A typical deployment
The August deployment produced both success and failure from our 1st-generation temperature strings. We still managed to get both of those older units back on their feet with fresh batteries, and we add two beta units to the set. I really had to scramble to get the new chains ready in time because we are spending more time on testing and calibration as I try to squeeze the best possible performance out of these humble DS18b20’s. It takes me about a day to solder and epoxy a section with 8-12 sensors, so these instruments also represent a significant amount of build time. (note: the length of the wires in between nodes does not affect that time very much)
Although both of the alpha loggers passed the overnight tests following their first run, the shorter (25cm) chain developed a reading problem as soon as it was powered up. The fact that this error occurred before the unit went near the water tells me that it was either a sensor failure, or a problem with the connectors. I have been using Deans 1241 micro connectors between the segments because they seem really robust, but my gut tells me those break points could also add some signal reflection problems.
We went hunting for potential deployment locations by dangling a 24m chain from a life jacket and moving it around the cenotes to generate profiles. I think I will put a display screen on one of the next units to make these ‘prospecting’ trips easier in the future.
The logger itself ran for the duration, but the log data was a string of the dreaded 85C (ie: 1360) and ‘-1’ read errors. Since these numbers are fairly distinctive, I will put an error check in code on startup to see if I can intercept this kind of problem in the future. At least the pressure record from the MS5803 on the housing survived intact, and that sensor seems to be working again now that I have removed the Qsil silicone coating that I had over top of the sensor on the previous deployments.
I isolated the read fault to the first segment of the temperature sensor chain, and when that section was removed the rest of the sensors ran well enough. We decided to re-deploy the parts that were still running (although the chain is now less than four meters long) and I brought the dodgy section home for some forensic testing. I am suspicious of the U-09LV urethane that I used on a few of the nodes, thinking that it’s higher moisture resistance might not compensate for the stiffness and overall durability of E-30CL.
Fortunately, the longer chain that we deployed in the deeper inland site performed well, giving us another record with sensors spanning the halocline:
Two months of raw DS18b20 output, Logger 46 (10m cable with 20 sensors). Note: the warmer temps shown at the top of this graph are from sensors deeper in the water column, while the cooler temperatures are from shallow sensors in fresh water
Even with relatively long 50 cm spacing, the large rain events of the season pushed the fresh/salt boundary around so much that several sensors (indicated here with 48pt moving averages) switched from the saline, to the fresh water, and then back again. It will be interesting to see if those bands tighten up, or spread out, after we apply our normalization factors.
This 6m x 24node chain has a 10m extension, allowing us to change sensor positions in the water column by simply tying off the excess.
After several meetings to obtain permission from the landowners, we managed to install our new set of DS18b20 temperature strings. We decided to co-deploy a combination of high and low spatial resolution chains, so that we still have a good chance to get data, if one loggers dies. Due to memory limitations, etc. I built them with twenty four sensor nodes per logger, and even with those spread out over 24m of cable, 3k3 pullup resistors are enough for the one wire communications. That’s aggressive enough to give me some concern about self-heating if I was doing multiple readings, but I figure that with the bus at 3.3v it probably just comes out in the wash.
This deployment site had significant amounts of hydrogen sulfide at depth which forms a visible layer that is shown well by these photos from Angelita. It will be interesting to see how the chemistry affects our sensors. It certainly had an effect on me, as I was a little worse for wear after that dive.
Underwater Arduino Data Loggers 