I was keen to see if the Pearls were sensitive enough to track the slower deep water circulations and Trish has an ongoing collaboration with some UNAM researchers studying cave organisms in a system called Ox Bel Ha. With the new build, I was confident that we could push the envelope a bit, so we planned a dive to a deploy the remaining two Cave Pearls in the deeper saline zone of that system. (around 20m)
…thirteen…slap!…point…rrrrgh!…seven…
Once again our friend Jeff loaned me some of his dive equipment, and even better, both he and his partner Gosia, were able take a break from their busy instructor schedules to join us for the installation. Jeff had often offered his services to researchers in the past, and I think my nerdy enthusiasm amused both of them. As with previous installations, I calibrated the buoyancy of the sensors at the surface with a small hand-held postal scale. Deeper systems tend to have slower flows, so I adjusted the Pearls to only 10 grams negative buoyancy. This was pretty close to the wire for a system at full marine salinity, but with flows down in the 0-5 cm/s range I was hoping for the best sensitivity I could get. With our kit sorted we put in at a rather boggy zero-vis cenote whose large population of mosquitoes & tábanos which the pre-dive checks at the surface a trial, despite the fact that they had already feasted on me while I did the buoyancy calibrations.
I was sad that we had to leave my little waterproof point&shoot at the surface, because it was a beautifully decorated system, with intersecting passages at multiple levels. Three of us followed Trish’s lead out to a nice wide section where we waited patiently on the line while she inspected the cave with her hydro-geologist’s eye. She found a spot, with a roof profile suitable for our bungee anchors, and instructed me to connect three 50cm segments to pivot, putting the meter in the center of the passage, at 22m depth. With the supports connected I returned for the first flow meter, only to make the unwelcome discovery that both of the sensors were now positively buoyant. Arrrgh! I had cut it too close by calibrating to only -10 grams in the fresh water of the cenote! We transferred a couple of five gram ballast washers over from the second unit, but we still had a slow persistent rise to the ceiling. Trish provided a temporary solution by adding a metal dog clip to the support rods, and since we only had the one spare clip, we called the dive with our second flow meter still in the bag.
Despite the buoyancy problem, everyone was happy with the overall simplicity of the installation procedure, and Jeff graciously offered to re-calibrate and install the orphan meter the next time he was in the system (and he wanted his clip back 😉 )With our shortened trip schedule, we took him up on the offer, and after a celebratory cerveza in Tulum, we gratefully left him with all the pieces he would need for the second installation.
Underwater Arduino Data Loggers 