Tag: Adafruit

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Underwater Testing

Sometimes its one of those days when nothing goes your way (of course you are still happy and healthy but the given goals for the day are unachievable), I set out last week with the aim of thoroughly testing the survey box underwater and having a good ride around on my scooter before the onslaught of a dry Christmas.

A short line course was laid underwater and surveyed by a friend with a Mnemo survey device (part of which is the inspiration for my own creation). I began taking foresights and backsights with my box (having added two line clips to allow hands free alignment with the line. Unfortunately on logging shot number 13 (unlucky for some) the display somehow went back to front then froze and refused to continue to work. I had planned to survey the course three times for further device assessment and comparison against the Mnemo but was unable to complete one entire loop.

Survey box line clip and count down modifications.

We were planning a dive of around 2 hrs, a bit of surveying then some scooter fun, having had to abandon my surveying after 20 minutes I decided to take my scooter for a run wanting to assess how much battery life I might have available in its top speed. Clipping off the tow cord I took off but every few seconds the motor cut out then restarted, making the already distinctive Aquazepp sound even worse. I quickly decided that it was inconvenient but didn’t seem to be doing any harm so stopped next to a known landmark then took off for a lap on the turning of a fresh minute of the dive time on my computer.

It took 20 minutes to complete a lap which is known to be about 1100 m at the depth is was taken at. Arriving back at the line course I decided that I was against any further scootering but would surface quickly, reboot the survey box then head back to the line course to complete my goals.

Surfacing went fine, kept kit on, isolated my oxygen supply, quick minute with a handy screw driver to open the power port on the survey, rebooted and re-calibrated it, all fine so closed it back up. Mask on, oxygen back on… PSSSSSSSSSSSSSSSsss the hose had popped off of my Kiss MAV 🙁

Kiss MAV thread failure

I thought it might have just come undone to start with but after a few tries it was clear the threads had gone and that was the end of the rebreather diving for the day, I had left my scooter underwater so went back in on my open circuit bailout to retrieve it.

To top the day off there was a power cut so no hot drinks or food were available 🙁

The list of annoyances goes as:

i) Frozen survey box display and Arduino after 13 shots (hopefully a random occurrence)

ii) Scooter not running smoothly (investigation ongoing)

iii) Kiss valve thread failure (its quite old and known to happen so…)

On arriving home and sorting out gear I was still able to download the 13 shots I had taken before the device froze, after the addition of the line clips I’m happy with the few foresight and backsights I was able to take, the first belay was metal as well as one in the middle so those are expected to be bad but the rest agree quite well I think, further testing to be done to confirm.

Foresight – Backsight comparisons

So taking away some positives…. I was able to complete a ‘fast lap’ on the scooter and after charging the batteries have discovered that 24 minutes of use used 2500 mAh from each battery giving about 160 minutes of burn time at top speed. The data I was able to collect with the box is encouraging and it seems to be able to record foresights and backsights to within a few degrees away from large metal objects. The Kiss valve failed on the surface not underwater. Further testing planned….

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Underwater Survey Device Assessment

One of the previous posts on this website details the device I have assembled in the hope to speed up underwater cave surveying and at the same time make it more accurate than using the traditional divers compass, depth gauge and slate.

Using the Adafruit BN0055 ‘9 DoF IMU’ inside a waterproof housing as the tilt compensated compass should give a reasonable degree of accuracy but just how accurate is it going to be ?

To find out I ran some tests using a DistoX2 for comparison.

A small wooden jig was constructed that allowed easy foresight and backsight alignment of the home built device and the DistoX2 so that comparable shots could be easily collected.

Disto X2 in wooden Jig, Plastic pegs used for alignment
Survey box in wooden jig, axis of BN0055 co-incident with alignment of plastic pegs

The sizing of the recess in the wood is such that when the box is rotated for the foresight/ backsights and pushed up against the right and left hand edges the sensor of the BN0055 is in approximate alignment with the plastic pegs used to align the DistoX2, the BN0055 is mounted around 90° out from the long axis of the box so its raw reported bearings is around 90° different.

Forty comparable foresight and backsight shots were taken with both devices and the data entered into a spreadsheet. The first task was determining the average difference between the DistoX2 data and the box data (I should come up with a decent name for this device…) The average difference between the two was 89.56°.

The Raw data from the box was then corrected by 89.56° and re-compared to the DistoX2 data. Average difference to Distox2 and Standard deviation values were calculated.

The foresight and backsight differences were also calculated to give a quality check for the shots as the jig wasn’t moved until foresight/ backsights were taken with both devices. The DistoX2 foresight/ backsight differences were far smaller than those calculated for my home made device.

These tests were conducted on a flat surface so further tilted tests will be done to assess this aspect, overall I am happy with the results so far, I have been able to buy an off the shelf sensor and without any complicated calibrations or maths have a sensor that is able to report magnetic bearing to within a few degrees of a DistoX2.

Assuming the tilted performance isn’t much worse then any large errors underwater will come from the ferrous metal equipment carried by the diver (or in the sump) and the ability of the diver to align the device with the dive line which is another challenge itself which needs thinking about.