles down on a piece of paper, and ran in to my wife’s office at Northwestern University to discuss it with her. At the time, she was conducting a lab session with her instrumentation students, so they also got an earful of my enthusiasm, which amused everyone.

I don’t have any of those original “back of the envelopes”, but I did find one of my early concept drawings from some time in February 2011:

Original concept sketch from 2011

Original concept sketch from 2011

As you can see from the picture I figured I was going to need a really big battery in a heavy enclosure resting on the floor of the cave, so I initially conceived of the device using a float rather than a pendulum. I was still trying to figure out some way to actually measure the angle of deflection without breaking the integrity of the underwater housing because I knew each gasket or o-ring was just another potential point of failure. I had some idea of just attaching the bobber to a joystick mechanism, and one friend suggested putting magnets on the string and using hall effect sensors in the case.

But I was still trying to figure out how it might be possible to put everything, including the sensors, inside the float itself. I soon discovered from the Arduino forums I was rummaging through that there were plenty of people using accelerometers in tilt sensing applications very much like this, for robots, quad copters, and even one fellow who put a datalogger with an accelerometer on his garbage can.

One of the prototypes from 2011. (held together with hot glue.)

One of the prototypes from 2011. (held together with hot glue.)

So I set to work building prototype data logger / accelerometer combination, bootstrapping myself on the arduino micro-controllers with the many helpful tutorials at Adafruit Industries , and of course the Arduino playground.  I was, and still am, beholding to the many people who share their expertise so freely in the open source hardware community. And I managed to cobble together a couple of dry prototypes (that actually recorded data) near the end of 2011, which I dragged around at Christmas of that year, doing show & tell sessions with my more technically able friends. My wife, bless her, put up with “Ed’s latest project” evangelism, even though I had melted my way through most of the drinking cups in the kitchen, and the house was beginning to take on the distinct bouquet of poor soldering & burnt plastic. At the time I was using a vanilla Arduino Uno, with an Adafruit data logger shield.  To that I had added the MMA7361 from modern devices as the accelerometer, and the whole thing gave me a whopping 24 hours of run time out of 6 AA batteries.  Not exactly the year’s worth of data we were hoping for but I had managed to cram all of that into a hunk of pvc pipe from Home Depot, so the “all in the float” approach was looking like it might just be possible.  I still had not given anything a “dunk” test yet, but at least it was a start.

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