How does one measure visibility?

[fuzz]

I know people often talk about visibility in objective terms - but in my experience it is anything but...

Is visibility the distance you can make out a sillouhette? The distance you can define a fish? The distance you can see scales?

The distance from which you can see the bottom? The distance from which you can distinguish between types of macroalgae?

I often see visibility chart estimates or hear people talking about visibility on a day's diving, but not sure how to properly define it.

Any thoughts?

 

[Abriapnea]

HOO ! ; that's a good one ...
IMO vis. ranges are handed down by old salts to newbies : the experienced diver tells you "it's 12 m. today" and you adjust your perceptions accordingly . At least that's how I learned and pass it along .
Mind you , wouldn't be surprised to find out that some of our tech- minded colleages swim around with yardstics/laser thingies or even little calipers in the case of our brethren in NoCal ...

 

[Shadowkiller]

Originally posted by Abriapnea
little calipers in the case of our brethren in NoCal ...

Are we still talking about visibility?:mute :blackeye

I usually define viz as the distance I can still make out the bottom clearly enough to spot fish...

 

[icarus pacific]

now that was smooth!

Originally posted by Abriapnea
in the case of our brethren in NoCal ...

Hey!, OWW!!:blackeye jeez...

Actually Abri's correct for the second time in as many weeks- I looked down and thought, "This is gonna suck!", and heard the experienced types just whooping it up over the great viz! Thing is I was hoping for that TV style, Sea Hunt, Jock Clouesau kinda stuff... :head

After a while you figure, well I can see the rivets on my fins and that's 6 feet away, the bottom is at 30 and I can see the kelp holdfasts, that wreck is in 218 and the current keeps it clean and the stern is 100 feet away...

I have guys that I dive with that swear that the viz was 10 more than I thought- and we were 4' apart and couldn't see eachother!

I need to see some detail before I call the viz, not scales so much but if I can tell the difference between a whatever and a whatever from where I'm at, sweet.

little calipers... :hmm


sven

 

[unirdna]

For anyone interested, here's what us 'old-school' science types use to measure water clarity. It's called a secchi (rhymes with "becky") disk You tie a marked (meters, feet, etc) string on it and lower it into the water. Throw on your mask, stick your face just below the surface and lower the disk until it completely disappears (note this length). Then you raise it until it reappears (note this length). Then you average the two. There you have it. The cheapest, 100+ year old, highly controversial (in the arena of limnology/oceanography that is :hmm) method of measuring vis.

Ted

 

[fuzz]

Originally posted by Shadowkiller
Are we still talking about visibility?:mute :blackeye

I usually define viz as the distance I can still make out the bottom clearly enough to spot fish...

This is what I mean - I can see fish a few feet before I can determine what they are by shape of its sillouhette - then another couple feet till I can tell patterns, etc.

In poor visibility, those few feet between those 3 options cause a very high variance in distance - say from 10 foot visibility to 4 foot visibility!

 

[fuzz]

& unirdna... don't usually carry a secchi with me when I go diving... :hmm

 

[Jon]

Once the zebra mussles started clearing up the water on Lake Michigan people just wouldn't believe me when I told them how good it was.

I finally resorted to taking a fiberglass tape measure with me and attached one end of it to a shipwreck and swam out with the other end to record the distance. 85' was the best that I have gotten so far and that was taken in January on a wreck in 100' of water. Pretty amazing considering only 15 years ago 10' was considered a good day. Another way is to reel out my wreck line which is marked with knots every 10'.

I know that not everyone dives with a tape measure, but we have them on us while doing shipwreck surveys.

Another way that we check vis is buy measuring off the size of the windows, or hatches, on a shipwreck. Once we get to the bottom we can count out how many windows, or hatches, we can see and tell everyone back on the boat what the vis is like.

I usally do this while freediving. I jump in as soon as we get on site and tell everyone where the thermocline is and what the vis is like on the bottom, since it "opens up" once you drop below the thermocline.

Jon

 

[Jon]

Like this.

Jon

 

[Dogmatrix]

I was always under the impression that when rating vis it was the distance you could clearly see the outline of a sheet of a4 paper...

Dunno who is going to give that one a try... might have to do some laminating...

Anyway all the best... I will try to get some pics in PNG of "good vis"

 

[Iyadiver]

Assuming the viz of the water from surface to the bottom is the same, I measure visibility by being able to see the rock or whatever I am familiar with at that depth and compare my depth gauge reading. I must be able to see that reference object quite clearly. Diffferent color can be seen differently, distance wise. If the water is really murky, I drop my shaft and whatever distance I can see the Slip Tip, I can measure with my shooting line.

Anyday that I can see my shaft flying all the way and miss the fish is a decent viz. Sunlight play the effect too. I prefer a viz of 2.5 times my shooting range. Any clearer than 100+ I feel stupid as I can not gauge far away fish accurately, unless I get 100+ viz 30% of my dive.... but ..I don't:head

 

[fuzz]

Originally posted by Iyadiver
I must be able to see that reference object quite clearly.

This & other similar comments is what I'm referring to - concepts of what people deem as "clearly" and subsequent overall visibility is(& will remain) a very subjective issue.

 

[ivan]

hi

Vis is how far sown you can see the coral from the surface. Today it was 15ft He He

cheers

 

[Iyadiver]

Here is my version of 5 meter (16.5 feet )viz. Below is a captured video image of a propeller shaft and the "V" bracket/strut. In video mode is a bit better but when freezed/captured the quality degrage a bit. Look at the "V" bracket furthest away leg, that is about 5 meter distance. If there is a trevaly there, I still could see it's shiny body a bit. The bracket height is about 2.5 (8 feet )meters, the propeller shaft is about 60cm in diameter ( 2 feet).

 

[Griff]

My measurements are made in large increments, like its 1m (cant see the end of my gun) . 5m, cant see the rocks from the top , 10m, can see the rocks from the top (not too much definition) . 20m+ you can see the crayfish that you just dropped walking along the reef back to his hole 15m down :waterwork .

and Iya, 16.5ft? do you also own one of those calipers?

I always laugh at the scoobies when they say, ja, the viz isnt too good today, when you can see the sand meeting the rocks 20+m down. mind you, they get pretty spoilt at sodwana. i sometimes can watch the painted surgeons and moorish idols milling around the reef while lying on the pontoon of the diveduck. thats when i'd say its 25m+.

i've got a question, how far is it possible to see underwater? because the light is refracted and abosorbed of whatever, by the cleanest water, what is the best viz physically possible?

 

[Iyadiver]

Griff,

I carry a floating stringer that is 12 meter long. The first 1-7 meters, it has a knot at every 1 meter. I use it to measure my target shooting distance all the time, thus technically I do have a measuring tape with me every dive.... he he he he

 

[Griff]

well bud, if your tape is in 1m increments, you are deserved of the title Mr Long Post

 

[aquadog]

vis= how far away you can read your beer bottle clearly on the ride in. vis decreases with the more beer consumed.
dive hard drink safe
aquadog

 

[Cingene]

Fuzz,

you had to ask didn't you :t

 

[fuzz]

Yeah, sucks when people say vis. up north is 8 feet & I can't even see my ab iron