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.