We planned on retrieving the deeper system units first, so after our customary visit to Bil’s dive center in Tulum, we headed out to one of the sites that our friends Jeff & Gosia had installed for us back in October. But a cracked sleeve on one of the high pressure hoses stopped the dive while we were still dry, and we spent a couple of hours hunting for a replacement in town. By the time we were ready to go again, a long dive was out of the question. So we chose instead head over to our primary test site on the coast. It was a short, shallow dive, and I had a new suspension rig that I was keen to put on the ceiling of the cave to bring those flow sensors closer together. We only had one new sensor ready for the site, but we could always swing by later to put the other units in.
The tide was with us, and we were at the site moments after leaving the surface. I did the now routine inspections, noting a bit more wobbling than I wanted to see on the suspension rods. I also spotted some discoloration on the white thermal-conductive epoxy I had used for the temperature sensors. I checked my watch, then the unit, watch, unit,…and saw no LED pips. Now that was a real cause for concern, but there was nothing for it at this point. So we collected the old flow sensors, removed the anchors, and I set about constructing a new connection rig from the various pieces of PVC I had in the mesh bag by my side.
A little extreme underwater plumbing, and an improvised extra support for the center of the rod (thanks to my old nemesis: vortex shedding) we had it installed. We connected the one new sensor we had with us, and were somewhat surprised that it took almost 180 grams of ballast to make it neutral (?), then I remembered that I had lithium batteries in this unit. High power/mass ratios are not as advantageous as they might seem in underwater applications. After returning to the surface, I cradled the Pearls as we drove the tanks back to Tulum, watching for any signs of life, but it was starting to look like all of the units had expired. I was pretty unhappy about that, especially since C1 was a “Rosetta stone” build, with both a BMA180, and a BMA250 acclerometer inside. I planned to use that data to develop a transfer function that could merge the data from the different build generations. Now it depended on how long that logger had operated before the epoxy let go. If water had entered the housing, there might not be any data at all. I was also cursing myself for putting an untested adhesive on the pressure logger, as that was our only reliable tide record for the site.
Back at base, I had a chance to examine things more closely, and the news did not get any better. The new epoxy had degraded into a flakey, rubbery mess, and rust had devoured my temperature sensors. My only hope was that the plastic weld putty around the wires passing through the hull had provided some measure of protection in the shallow water. Once we had photos of the damage, I started opening them up.
I was not expecting much, so I was pleasantly surprised to find that the loggers with the white epoxy had no water in the main housing. Both C1 and the pressure unit had small battery leaks, because the power module shorted out when salt water bridged the contacts, and alkalines usually pop if you drain them completely. The data files on the SD cards were intact, showing that C1 had two weeks worth of data, while the pressure sensor ran for a month before it lost power. I copied the files over to Trish, and moved on to other forensics. As with the Beta units in the Akumal Bay, the RTC’s had lost between 30-40 seconds of time over the three month deployment.
Then I turned to C2 and C4, which had been spared the bad epoxy. I had hoped for a full data set from at least one of them, but the log showed that they barely squeaked into October before pulling their batteries below the 2.8v cutoff. That meant we now had a month long data gap for a system that we had been monitoring continuously since the first alpha units went in. The C2&4 units power curves were so spectacularly bad that I immediately restarted them on the meter, and discovered that both of their SD cards were terrible, with one of them drawing > 7mA while the logger slept. (That’s probably some kind of record, and I am temped to mail it to Bunnie, to see where it came from.) And just to pour salt on the wound, the 7-8 month lifespan projections from the previous generation made me pretty bravo about power consumption back in August. So I left the C’s running on a short 5 minute sampling interval, taking three times as many data points as we actually needed. Had I set them to a more pedestrian 15 minute sampling schedule, they might have pulled though. Arrrgh!
But in the end, we had something to work with, and that’s all we really need from these early builds. While I was grumbling about crap SD cards, and adhesives made from leftover chicken parts, Trish had been click-clicking away happily on her data. She was in a much better mood than I was, so I asked her to cheer me up with a quick peak at some of the raw Z axis records out of C1. In theory, the 14-bit/1g bma180 (in blue) should outperform the humbler 12-bit/2g bma250 (red) which I had used on the earlier builds:
That 250 data is more stratified, but not nearly as much I was expecting, and the difference in signal magnitude is almost negligible. Huh…perhaps that inter-generation data translation is not going to be as tough as I though.
By this point (2 am? ish?) my own batteries were running low, so we called it a day. Not a great day mind you, but sometimes that’s just how it goes.