Portable sonar for freediving

[Sbiriguda]

Some people use portable sonars like this one in freediving
https://www.amazon.com/Vexilar-Inc-...=UTF8&qid=1546938159&sr=8-12&keywords=vexilar
Does it make any sense for you? Do you use one?

 

[rjack321]

I've used one for cave surveying - to better estimate wall and ceiling distances. What are you trying to accomplish with it?

 

[Sbiriguda]

By now I am just curious. I don't think I really need it and I didn't see people who use it (by now)
As far as I understand, you actually used it for mapping caves so to have good dive planning, not really to get info on the cave during the dive itself

 

[rjack321]

By now I am just curious. I don't think I really need it and I didn't see people who use it (by now)
As far as I understand, you actually used it for mapping caves so to have good dive planning, not really to get info on the cave during the dive itself

It can be hard to quickly and efficiently measure the width of a cave when it's many meters across. So I have used on handheld sonar to measure left and right (and up/down too). You can use a tape measure, but it takes a dedicated person swimming to the left wall then back to the right side. So that's pretty slow. The sonar works ok if you have a flat surface to reflect the soundwaves, if its really irregular/lumpy it doesnt seem as consistent to me.

 

[Sbiriguda]

The sonar works ok if you have a flat surface to reflect the soundwaves, if its really irregular/lumpy it doesnt seem as consistent to me.

Interesting remark

 

[rjack321]

Cave walls are a lot more irregular than the ocean or a lake bottom. The beam of the sonar is not a lazer, it's a cone. When the structure is irregular and of varying densities and reflectivity like a wooden shipwreck with a mast sticking up, on a fishfinder/sonar you will see a "hard" structure with a "fuzzy" or soft top in the less dense colors. This unit has no way to show (eg) that portions of the beam are coming back from what looks like 30m but the majority of the beam is taking longer and returning from 35m away. So instead of getting to see that there is a hard structure at 35m with some sort of less dense structure sticking up 5m (cabins, masts, kelp) you get a changing depth based on the last few returns and your exact hand position. 35m then wait a second, now 32m, a few more seconds 34m, a few more then 31m etc

For mapping caves it worked best up to maybe 10m. Beyond that the cone is too wide and you aren't sure where its actual reported measurement is since there no spot on the wall.

 

[JohnnyC]

I've used one for cave surveying - to better estimate wall and ceiling distances. What are you trying to accomplish with it?

Can't remember where, but I saw some european dudes using a Leica Disto in a purpose built housing. Does distance, azumith, inclination, etc. Much more accurate than sonar.

 

[rjack321]

Can't remember where, but I saw some european dudes using a Leica Disto in a purpose built housing. Does distance, azumith, inclination, etc. Much more accurate than sonar.

Disto's, as currently built, don't really work underwater due to the differences in refraction. Underwater Surveys Using A Disto - Surveying - UK Caving

And the distances are quite limiting ~5m. You can correct for the refractions but its not really worth it since you can measure <5m pretty quick anyway and its the long LRUD shots that take so much time to measure with a tape.

 

[Sbiriguda]

Disto's, as currently built, don't really work underwater due to the differences in refraction. Underwater Surveys Using A Disto - Surveying - UK Caving
And the distances are quite limiting ~5m. You can correct for the refractions but its not really worth it since you can measure <5m pretty quick anyway and its the long LRUD shots that take so much time to measure with a tape.

The remark is interesting though... Leica Disto is an excellent tool, very reliable out of the water, but not designed and engineered for underwater usage. Does a similar laser based technology exist also for underwater measuring? I haven't heard about it

 

[rjack321]

The disto won't work in very high humidity "misty" conditions, there's too much scattering of the light beam. Same issue happens underwater, its not possible to use lasers due to refraction, diffusion, and beam scatter.