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….

Rebreather Oxygen Sensor Isolation

Sharing the output from one oxygen sensor to two or more monitoring systems on a rebreather is a popular way of increasing redundancy (and complexity).

This does not cover cell failure issues or corrosion problems but if done correctly allows monitoring redundancy to be added in a shared cell monitoring system.

A few commercial options are available from the likes of Revo, Narked at 90 and Ocean Opportunity. Some include a single resistor, some use two resistors I thought an experiment was in order to check what the difference might be between the single and double resistor approach.

I used an old home made PP02 monitor I made some years ago courtesy of:

http://www.ppo2.com/Displays_1.htm

It uses a very sensitive DPM and has three digits after the decimal place unlike most modern dive computers (the dancing digit).

Three circuits were tested, the first with no isolation to prove the need for it, the second with a single resistor on each split and the third with two resistors on each split (one on the +ve, one on the -ve) the results can be seen in the video below:

Rebreather Oxygen Sensor Isolation test video

All tests were done in air close to sea level, results may differ with increased PP02

The display was re-calibrated each time a set of resistors was added.

The best redundancy comes in the form of 3 cells and 3 separate monitor systems like the original Kiss Classic displays.