The 2014 Cave Pearl Project ‘Year in Review’

With the last field reports of 2014 out the door, I feel I should post something about the project having completed it’s first year as a “serious endeavor”.  At the risk of sounding like A.J.Jacobs,  it has been “A Year of Living Inventorly”, and the fact that I had never done anything like this before did not stop me from trying to build one of the simplest underwater dataloggers ever made. Looking back, I am very pleased by how much the Cave Pearls developed, both in terms of the electronics:


and the physical build:

Cave Pearl Physical Build Tryptic

It’s been one long string of learning by doing, and with no “formal” training in electronics, I have been using a basic process of elimination approach to tackle each problem as I went along.  Each time I built a one, it became a little easier to see where the next improvements could be made.  I became fairly obsessed about the project and my wife, with her usual good humor, coined the phrase “nerdling” to describe the days spent cutting & soldering in the basement, instead of pursuing my other passions.  But left brain/right brain tropes have always sounded odd to me because inside my head both tasks feel pretty much the same.

The highlights of the year were the field deployment trips:

Date #deployed Logger / Sensor combination
Dec. 2013 2 Alpha Flow Sensors go under water for the first time.
Mar. 2014 3 Beta Flow Sensors   Tinyduino based loggers with Bma250 accelerometers, housing have no latch clamps
1 1st gen. Underwater Pressure Sensor
Aug. 2014 3 3rd gen. Flow sensors Rocket based logger platforms, Rosetta Stone builds to compare sensors, BMA180 accelerometers adopted, epoxy failure in this generation
3 Beta Flow sensors re-deployed  in the open ocean
2 Pressure Sensors    MS5803 based,   2bar & 5bar
6 1st gen. Drip Sensors    deployed at Rio Secreto
Dec. 2014 10 Flow sensors a mix of 3rd & 4th generation units, most go into deeper low flow saline zones, 1 commercial co-deployment and a fresh/saline co-location at one site
3 Beta Flow units 2 rebuilt for open ocean deployment, one unit has been underwater continuously ~ 10 months
11 Drip Sensors 1st & 2nd generation, @ Rio Secreto
2 Pressure sensors 2bar in water & 5bar @ surface
2 1st gen. R.Humidity, Pressure & Temp. loggers

The first time I make a machine, it usually takes a week or two of experimentation, and plenty of epoxy, wire & PVC goes into the bin. But now that the logging platform is more developed,  it is getting easier for me to come up with new sensor/logger combinations. While the underwater housing remains the most time consuming part of each build, the overall improvements in design, and in my own skill level, show clearly in the number of units we have been able to deploy over time:

Cave Pearl DIY housings made from PVC

So I guess that’s a report card of sorts, and I owe thanks to all the Arduino makers out there who shared their code & experience, because they really made it possible for me to pick it up as I went along.  But even with that help, there were still a few bits of information that would have been really nice to know a year ago, that I that just did not find anywhere on the web.  So I thought I would address some of those questions.

How much does it cost to build this stuff?

As a long time freelancer, I keep pretty detailed records. By the end of 2014, the cumulative hardware cost came to about $10,000  including all electronics, tools, and pvc parts. Yep. Really. The waste generated in the process, as you hunt around for the “right parts” is significant, and in the early stages I build at least two units that never leave the bench for every one that makes it into the field.  Now I try to buy “just enough” material to build the next 3 or 4 units, no matter how ridiculous the shipping charges are. (often more than the parts themselves)  If you try to order the cheaper parts from eBay you end up with 2-3 week shipping delays, which is a real impediment to your prototyping workflow.  The problem is that every design change leaves you with a handful of leftover bits from the “old” version, and I now have a substantial collection these orphans in boxes all over my workshop.  Even with diligent effort about 1/2 of the electronic parts I purchased did not get used. (at least not yet…the prototyping is going faster now that I have a critical mass of components on hand) Also, I would advise anyone to buy better tools right at the start, rather than wasting your money working your way up through crummy soldering irons, and low budget saws. A good table top band-saw & drill press are essential.

Should I make a blog for my project?

At the beginning, I really had no idea if it was worth the time and effort to maintain a blog like this. But I know several people who are smarter than I am, working on equally cool projects that no one will ever hear about because they have not put the word out.  So for them I thought I would post a snapshot of the stats from this little opus, which has not been subjected to any “How to promote your blog” efforts because I don’t have any spare time left.  I post links in forums when I have something useful to contribute, and a handful of people have given a nod to my project, but that’s about it:


In total there were 6861 visitors in 2014, generating 19,456 page views.
At the start of the new year, the stats look like this:


As you might expect, at least 1/2 of all traffic comes from the U.S. The UK and Germany lead the western European contingent, which adds another 1/3 of the total, while the remainder is sparsely distributed around the globe.  I used to get really excited when some new country generated a big spike in traffic to the blog. Then my brother explained how botnet attacks work. This went a long way to curing my daily compulsion to check the wordpress stats because the truth is they probably don’t care that much about underwater data loggers in the Republic of Elbonia.  Still, some stats are useful, like the fact that 99.9% of the traffic comes in from Google searches.  Ignoring the home pages, posts with content on “How to do X with an Arduino” are by far the most popular:


I was surprised to see the RTC page rise so high in the ranking and I was probably just the first person to gather that information together in one place.  When a page does rise above the one-per-day hit rate that just about everything seems to get (from search engine spiders?),  I maintain that page with regular updates, and that seems to be paying off. I also take the time to cross-link extensively between old and new posts, because several people have complained that they could not navigate around “my website”. There are still many people out there who do not know how blogs work, and almost no one uses the built in search feature.

Where does the project go from here?

On the physical side, I need to make the process of assembling the underwater housings less labor intensive, and I need to improve the overall response to flow rates less that 1 cm/second because the deep saline circulation in those caves is very slow. Wherever possible the design also needs to make in-water procedures for things like buoyancy adjustment faster and easier.

On the electronics side, I will hunt for ways to reduce sleep power consumption, while continuing to use cheap components in my basic “off the shelf” design criterion. I have to find or develop better sensor calibration methods, and there are still many code issues to sort out. With some basic improvements in buffering and data handling during processor up-time, I think I could still see significant improvements in the power budget. Using other data formats is one obvious approach, because it’s all basically integer data. But I will probably stick with ASCI for a while yet, as PString makes the code very easy to modify when I change sensor configurations. This is important because the “sensor caps” are interchangeable modular units, independent of the logging platform and the rest of the housing.

I am finally getting comfortable tackling new sensors at the datasheet level and I now have a general sense of how to get communication working between breakout boards and the Arduino.  I accept the criticism that so far I have been making DIY versions of sensor & logger combinations that are already available on the market; in essence trading build  time for money.  With a year of work under my belt, I would say that this trade is not worth it for equipment that you only need one of,  because getting that first prototype operational usually costs you more than you would have spent on the commercial sensor. It starts to be worth it when you reach about ten units, because by then you have all your parts sourced and you can build them pretty quickly.  From that perspective, this project is just starting to reach a break-even point, with the added benefit that we can make custom builds for a particular research question.  Of course, sorting out a new instrument is so immensely satisfying that I would be doing it no matter how many we actually needed.   My hope is that by the end of 2015 I have the chops to develop genuinely new devices that no one has ever thought of.

But looking at the page stats again reminds me that I probably shouldn’t spend too much time writing posts like this one.  I need to get back to the bench and figure out how to do more stuff with an Arduino…and then write about that.

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