I was happy with my Mark II design, but I realized that it was going to take half a day to make each one. The latch clamps pushed the material cost to about $60, and the labor involved meant they were still going to be expensive in the kind of numbers Trish wanted for her network of cave sensors. So I applied some thought to simplifying the design still further.
I had used some Fernco Quick-caps to convert the earlier botched housing bodies into anchors. It only took about fifteen minutes to create a one-piece housing, based on those, and I did not have to sand down any o-ring seats. Dayam! But my excitement was tempered when I put them in water, as the rubber end caps were so heavy that they rolled the units over, even with batteries in the PVC cap. So I needed to come up with a way to attach these guys to the floor anchor, that maintained a rubber side down orientation. But I could not get anything to bond the flexing, bumpy outer surface of that rubber (which turned out to be “elastomeric pvc”) In the end I threaded a few cable ties through pvc plugs, and suspending them under the pipe clamp. This worked but it only heightened my concern about the nature of the seal on these puppies. You see traditional O-ring designs actually work better on deep dives, because the added water pressure compresses the o-rings more tightly. This new design was super simple to build but it was also critically dependent on not one, but two pipe clamps made of metal, as I had to balance the mass of the clamp screw. Even with marine grade stainless, I had my doubts about the longevity of that seal. Nonetheless, I pressed on, and thought about a scaffold for the batteries and sensor package. A bit of ply, some hard foam insulation, and a touch of gorilla glue (an adhesive on my top five list of bodging materials) produced a battery compartment and electronics platform under 25 grams. I added a ballast mass post with an old Ikea door pull, and put one of my new Tinyduino stacks into place. The hold down screws they came with were not long enough to penetrate that wood, so I had to fashion a U bend out of brass wire to affix the electronics.
So now I had two different sensor housing designs. The more robust, o-ring design was expensive and took ages to make, while the simpler quick-cap housing, at about $10 material cost, it was definitely “Cheap as chips”, but it’s integrity relied on a couple of metal pipe clamps. I would not know if we had a winner until the next field work trip in late November. But even if this second design didn’t handle the water pressure at depth, I knew it was going to be handy for other cave research, because it was still a decent enough waterproof enclosure for less demanding environments.