Pressure testing has been on the to-do list for ages, but the rating on the PVC parts in our older housings meant we weren’t likely to have any issues. However, the new two-part mini-loggers fit inside a thin walled falcon tube, which raised the question of how to test them. There are a few hyperbaric test chamber tutorials floating around the web, and we made use of one built from a scuba tank back at the start of the project, but I wanted something a less beefy, and easier to cobble together from hardware store parts. Fortunately Brian Davis, a fellow maker & educator, sent a photo of an old water filter housing he’d salvaged for use with projects that needed pressure tests. Residential water supply ranges from 45 to 80 psi so could replicate conditions down to 55m. That’s good for most of our deployments and certainly farther than I was expecting those little centrifuge tubes to go.
I first tested 50mL ‘Nunc’ tubes from Thermo. These are spec’d to 14psi/1atm, but that’s a rating under tension from the inside. I put indicator desiccant into each tube so small/slow leaks would be easy to see and used a small bicycle pump to increase the pressure by 5psi per day. These tubes started failing at 25psi, with 100% failure just over 30psi. Multiple small stress fractures occurred before the final longitudinal crack which produced an audible ‘pop’ – often four or more hours after the last pressure increase. If 20psi is the max ‘safe’ depth for these tubes then the 50mL tubes can deployed to about 10m with some safety margin for tides, etc. This result matches our experience with these tubes as we often use them to gab water samples while diving.
As expected, the self-standing 30mL tubes proved significantly more resistant. All of them made it to 45psi and then progressed through various amounts of bending/cracking up to 100% failure at 55psi. Where the caps were reinforced (by JB weld potting a sensor module) the rim threads of the cap sometimes split before the tube itself collapsed:
So the 30mL tubes have a deployment range to 25m with a good safety margin. The plastic of these tubes was somewhat more flexible with some crushing almost flat without leaks. This implies we might be able take these a little deeper with an internal reinforcement ring (?)
The next experiment was to try filling the tubes with mineral oil to see how much range extension that provides:
The bag was included to test the ‘naked’ DS3231 & 328p chips. We’ve had IC sensors fail under pressure before (even when potted in epoxy ) Although it’s possible the encapsulation itself was converting the pressure into other torsional forces that wouldn’t have occurred if the pressure was equally distributed.
Again we moved in 5 psi increments up to 80 psi – which is the limit of what I can generate with my little bicycle pump. At 50psi some mineral oil seeped from the bag and at 70psi the ~1cm of air I’d left in the 50mL tube caused similar leakage. On future tests I will spend more time to get rid of all the bubbles before sealing the housings.
The loggers continued blinking away for several days at 70, 75 & 80psi, but eventually curiosity got the best of me so I terminated the run. We were also getting uncomfortably close to the 90psi maximum test pressure on that polycarbonate filter housing. I was hoping to have some weird artifacts to spice up this post but no matter how hard I squint there really were no noticeable effects in the data at any of the pressure transitions – basically nothing interesting happened. I thought the resistive sensors would be affected but the RTC & NTC temperature logs have no divergence. The LDR looks exactly like a normal LDR record with no changes to the max/mins outside of normal variation. The battery curves are smooth and essentially indistinguishable from ‘dry’ bookshelf tests on the same cells. But I guess in this kind of experiment success is supposed to be boring… right? With mineral oil these little guys can go anywhere I can dive them to – even if the ‘housing’ is little more than a plastic bag.
One thing of note did happen after I removed the loggers from the chamber: I accidentally dropped the 30ml logger on the counter while retrieving it from the chamber and a thin white wisp of ‘something’ started swirling around the clear fluid inside the logger. This developed slowly and my first guess was that the capacitor had cracked and was leaking (?)
After emptying that oil, the logger itself went into a red D13 flashing BOD loop for a while but by the time I’d cleaned it up enough to check the rail, the battery had returned to it’s nominal 3v. My theory is a similar off-gassing event was happening inside the battery – briefly causing a droop below the 2.7v BOD threshold. So it’s possible that while the loggers are not depth limited per se using mineral oil, components like the separator in a battery may still be vulnerable to ‘rate-of-change’ damage. After more than two weeks at depth, I had vented the chamber in less than a minute. Of course when retrieving loggers in the real world I’d have to do my own safety stops, so this hazard may only affect loggers that get deployed/retrieved on a drop line.
I’ll run these loggers on the bookshelf for a while to see if any other odd behaviors develop. After that it will be interesting to see how well I can clean them in a bath of isopropyl (?) as I suspect that the mineral oil penetrated deep into those circuit board layers.
Addendum: 2023-05-30
Although the units sleep current was the same as before the pressure testing, the battery in the 30mL tube barely made another twelve hours on the bookshelf before the voltage dropped again – well before the expected remaining run time. So it’s a safe bet that any deployment which exposes coin cells to pressure at depth is a one-shot run. Given how cheap these batteries are, that’s pretty much a given when deploying these little loggers even if they remain dry.
Addendum: 2023-12-01
Short 30ml tubes work well for single-sensor applications, but classroom labs needed to switch between different sensor modules easily. So we added 3D printed rails holding mini breadboards to provide this flexibility, and the 50mL centrifuge tubes provide the space for these additions. They may not have the same depth range, but they are robust enough for most student experiments.
Underwater Arduino Data Loggers 