Field Report 2014-03-22: The next generation of flow sensors is deployed.

The Field Testing Station.

The field testing station. The polished O-ring seats are covered with blue painters tape for protection.

After the successful retrieval, I set to work on scripts for the next generation of sensors. It’s amazing how the kind of focus that coding requires can really mess with your perception of time, leaving you feeling that everything is being done at the last possible minute, though you have been working on it for several days... But after some datasheet slogging (thanks once again to the folks at Turtle Bay Cafe for their patience), the units started to produce reasonable
numbers, and on the morning of the 20th we were “all systems go”. I had three pendulum units (plus one backup) and one high resolution pressure sensor ready to deploy.   The pressure sensor would be stationary to record the water level, and I did not want it swing around on a pendulum until I get a chance to do a bit more homework on the calculations required to compensate for that motion.

As usual, we had one unit misbehave on the bench so badly it needed a complete "brain transplant".

We had one unit misbehaving so badly it needed a complete “brain transplant”, but the modular design of the system meant this was pretty easy to do.

The low power consumption of our bench tests gave me the confidence to set a couple of the loggers to 5 minute sampling intervals, while leaving the third on a more conservative 15 minute schedule. (in case the faster loggers run out of juice before we can collect them). Then we sealed everything up and set out to collect the tanks, etc. from our friend Bil Phillips at Speleotec dive shop in Tulum. On the way there I monitored the heartbeat LED’s.  But unit 3 did not pip, so while Trish sorted the dive gear I cracked it open to find that indeed, it was not logging (I suspect because of a loose RTC alarm/interrupt line).  As luck would have it, a couple of researchers working with a group from Denmark/Austria  (who have done some impressive work ) arrived to prepare for their days dive. They were testing some newly developed 3D scanning equipment, including a flow meter using an optical method based on laser tracking of particles. A good nurtured discussion ensued about the pros & cons of different measurement methods: “How will you calibrate?” “That’s going to be really non linear..” “Yep, but I have no problems with bio-fouling, and no issues with salinity/refractive index…” I will skip the rest of the nerdy details, but let’s just say there’s nothing like a bit of friendly competition to motivate…

I used the deflection of an 8 inch cable tie as a rough field balance. Units were tuned to approximately 10-20 grams negative.

I used the deflection of an eight inch cable tie as a rough field balance. The units were tuned to  approximately 10-20 grams negative.

Once out at the site, I tried to standardize the buoyancy of each unit. The beta’s had significant variation in their response to water flow, and my goal on this build was to achieve a more reasonable amount of inter-unit consistency. Even with stainless steel bolts on the housings, I still had to add about 150 grams of ballast to each logger. (weighted towards the top of the units to offset any torque from the internal mass of the AA batteries)  I am not happy about all that hard iron near my compass sensor, but the data will tell me if it causes a serious problem, as compared to all the other factors, like the batteries, etc. My humble budget will not extend to a degaussed power supply!

They are deployed quite close together, to allow me to assess inter-unit response for this build.

They are deployed quite close together, to allow me to assess inter-unit response for this build.

Low channel flow meant that the deployment dive was pretty easy, and we re-occupied the previous logger location for a continuous data set. The new bungee cord anchors are much easier to attach to the ceiling of the cave than the knots of nylon string used earlier, but of course we don’t yet know how long the rubber will last. Despite my surface testing, I still needed to transfer a few ballast washers to achieve a similar angle of inclination on the pendulums.  During this operation I was promising myself that the next units will be much more compact, and have no metal parts on the outside.  After a final inspection swim, with the capture of a little video, we were done.  Although the whole installation went smoothly, the earlier delays from Unit 3, and my buoyancy calibrations, made for a very long day, so it was well after dark when we finally left the water. After so many months of work, I could finally relax a moment and take it all in – my little cave pearls are starting to feel like a “real” scientific monitoring platform:

(Yeah, shakey cam: but our WG-3 croaked last year and the Heros are not great in low light, so this was captured on a little Powershot D10, that’s nearly 10 years old)

It will be a while before we see data from the new units, but I am confident we will see good numbers from them. (…still have my fingers crossed though!) I think I need to go have a moment on the beach, before my brain starts chewing on all fixes for the next build. I have homework to do before I get a good electrical conductivity sensor in the mix that can cover the entire fresh to marine range (standard electrodes are not designed for this) but I wonder what else I could add to the little loggers?

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