Category Archives: Developing a FLOW ≋ sensor

A hydrometric pendulum using drag-induced tilt angles to infer water flow velocity.

For a case study showing the kind of data we collect from these sensors, see:
A Flexible Arduino-Based Logging Platform

Field Report: 2015-03-23 Flow Sensor “drag fins” tested

Our deployments last year saw only modest instrument response in slower systems, especially those where the water was flowing slower than 1 cm/second. Most of the deep saline circulation fell into this category, and we really wanted better data from those caves. So I came up with a add-on attachement for the flow meters, hoping to dramatically increase their surface area without affecting buoyancy that much.

Officially this was an introduction to beach facies mapping, but it looks more like geosci Kung Fu to me.

Technically, this was an introduction to mapping beach deposits, but to me it looked like geo-scientist Kung-Fu

I had a couple of these new fins on this trip and I asked my wife, who was busy leading the Northwestern University earth science students around the peninsula, when I might sneak away from the group for a few hours to see if they actually worked. She suggested instead that we do an actual deployment, using the opportunity to expose the undergrads to the aspects of real underwater fieldwork.  I was instinctively cautious about this idea, having seen a fair number of tech demos go wrong over the years, but I have also come to realize that Trish’s enthusiasm is an unstoppable force, so we added the dive to a schedule that was already bursting.

The new "parallel" anchor rig make the differences between the instruments obvious...

The new “parallel” anchor rig made it easy to see differences in instrument response from the last deployment. It’s hard to achieve consistent results with all the changes I make from one generation of underwater housings to the next.

With all the other activities on the go, it was mid- afternoon before we actually donned our gear, while answering questions from the students about the double tanks, and doing little demos of the other cave-diving kit. Then we waddled off to the waters edge, festooned with the mesh bags of loggers, cables, and other bits that accompany a typical deployment. I’m not saying we looked bad, but it was probably clear to the students that we weren’t going to peal off and reveal a freshly pressed tuxedo after the dive 😉  Once in the water, we had a short swim along an open channel to the cave entrance with a gaggle of snorkeling students following our progress at the surface. One of our primary lights started acting a bit flaky on the way and we had another impromptu Q&A session floating among the mangroves as one student paddled back to fetch our spare.  A bother, but it did put a point on what we had said earlier about all the redundant equipment we were carrying. When the extra light arrived, we started the dive, and I made a note to myself to take more photos than usual for the debrief that would occur at the end of the day.

Here I am adjusting the ballast on one of the flow meters before deploying it.

Here I adjust the ballast mass on one of the flow meters before installation (with the new drag fin in the foreground)

Once on site, Trish set our mesh bags up in a little work area, and I swam out for the usual round of inspections. North? check. Epoxy OK? check. Vortex shedding? etc. Once that was in the dive notes, I began the one-by-one exchange of the old units.  The indicator LED’s pipped right on schedule, telling us that we had no epoxy failures this time round. Once all the flow sensors had been replaced, I took a few photos and noted that the unit closest to the main line was not being deflected as much as the other sensors. I then added the new drag fin to that heavier unit.  I also had a pressure sensor to install, and while I switched that out I could see that the sensor with the new drag fin was now almost horizontal compared to the other sensors:

I don’t know about you, but I am calling that a success.  In faster systems the fin might clip the high end, although the cross sectional area now changes quite a bit as the unit approaches 90 degrees. Any approximation with drag on a sphere has also gone out the window, but I already knew that empirical testing was going to be necessary to get point velocities. As I refine this idea, I will come up with different sizes, and integrate the baffles more elegantly with the ballast mass adjustment. Wheels are already turning in my head with possibilities.


As part of the extra video we captured for the students, I recorded a short clip of our exit from the cave. With the water at high flow, there was significant mixing at the fresh/salt water interface, producing an optical consistency similar to salad dressing. This is limited to the mixing zone region, and you can see this when I placed the camera below the level of the interface where it obtains clear visibility again.  While cave divers run into this kind of thing frequently, it’s probably something that regular divers don’t experience very often. So I thought I would post the clip just to show people what it was like:

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Field Report: 2015-03-18 One logger sacrificed to the sea gods.

DIY Cave Pearl data loggers based on Arduino Microcontrollers

The B4 unit has been a star performer despite the fact that it was one of the earliest logger units I ever built. It has been running continuously since its first cave deployment in March 2014.

I was happy to see Gabriel from CEA the next morning to discuss retrieval of the open water units, but he delivered some unwelcome news to go with my morning coffee: the logger at the mouth of Yalku Lagoon had gone missing. Loosing the unit itself was irritating, but loosing four months worth of data – that hurts!  Another pivot joint on one of the loggers in the bay had failed the week before, and one of the reef volunteers spotted that unit while it was still hanging from the backup bungee. I received that news before we headed south so I had quickly crafted some stronger universal joints from pvc to fix the problem. It was salt in the wound to know that these new pivots were sitting in my suitcase, having arrived a day or so too late to save the Yalku unit. Darn!

Oh well, we can only try again, and there is some solace in the fact that we are not the only ones to see equipment suffering this fate.  There is still a small chance that someone will pick it up further down the beach, try to figure out what it is on Google, and send us an email to say they still have the SD card. From this point forward, I will be labeling the inside of my loggers as well as the outside, and I will add a little “If found please email…” blurb into the data files.

Marco cuts B3 from it's mooring

Marco cuts B3 from it’s mooring on the south side of the bay. Marco has been doing regular checks on the loggers since the beginning of the open water experiment.

Gabriel had meetings to attend, so Everett, Marco and I popped on some fins and swam out to recover the units in Akumal bay.  As usual they were covered with a crop of algae & other critters but both B4, and the B3 unit that I rebuilt on the last trip, were still running smoothly. The unit in shallow water was so encrusted that I told Marco to pull the whole assembly, including the anchor plate, because there was no way to inspect it through all the accumulated cruft. That bio-fouling likely increases the drag and the buoyancy of the meter over time.


I left one of the new pivot joints on the B4 anchor plate. Hopefully these are robust enough for the constant wear and tear.

This is one of the new universal joints – hopefully robust enough to save us from more losses.

These two loggers have now been in the open ocean for seven months, and once ashore I began a familiar routine, cleaning them with green scrubby pads and copious amounts of rubbing alcohol.  The Loctite E-30CL epoxy on the LED’s is holding up well although the JB weld on one of the DS18b20 temperature sensors definitely has little patches of rust showing through. The stainless steel ballast washers appear to have fused together, but the O-rings are still looking good. The nylon bolts are sounding crunchy, perhaps indicating that they are starting to get brittle so I might replace them on the next round.  Now that the new pivot joints are more robust, I probably need to think about upgrading the rest of the connections as well.


JB weld on one of the DS18b20 sensors

The JB weld on the DS18b20 sensors is getting a bit crusty.

Once the data was downloaded, I reset the RTC’s, and checked that the sleep currents were still the same as they were in December. These units have Tinyduino stacks, so they run with fairly high sleep currents. (around 0.7 mA) With six new AA’s in the power module they should still deliver 6-9 months of operation.  After adding a fresh desiccant pack they were sealed & ready to deploy with code that crops the highest and lowest off of 13 readings, spaced 8 seconds apart. I average the remaining 11 readings to filter out the high frequency wave turbulence, and get at the underlying flow direction. So far this approach seems to be working well.  I also gave Gabriel a new logger to replace the one we had lost at Yalku lagoon Hopefully the sea gods will smile upon this new deployment.

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Field Report 2014-12-18: A Water Flow Sensor Co-deployment.

Heh gringos! No Cueva!

No cueva aqui…

Over the next couple of days, we managed to double the number of flow sensors in the core systems along the transect that Trish was studying as part of a biology project headed up by Dr. Fernando Alvarez from UNAM in Mexico City. We revisited our test rig at the coastal site, leaving a set of three flow units there along with our 2 Bar pressure sensor. All had been tested showing good sleep currents, so hopefully we would not loose our data this time.  Those dives went smoothly, and we ended up with a rare day on our fieldwork schedule that was not already booked.

We knew there was a system in the area were some biologist friends had a Lowel Instruments flow meter (the first time I have seen a commercial unit use the tilt/drag principal…) installed beside their monitoring equipment. But the meter was scheduled for removal in January of 2015 – so this opportunity was brief.  If we could make it out, a co-deployment would give us a chance to compare point velocity numbers from a commercial unit with our DIY project data.  On the down side, it was quite a haul to get out there, on a road that our little rental car was not really capable of handling.  But, thanks to my marathon rebuild session, we still had three flow sensors that we could deploy…

We decided to go for it, and just bring everything along in case the system looked interesting. Thirty minutes of slowly bumping, clunking and screeeeeeeeeetttcccching past the machete stumps, and we made it to the walking trail. From there we still had a good march out to the cenote, so we donned our gear and set out with doubles on our backs; placing each foot as carefully as we could on the uneven trail, while also trying to move fast enough to outpace the mosquitoes.  By the time we reached the water, it was getting pretty hot in those wetsuits!

Yep, we were not going to smell pretty after a dive in this stuff.

Yep, we were going to smell mighty good after this dive.

After we had cooled off , we did the pre-dive checks, secured our mesh bag full of sensors, and set off along the line.  And the water was…. brownish green…(?)  Much more than the other cenotes we had been diving in. But then I noticed that some of those bits of perc were actually moving around, under their own steam, and I understood why a group of biologists would select a site that was so darned awkward to get to. It was heaving with little critters!  When we found the site they had chosen for their monitoring equipment it was at the top of a trapped dome, probably perfect for monitoring things like nutrients, but almost certainly with too little flow for my Pearls to register. I set up a sensor anyway, thinking that if it was a zero flow location, we were not going to have much data to work with for that calibration. Who knows, perhaps we would get lucky and catch a big rain event flushing out the system during the overlap of the two meters?

Then we explored further into the cave.  The line soon descended below the limits of our humble point and shoot camera (so we had to leave it behind), but the visibility opened up below the tannic water showing a spectacularly wide passage, with the fresh/saline transition smack-dab in the center. Trish immediately became very excited, pointing out that there were ripples in the sediment.  We swam further into the passage, and she hand signaled her intent to find a location for a dual installation with our last two units.  I waited on the line while she searched, watching the slowly undulating halocline as it scattered our dive lights across the walls of the passage like a fun-house mirror. Once a site was selected, we configured one unit as a float, for installation on the floor of the passage, and directly above that we hung our last unit from the ceiling; up in the fresh water zone. After an inspection swim to check connectors & compass bearings, we shook hands in an exaggerated ceremony before following the jump reel back to the main line. I don’t think we could have picked a better place for our last installation of the trip if we tried. Worth the hike, and the bugs, and all the scratches & dents we put on that rental car…which now all seemed quite minor…really…no problem at all 😉

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Field Report 2014-12-16: Rebuilding & Reloading the Old Sensors.

The depth limits on our point & shoot camera prevented us from capturing photos from the dives. But a pile of 1st & 2nd gen units was slowly accumulating on the floor in our room...

1st & 2nd generation loggers were accumulating rapidly…

Each of the next few days started with a dive to replace old units that had been installed in deeper passages along System Ox Bel Ha, with the new generation of flow sensors.  To avoid the ballast problems we had on the last round of salt water deployments,  we decided to adjust the buoyancy of each flow sensor in-situ, while it was hanging from the support rods. The connectors themselves contributed varying amounts of negative buoyancy depending on their distance from the pivot joint, and some of the deep sites needed up to four rods (~2m) to get the flow meter into the right location in the water column. This required more time at depth than I would have liked the first time we tried it, but over the next few deployments we got reasonably good at weighting the units so that they were sensitive to the gentlest water movements. I need to put some more thought into making this procedure easier to do.

And we knew how important this fine-tuning was in the deep saline zone because each unit we downloaded told us that the August flow sensors were far heavier than they should have been. Ten grams of negative buoyancy is fine in a coastal discharge that races along at 15 cm/second, but when the fastest flows are below 1 cm/s, the pearls needed to be as light as a feather.  Semi-diurnal tides that jumped off the screen when we plotted the data from high flow sites, barely rose above the ADC noise in systems like Maya Blue and Jailhouse.  Of course there were more epoxy failures, and we continued to see units brought down by fake SD cards. The combination of these factors meant that we lost most the data from the last generation of flow sensors. I will never trust retail packaging for electronic components again.

Each rebuilt unit needed a 24 hour test run...

Each rebuild needed a few days of testing to catch code bugs…

And for the first time, we had so many sensors returning from deployments that refurbishing & reloading them was turning into a major part of the trip logistics.  That sounds pretty obvious in hindsight, but I was so used to having the opposite problem: where we concentrated on squeezing every possible  dive out of the “precious” YSI Sonde or Hydrolab, that having to triage old data loggers had never happened to us before. I started migrating parts from the younger units with failed epoxy, into the older generation builds with sound housings. Then every logger had it’s SD card replaced with ‘good sleepers’, and I tested them over again… just to be sure.  I completely rebuilt two of the Beta generation units for CEA’s open ocean deployement, and finally got around to putting the bma250’s they carried into a low current sleep mode.  I even melted grooves into the housings so that Marco could check the sensor orientation “by feel”, after they turned into floating algae farms.

Good enough for a "surface" deployment

I hope it is sealed well enough for a surface deployment…

Things proceeded well: All clocks on UTC? Check.  Replace old style battery connectors? Check. Good data saves from test runs? Check. Every few hours saw another unit up and running with reasonable sleep currents.  But the failed pressure sensor posed a bit of a problem. Bad epoxy or not, we needed two pressure units running so we could subtract the barometric from the combined signal that the under water units were reading. In the end I decided to re-submerge the older 2bar unit I built back in March, despite the fact that it had already done a long stint underwater, and I would leave the newer 5-bar pressure unit on the surface after sealing the hole with some glue from the local hardware store.

I was so zoned getting all these little Frankensteins going that for a while I lost track of the days.  I think it rained…or maybe it was sunny…because I was in Mexico…right? Fortunately while I was going non-verbal, Ben Schwartz and his crew of avid cavers arrived in Akumal. Being somewhat occupied, I hardly noticed the time Trish spent talking to them about to the region, and it’s wonderful cave systems.  They got the two-penny tour of our humble field station and endured my Cave Pearl “elevator speech”, which was still embedded in my brain from the GSA. Good thing too, as scripting & sleep deprivation had crowded out most of my other brain functions by that point.

And at night our room lit up like a Christmas tree every fifteen minutes, because all the little LED heartbeats were blinking in rough unison as loggers ran their overnight tests…

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Field Report: 2014-12-12 Retrieve the Costal Discharge Flow Sensors

The C generation units ready to come home.

The ‘C’ generation units, ready to come home.

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.

Uh oh, what is that brown stain on the temperature sensor?

Uhhh, what is that brown stain on the temperature sensor?

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.

It looks more exciting in photos than it does in real life...

It looks more exciting in the photos than it does in real life…

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.

Pretty bummed out at this point...

Wanna see a maker cringe?  Show them this

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.

The test rig in place

The parallel deployment rig after installation.

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:

StratifiedValuesforTwoacc_redisBMA250That 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.

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Field Report 2014-12-10: Collecting Salt Water Pearls

We did not have scuba gear, so the fastest approach was simply to cut the anchors away.

Without scuba gear, the fastest approach was simply to cut the anchors. I wanted to inspect  those connectors anyway.

Our first day with boots on the ground, and we were quite keen to see what we had on the loggers that were deployed in back in August.  A fresh batch of data is always a great way to motivate yourself for fieldwork.  So Trish headed out to Rio Secreto to collect the 1st generation drip sensors, while I met up with Gabriel and Marco from CEA to see about retrieving the Beta flow sensors we put in Akumal bay back in August. As we waited for a boat to become available Gabriel showed me the fantastic records that they had kept, and the locations they had selected for the deployments of the other two flow meters. One had been placed in the shallower south side of bay on October 13th, and the third unit was deployed at the mouth of Yalku Lagoon on November 27th.


This was discovered on Aug, and the unit was re-installed on Nov 21st.

This pivot joint failure was discovered on Nov 7th, and the unit was re-installed on Nov 21st. (Photo courtesy: Centro Ecologico Akumal)

Opportunistic photos of the units every couple of weeks revealed that the constant roil of the surf had taken a toll: with both of loggers in the bay suffering failures on the anchor rods & pivot joints. I had designed the pearls for much gentler cave environments, so this was not unexpected. I was just thankful that the folks at CEA had been around to catch the problems while the “backup” bungee cords were still in place, or the loggers could have simply drifted away.  Sometimes all you get from the first deployment is an understanding of how to do the next one better, and patchy data is still 100% better than no data at all.  Of course, As I reviewed the photos with Gabriel, I could not help but wonder if the electronics had survived all that knocking about.


There was even a few small crabs crawling around on the surface.

A marine bio project if I ever saw one

Gabriel had other pressing business that day, so when the boat became available Marco and I set off to retrieve the flow meters.  At each site we did a quick check that the north orientation was still correct, and that there were no obvious signs of physical damage. It was a gorgeous day to be out, but the bright tropical sun made it impossible for me to determine if the leds were still piping.  The first unit in the bay looked great but the second unit (in much shallower water) had suffered an incredible amount of bio-accumulation in only two months. I had never seen this on a sensor in the caves and it made wonder it if would even be possible to deploy ambient light sensors on a reef without some kind of rigorous cleaning schedule. By mid afternoon we had all the babies on board, and were heading back to shore.

The Oring seats were still clear. I guess PVC tastes better than EPDM?

The 0-ring seats were still clear. I guess PVC tastes better to sea critters than EPDM?

I spent a couple of hours scraping the gunk off of the housings with isopropyl alcohol before I dared to break a seal. And it was tough going, even with a pot scrubber. During the cleaning I could see that the LED on unit three was lit, indicating that it had gone into some kind of error state. Unit 4 piped on schedule, but I saw no flash from Unit 5. After the cleaning, it still took a wrench and some colorful language to loosen those bolts.

Once they were open I had a chance to look at the data files.  All of them had saved at least 10,000 records, but unfortunately the data from Unit 3 consisted of the same four numbers, repeating over and over again.  Inspection revealed that the SCL line on the I2C bus was broken. This had terminated the internal communications, although the RTC interrupt continued to fire on schedule for at least a month before it got confused and reset itself. So the logger from the south of the bay did not give us anything useful.  Unit 4, the first to go in, was still running when I disconnected it and I was keen to how much power it had used in three months. (see: mV vs time in the graph below) These beta generation units were running some pretty hairy old code, and I knew they were probably pulling a few mA the whole time. I also had Unit4 on a five minute sample schedule, so it had saved almost twenty eight thousand records to the SD card:


No surprises there, with another 2-3 months of operation before this unit powered down. But it is worth noting how much spread there is in the voltage reading. This generation of loggers sported a TinyDuino stack so I used the AVR’s internal 1.1 vref to monitor the battery, and I was not expecting to see so much variability with the bandgap voltage method (>70 mV of noise?). When I use a voltage divider to read Vbat on my other builds, the readings are much more stable. 

It will take me a while to chew the compass and accelerometer data into something useful but the temperature record really jumped out at me:


 *repairing the anchor rod failure left a two week data gap in Nov.

For almost two months the night-time lows stay above 28 C, with some of the highs reaching 31 degrees. And this sensor (DS18B20) is not on the surface, but down in the middle of the water column at about 3m depth, pretty close to that reef. I’m no biologist, but it seems to be getting a little toasty down there…

We had a little farmer tending the crop on unit3.

We had a little farmer tending the crop of algae that bloomed on Unit 3

Unit 5 was still running, although the LED ground line had been shaken loose, which I why I did not catch any pips. This build also had a 3.3v regulator one the power module, so I don’t have a battery voltage data to analyse. And finally, this unit did not go into the water till Nov 27th, so it’s flow data record is quite brief. However there was one other thing I could look at, before calling it a day: How much did those cheap eBay RTC’s I was using drift over the deployment?  I found a lag of about 30 seconds in the RTC on Unit4, and about 40 seconds had been dropped from Unit5. I probably caused some of that delay myself as I was not setting the clocks very carefully back then, but it is still gives me some indication that these RTC boards should be good for a year long deployment. Not bad for a board that only cost two bucks.


Beta Unit 4 has now been under water for almost 10 months. The JB marine weld & Loctite epoxy are starting to show their age, in fact if the units were not under water the whole time, I’d say they were suffering from UV exposure.  But I think they should still be water tight for a while, despite the fact that I exceeded any manufacturer specifications quite a while ago.  The plan is to keep these early builds in service till the housings finally fail, but I would like to lower that sleep current before I deploy these units are redeployed.  If memory serves, I never did get around to sleeping that bma250 in the Beta generation code (?)

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Project Update: Gearing up for field work 2014-11-26

Using Loctite E00-CL this time round.

Using Loctite E00CL this time round. The epoxy is weaker overall, but claims higher shear strength on PVC than E30-CL. The faster epoxy gets hotter, and contracts more, so there may be some risk of lifting the components. And the numbers in the individual data sheets do not exactly match those in Loctite’s own Plastics Bonding Guide, so who knows?

Another field trip is rapidly approaching, and I am scrambling to finish the bench tests before we have to stuff everything into a suitcase. The last three months have seen the project migrate away from the unregulated TinyDuinos as the heart of the data logging platform, to RocketScream based builds. Most of my sensors require a regulated 3.3 volt supply, and with only one MCP1700 voltage regulator in the mix, the Ultras have been delivering better sleep currents overall.  The MCP also gives me the ability to use lithium batteries in a pinch (who’s over-voltage would fry the unprotected processor on a TinyD board) , and it delivers up to 250mA if I end up with really power hungry sensors later on. Now that I have the same core logging platform in all the different Cave Pearl models, it is easier to shave down my code, as the compiles keep bumping up against the 32k limit for multi-sensor configurations like the pressure/temp/RH unit

But I have not forgotten how the TinyDuinos catapulted this project into viability back in 2013, and I am waiting to see if they release a generic I2C driver shield. Despite my rough handling of those early Tiny-based builds, most of them are still chugging along after months under water, a tribute to the quality of their build. I enjoy soldering my little bots together, but anything you have to do a hundred times begins to loose it’s luster.

Cave Pearl Flow Sensor

With new 32k EEproms in the mix, space on that logger platform is getting pretty tight and I have to trim the groove hub pcb to make more room.

Bench testing over the last few months has seen more sensor failures than I can throw a stick at, and I am sure that there are more to come if I keep using cheap eBay vendors.  The best overall diagnostic to identify good breakout boards continues to be shutdown mode current. If it’s on spec, and the board delivers a stable reading after wake-up, your golden.  Along the way, there have been so many little code tweaks I could not even begin to list them all. Some, like having the sensor reading LED pip change color to also indicate battery status, were effortless. But others, like determining the optimum number of times to use precious power up cycles to check that battery status, still have me scratching my head. We have more than 12 new loggers to deploy this time round, and I will be embedding plenty of little mini-experiments in the code to give me some empirical data for those questions.

You need at least a week of dry runs, as some sensors fail after a few days of operation.

You need at least a week of dry tests, as some sensors don’t fail till they have been running for a few days.

At this point I am focusing on micro amps, not milli amps, and the best drip sensor builds are coming in with sleep currents in below 15 μA (if I get all the sensors in to their low power modes and pin-power the RTC) That’s a heck of allot better than I was expecting for a few jumpers connecting off-the-shelf breakout boards. Even with the physical build coming together well, I still have a huge sensor calibration to-do list hanging over my head. But the tickets already bought, so that will have to wait till after the next set of field deployments. I also need to develop a new bench testing method that gives me the ability to discriminate how relatively subtle code changes affect a micro-power budget. Oscilloscopes seem to capture a time window that is too brief for the complex duty cycle of a data logger, and the power use ranges from a few μA of sleep current to many tens of mA for SD card writing during each cycle.


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