Maybe dumb ? about visibility

[dannobee]

Thal: The only time I've seen or used a Succi Disk is for vertical visibility. Is there a recognized relationship (or rule of thumb) between vertical and horizontal visibility?

I have a Secchi disc on my anchor line, just above the chain, with marks on the line every 10 ft and a hash mark every 5 ft. I can lower it into the water and check vis before committing to diving a spot, or once throwing out the anchor, can work down the anchor line and get an accurate measurement of the vis on the way down.

Since boats get blown by the wind and currents, the anchor line is rarely straight down, so using the angle of the rode gives you an idea.

I had someone actually argue with me about the vis using this method. His "estimate" was a lot more optimistic than what I measured with the Secchi disc. Go figure. Sort of like "measuring" temperature with your hand while someone else is looking at the thermometer reading.

The Reef Check way works, too. But I think you're supposed to hold up your hand, not just the fingers.

 

[Greendiver]

Fun discussion!

Around here the vis can change quite a bit with change in depth. Descending through the thermocline can reduce vis to a foot or two but then can open up to 50' or more after dropping through the suspended particles. This can be within a 10' change in depth. Very dramatic.

Ah, 100'+ vis... The promised land!

 

[The Chairman]

A real visibility measurement is done with a Succi Disk, a round white disk like a Frisbie on a measured line.

I have always heard of it as a Secchi Disk and it was named after its inventor.

This is actually used from a boat and has little to no bearing on your own personal vis. For me, the vis is equal to the distance where I can still distinguish my buddy! Here in the Keys, that's often 100 ft or more though I never wander that far from my buddy!

 

[DivemasterDennis]

Measuring or estimating visibility at 20 feet and under is not very difficult, people, as we all know. It can be done, assume you dive with a buddy, in "diver lengths." But I think there can be more than WAGing to determine visibility of greater distances. Once again, diver length's is useful- I call a diver length 6 feet (plus or minus a foot) and that measure can give you a pretty good estimate up to 8 or 10 diver lengths. One could use a marked line (and I have done this, but rarely). I think familiarity with a location and conditions allows a more accurate assessment. Vertical viz can be a good indicator, but as noted early in the thread, conditions can change during a dive with depth changes, location changes, as in cornering an out cropping, and of course, sun light penetration, meaning vis is better at high noon than in later afternoon or early morning, and drops with cloud cover. Interestingly, accurate measurement of viz is rarely crucial. You really only need need a few units of measure. In the ocean, my scale is this: can't see a thing (less than 3 feet); extremely limited (3 to 6 feet); poor (6 to 15 feet); limited (15 to 25 feet), pretty good (25 to 40 feet); good (40 to 60 feet); and great (60 feet plus). But in the local reservoir where we train, the scale is different, and looks like this: a bit limited (0 to 2 feet); pretty good (3 to 6 feet); good (6 to 10 feet); great! (10 feet plus). The of course, there are location where 60 feet of viz is considered poor- those are my favorite locations! Anyway, here's wishing for all of you this holiday season warm waters, calm seas and great viz!
DivemasterDennis

 

[friscuba]

I'd believe 150' + on the dives I've had at Molokini. I used to get in viz agruments with one of my employees, he said I usually underestimated them. I took down a hundred foot reel on what I called a fifty foot day and could see people at the other end, could read fingers at about 75'.

I can only think of a few occasions that I haven't been able to see the back of my boat from the mooring line on a safety stop, that would be maybe fifty feet with my boat, usually it is much much farther.

I have always wanted to go down with two divers with 150' reels on a great viz day to see what it really is.

 

[DivemasterDennis]

Been there- and it is 150' plus. I've enjoyed Kona diving too, and my wife and I are big Hawaii diving fans. It is paradise- above and below the water!
DivemasterDennis

 

[sambolino44]

in tx we estimate visibility the same way we weigh a hog.

You get a big long plank of wood and a pile of rocks. You pick out a good round rock and place it under the middle of the plank. Then you tie the hog to one end of the plank and start piling rocks on the other end until it balances. Then we estimate the weight of the rocks.:d

aggie!