Trish ties unit one to the ceiling of the cave.

Although our field tests so far had uncovered buoyancy issues, both designs had remained water tight. So I was feeling brave enough to think about putting the electronics into the housings. And the UNAM crew was arriving in Tulum today, so I was about to loose my dive buddy to “real” research. We decided to go for it and install the little loggers, in a cave system, and leave them there for a few days taking readings every five minutes.

For this first deployment we chose a system that Trish had already calibrated for discharge many times over the years, allowing her to review any data that my little DIY units might produce within that context.  But given the buoyancy issues we had seen over the last few days, we decided to test everything in open water at the entrance.  And it’s a good thing we did as, once again, the o-ring design sank*… as you might imagine, I spent the next few minutes making some rather large bubbles.

…Once I regained my composure, we decided to make lemonade. If that unit was not going to float, then we would simply install it upside down, as a pendulum. The magnitude of the displacement would be almost the same, and all I had to do was change the sign on a few of the readings.  So we grabbed one of the anchors, and a bit braided line from the dive kit, and made our way into the cave.  I carried the anchor and poles, while Trish ran the dive reel leading us into the dark of the cave.  I have to admit I winced a few times as the mesh bag carrying her unit occasionally bashed into the nearby rocks, while her attention was focused on the line. I had visions of those Tinyduino stack connectors coming apart, “But hey” I told myself, “that’s what a this is all about.”  They would either survive the real world, or I would have to go looking for a different electronics platform.

We made our way to a location where we had installed one of the old RCM Aanderaa sensors, many years before. And while I found a place where the anchor didn’t sink elbow deep into the piles of organic mung, Trish tied off the pendulum unit. We did a few laps round the installation with the waterproof camera, to capture a little video, and then made our way back to the entrance. Our loggers were now out in the wild, collecting real flow data! The schedule was pretty busy for the next little while, so it was going to be a few days before we would be able to retrieve our units, to see if they worked. It will be interesting to see how the readings compare to each other.

I have my fingers crossed!

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*Some time later, we found that AA batteries vary considerably in their mass, and I had switched brands just before this deployment, throwing our buoyancy off again.

I had some serious concerns about the wobble behavior observed during the early housing tests, and that vortex shedding might still kill the whole idea of measuring tilt as a proxy for water velocity. But without some real world observation there was no way to know if this was a lethal design flaw, or just an artifact of plain old surface turbulence. The support poles I had given the student to try out earlier were only 50cm long…

The new housings get their first real world test.

So, even though we started our trip with a lovely bit of dry caving, I was very keen to get the new housings in the water before Trish started diving with her grad student and the diverse group of UNAM & Texas A&M researchers that would be arriving soon.

We piled all the bits into the rental car, and headed out to a local coastal outflow that was easy to access from the surface. I reasoned that if the units didn’t respond in the high flows at that site, there was no point in trying them out in the slower cave systems.  And, as this was to be the first test in anything deeper than a laundry tub, I decided it was safer to leave the electronics behind, and test them with simple calibration weights inside. We passed the usual gaggle of snorkelers on the way in, who stared curiously at all the plumbing we were carrying. A few continued to circle around above us as I set to work getting the anchors in place,  threading the poles, and attaching the housings.  At least I had proved that the thread plugs were easy to attach under water!

Right from the start the units appeared to be working: they were leaning in the direction of flow, and by 45 degrees (or more). I knew that the 2 G rating on our accelerometer meant we were using less than half the sensors range, but there wasn’t going to be any problem reading a signal that strong. And although we could see some of the wobble that the student had mentioned in the support poles, it was fairly mild except in the fastest flow areas. So the dreaded vortex shedding problem turned out to be far less serious than I had feared. I was happy!

Trish and I spent the next little while swimming around the units, trying to see them from all angles, while the curious onlookers tried to figure out what the heck we were doing. But as we continued discussing the floats, working on where to move them next, a problem was slowly developing. Over the next 15-20 minutes, the o-ring housing, slowly, inexorably, sank to the bottom. The only logical conclusion I could think of was that the seals had failed, and that the Mark II, which I had so carefully assembled, was a failure.

Later, after we had retrieved the units and dried them out at the surface, I cracked open our sinker, preparing myself for….Nothing! It was bone dry inside! How does something sink “slowly” without leaking? I put the cap back on and marched back over to the water, to dunk it in. And it floated, just as it had before, with just a small bump of pvc cresting the water. Hmmmm.

Then I grabbed the hollow support poles, tossed them in, and they floated too.  What was going on?  I moved the tubes around a bit, and spied a few small bubbles leaking out one end…Aaaha! They were not as buoyant as I thought they were. A bit more shaking to fill their internal volume with water, and I managed to get the poles to sink, very, very, slowly.

Then I jumped in, and pushed the housing down to the bottom; to observe it at depth. On the surface, it floated happily, but down around 15 feet, it rose much more slowly.  It dawned on me that I had been compressing the new o-rings for the first time! So the bubbles in the support poles were draining out, and, the internal volume of the housing was also changing with oring compression!

So today was not a failure at all!  I had simply shaved the buoyancy budget too close in my quest for more response to water flow. I could fix this…

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