Visibility

[Hostage]

Anyone have a good way of figuring out how far something is away? Does visibility start at when you can barely see a diver outline before they disappear (not your dive buddy)? It seems a lot easier to judge distance on the surface than under water.

-Hostage

 

[TSandM]

Visibility can be measured formally with what is known as a Secchi disk, which is a disc divided into black and white sections. Viz is defined as how far away you can distinguish the two areas from one another, but almost nobody goes to those lengths to determine was viz is. Other than that, the determination of visibility is somewhat subjective. I personally don't consider that I can "see" someone unless I can recognize who they are -- just because I can glimpse tanks or bubbles doesn't mean I can really "see" that far. I have to be able to make out details. And then you have the issue of trying to estimate how far away the person whose details you can see really IS . . .

I suspect that what I hear around Puget Sound is kind of along the lines of: if it's REALLY good, it's 40/50/70 feet; if it's pretty good diving, it's 25, if it's typical diving, it's 10-15, and if it's so horrible you wonder why you're there at all, it's "THREE FEET!" or less.

 

[Pane]

I used to log estimates for viz (usually based on the dive guide's estimate) but I gave up on that. Now I just use terms like "okay", "pretty good", or "terrible".

logged a fair few terribles during our most recent trip, which was to Fuerteventura

 

[DivemasterDennis]

One function of keeping a dive log of every dive is to develop a means for pretty good guesses at how far you can see. On most dives, I will select an object at the limit of vision, and then count kick cycles to it. Another method is to use your depth gauge, and see at what depth you can still discern surface waves. The deduct 10% for lighting difference's impact at depth. I find that method to be fairly accurate as well. My categpries for viz are these: less than 10 feet, "about 20 feet" then ten feet increments up to 70 feet, then "more than 70 feet." In some totally clear places ( Back wall of Molokini in Hawaii, certain day and conditions in the Caymans, and elsewhere in the Caribbean, I have noted "100 +".
DivemasterDennis

 

[tdtaylor]

Great question. We were sitting around Saturday on an SI and discussing this. No one agreed on a distance, but there was more agreement on "poor" and "not good." This is one subject where YMMV.

I for one have utilized viewing the surface as Dennis mentions above.

 

[Dhboner]

The vis was always better on your dive than someone elses...."You should have been here yesterday we got 150 feet"

 

[socaljohn]

Visibility can be measured formally with what is known as a Secchi disk, which is a disc divided into black and white sections. Viz is defined as how far away you can distinguish the two areas from one another, .

I remember this from the "floating classroom" field trips out of Dana Point when I was a kid. But I have always wondered what is the definition as regards how much detail you can make out vs just seeing a shadow. Here in So Cal our shore dive viz is divable at 10 and great at 20 feet.(Probably a sep thread I will look for)

 

[gcbryan]

I think determining the distance becomes easier as you dive more as you have more of a standard reference point. If it's usually 15' where you dive that distance soon becomes the standard by which you judge everything else.

A good day is more than that. A poor day is less than that. Vis starts for me where I can recognize something.

 

[Hawkwood]

The vis was always better on your dive than someone elses...."You should have been here yesterday we got 150 feet"

In my experience it is invariably the other way around.

 

[RonFrank]

We dive some fairly crummy sites. If I can see my buddy 5 feet away I am a happy camper. If I can not we may end the dive early. If we have to hold hands vis is worthless and a second dive is not forth coming.