Crunch time for DiveLine

[Manjula Ellepola]

LViz, thanks for the real world experience. It is indeed challenging to make something reliable that will work in such highly variable environments. The idea with the DVL we are looking at the moment is that the diver can measure his relative velocity through the water column. Knowing velocity one could estimate the travel distance. However as you point out in strong currents the upward looking DVL will have problems as the diver will drift with the current. The solution might be to have a downward looking DVL where you can bounce off the bottom. If we were to have a down ward looking DVL where would you ideally locate it on the diver?

 

[lowviz]

I was pretty sure that you only had one variable. OK, so the problem is that you can only measure water flow rate past the diver.

Do you have both magnitude and direction or just magnitude?

 

[Manjula Ellepola]

Let's go back to basics for a moment, if you will.

Imagine a tied-in boat over a wreck with a two-knot current at the surface and relative quiet at the bottom. I've seen down lines bent into an 'S' shape due to opposing currents at different depths. The ocean isn't just a big pool of water, rivers run through it. In addition, assume the usual really poor visibility. So, yes. Something else is needed. It is usually a reel. Not ideal but cheap and effective. It takes a lot of attention, though. Something less demanding (and highly reliable) would be nice.

So, begin at the beginning:
What independent parameters can you accurately measure? All of them, what are they?

Sorry forget to ask you why a real takes a lot of attention and is demanding? Your experience and explanation is valued.

 

[lowviz]

It is a major nuisance in any sort of flow, if you are swimming rapidly and then stop the spool's inertia will give you just enough more line to tangle it in the reel's body. And the final nuisance is that you have to use two hands to wind it all back up neatly on the spool during the entire return leg of your trip.

 

[Manjula Ellepola]

The DVL can give both magnitude and direction

 

[lowviz]

Direction relative to the sensor, I assume. So if I turn 45 degrees, did the flow change (now swimming partially into current) or did the direction change?

 

[Manjula Ellepola]

So in the scenario of the wreck with the high current, if you already have a dive line why would you use a spool? Can't you come up on the dive line? Links to a video would be nice

 

[Manjula Ellepola]

LViz, both parameters should change...

 

[lowviz]

Indeed. Can you tell with certainty how much of each changed? All that you are measuring is the speed and direction of doppler reflective particles past your sensor. I see a huge noise problem when you mount this on a dynamic diver. The two units (two divers) would be expected to drift badly with respect to predicting position just due to being on different divers with different diving styles.

I am all for pure research, used to do it. My suggestion is that you need at least one more independent variable that can be used to strengthen your positional calculations. Basing everything on water flow around the diver is a really big problem (IMHO).

 

[lowviz]

So in the scenario of the wreck with the high current, if you already have a dive line why would you use a spool? ...//...

You crawl down the tied in line ('down line' or 'dive line', you pick your terminology). Once at the bottom, you want to go somewhere so you use a reel. The reel has a spool on it, but when you just say spool most divers think of a finger spool for a DSMB.