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Results of the first deployment

A project log for Cheap underwater tilt current sensor.

Cheap (<$100) underwater current sensor that can be deployed at 70ft depth. Your thoughts, comments and advice are very welcome here!

Max KviatkouskiMax Kviatkouski 07/29/2018 at 08:205 Comments

So the buoy was deployed for about three days and already gave some meaningful insight into specifics of underwater current there. Main results are:

- Looks like current is going only one direction only during ebb.

- Meter needs to be more sensitive. At ~0.2kt current it was tilting for only about 10 degrees. My measuring range should be up to 1kt (currents stronger than that probably never occur at that dive site and also they would make the site not divable for most of divers).

- Vortex shedding made the buoy oscillate at somewhere between 0.5-3Hz.

- Mooring rope and clip were doing alright

- I've significantly improved log processing and log visualization. Data is available at the mukilteo.pnwdiving.org

Next steps would be:

- Increase buoy sensitivity. I can either increase drag area or decrease buoyancy. I chose latter. After thinking about how to add weight in the cheapest and easiest way without making buoy "bottom heavy" too much I chose steel rebar scraps. Experimenting with the buoy in a bucket of fresh water I figured how many of them will make the buoy slightly buoyant. It will be even more buoyant in saltwater but that's ok. I'm not trying to figure out perfect weighting. Just trying to make it better than it was and keep it simple at the same time.

- Update logging schedule to this: every 60s record 3s of data with 10Hz frequency. Here's why: 15s appears to be too frequent, current doesn't change that frequently. Vortex shedding was going at not more than 3Hz, so 10Hz sampling should be sufficient for proper averaging of the data.

- Calibrate buoy. I've emailed some Instrument Lab in Seattle that NOAA is working with. Hopefully they can help for a small fee. Another way to do it: tie up my buoy with small anchor to rolled out measurement tape that I have (300ft roll) and pull it back with different speed measuring time it takes, say, to fold 50ft of tape. That could be good enough simulation of current.

- Deploy updated buoy for a week or so.

Discussions

Edward Mallon wrote 07/30/2018 at 22:54 point

Before reworking anything try to just hang them from the bottom rope attachment point with a few zip ties and see how many you need.  (lead fishing weights might also work but the can be expensive)  or put some loops around the main body and zip the washers in sets of three distributed around the main tube.  Also - 316 stainless just barely makes it through a year in salt water, in fact most of ours are badly corroded after 6 months. So record how many you add, as you might need to replace them later with new ones over a long deployment.

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Jan wrote 07/31/2018 at 03:27 point

Or just use 1.4571 steel washers, so they won't get eaten by the sea :) barely more expensive, less hassle. 

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Max Kviatkouski wrote 07/31/2018 at 04:31 point

I was going to reply just here but my comment required a picture. Please find my thoughts in this log: https://hackaday.io/project/158083-cheap-underwater-tilt-current-sensor/log/150221-my-concerns-regarding-weighting-the-buoy-properly I will certainly go with smaller housing and weighing way used by Edward in my next v2.0.

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Edward Mallon wrote 07/29/2018 at 22:05 point

During deployment we ballast the Cave Pearl flow units with 316 stainless steel washers on the outside, adding washers till the unit is negative and then taking one back for approximately 5-10grams of positive buoyancy. You can see me adjusting that in the latest deployment videos on youtube (the saline intrusion video)

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Max Kviatkouski wrote 07/30/2018 at 19:50 point

Thank you Edward! That's actually very helpful! I will probably need to re-work my housing completely to allow weight to be added in a similar way but idea of using washers is very helpful actually. My ballast now is steel rebars which is obviously not a great source of stable amount of weight in saltwater. But I'll give it a try as it is now to see how much it will help with sensitivity and oscillation due to vortex shedding.

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