A Week of Repairs

Although I had hoped to get AMOS out at least once this past week for more testing, there were a number of repair-related tasks, mostly to do with the turbidity sensor that I wanted to take care of first. I also modified the grid collection software, as described last week, to alternate from East to West, then West to East as it collects points in a grid pattern, and cleaned up some of the other sampling code to get rid of occasional extra unwanted samples that were being collected.

The week before I had filled the turbidity sensor enclosure (read cat-food tin) with epoxy in a futile attempt to make it waterproof. Perhaps a different brand of epoxy, one that you mix together would have worked better. The kind I used required no mixing and was meant to cure when exposed to air, which meant that the bulk of the epoxy within the enclosure remained in liquid form, and then leaked quickly out under small cracks of the enclosure whenever it was tilted. The epoxy also made its way in through small openings in the top of the plastic turbidity sensor enclosure and covered the transmitter, receiver, and various other electrical components. So this week, I slowly picked away bits of epoxy from the turbidity sensor, but eventually discovered that the pins on the connector had weakened and snapped off anyway due to corrosion caused by exposure to water.

So I ordered a replacement cheap-o turbidity sensor and hope to have it in a week or two. It appears to be based on a design that is commonly used in washing machines. I'll have to try to cover it as best I can (duct tape?), and just hope that it doesn't get too wet. I was thinking that maybe by attaching some foam floats to the sides of the "sensor boat" it will lift it up a bit out of the water, and hopefully keep its top a little bit more dry. I also looked into some real industrial quality turbidity sensors and got a few quotes. The prices for decent models that are completely waterproof and capable of operating in sea water (titanium construction) are between $2K and $4K CAD. They also include "wipers" for performing self-cleaning of the optical windows. At the moment (although I'm tempted) there doesn't seem to be much point in buying one of these, as both the cheap $14 model and the $2K+ models have similar analog outputs that effectively look the same as far as AMOS and its software are concerned. Of course the $14 model is not waterproof, nor is it calibrated or guaranteed to be stable. It is also (unless externally shielded) affected by external light (i.e. the sun). But it will do for now.

I tried to replicate an issue with the software that occurred in the test from the week before, in which the software stopped writing to the ship's log after beginning the grid data sampling routine. I ran a number of tests in the pool to try to replicate the same sampling conditions, but was unable to replicate the problem: saving to the ship's log seemed to work fine no matter what.

I also bought a small sheet of rubber from Amazon and experimented with making my own gasket for use with the cheap turbidity sensor and various household containers. This was an abysmal failure. Even when I thought I had something that looked fairly tight, water leaked in immediately. Luckily I didn't waste too much time on it. Any waterproofing on AMOS will need to be bought from others.

While testing the turbidity sensor, I noticed that the A to D board was also glitching in and out. My wiring on this thing is admittedly pretty bad, but I think the root cause of the glitching might have  been a number of damp (blue) desiccant beads that had adhered to the surface and pins of the nearby compass module board. Both the compass module and the A to D board share the same I2C bus, so it's conceivable that communication problems could result from an electrical fault on one or both of these boards. On two separate occasions, cleaning off desiccant from the pins of the compass module brought the A to D board back to life. At no point though was the compass module ever unresponsive, so the desiccant theory is really just a guess.

As a step in the right direction towards better protecting the circuit boards on AMOS, I designed some enclosures and sent them off to 3DHubs for printing. Here is the enclosure that will house the main Raspberry Pi Compute module, the GPS module, the leak sensor, and the two thruster speed controllers:

The enclosures are not waterproof, but rather are intended to organize the boards and wires and keep things from shorting out. They also have lids to help protect against any water that might splash down from above.