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.