My concerns regarding weighting the buoy properly

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 07/31/2018 at 04:302 Comments

I didn't have time yet to lay it out properly with static diagram of forces but that's the problem I have. @Edward Mallon advised to use washers as a convenient way to add proper amount of weight to the buoy. That solution looks very elegant to me. At the same time my buoy is quite elongated and I think I cannot just add enough weight to the bottom of the buoy. If center of buoyancy and center of gravity are far enough there may appear rotational force that will try to keep the buoy upright.

Somehow I feel it's best to keep buoy axis aligned with tilt line.

Going forward I'll try to make the buoy more compact and that should allow me to use less weight and have the buoy weighted better.

For now, that's my very crude solution which, I hope, distributes weight good enough so my center of buoyancy and center of gravity are close enough.

I don't have any solid proof to my considerations (as to if my concerns above are valid at all) so if you have any thoughts, please kindly share with me.


Ed Mallon wrote 07/31/2018 at 17:52 point

I think you are right about distributing your weights more evenly but it's going to be a little challenging to fine tune those big rods... Our system uses rigid poles so its a tad more forgiving of the offset between the ballast mass and the buoyancy force. Because we deploy in both fresh and salt water in a single dive, we had to have a system that could be buoyancy-tuned on site.

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Max Kviatkouski wrote 07/31/2018 at 19:12 point

Yup, I saw your way to tether the buoy and it makes a lot of sense to me since rotation of the buoy was bothering me. Rotation concern could be eliminated by using accelerometer + magnetometer but that's more expensive and sophisticated than just using proper tether.

One more question Edward if you don't mind. I've read how you were calibrating accelerometer itself. Did you get to scale your flow meters in velocity units (kt, cm/sec) rather than measuring tilt angle? If so, can you please help me find corresponding entry in your blog? Thanks in advance!

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