Determining Visibility

[Dive-aholic]

I estimate visibility on whether I can see hand signals or not. If my buddy were to be 20' away and I can make out a hand signal, then that's the visibility. If it's 50' and I can make out the hand signal, then 50'. If I can see my buddy's outline but not make out a hand signal, then the visibility is less than the distance. And I estimate the distance, which isn't always easy under water.

 

[ClayJar]

Do they make an underwater range finder - I mean, kinda like a depth sounder device.

I have a HawkEye DF2200PX sonar range finder. It comes in handy when I'm paddling to a dive site in a lake, and it can also be handy for maintaining spacing from a wall/wreck in low viz. It's also fun just to use it for random things.

The nice thing is, you can get a refurb one for $75 from the Norcross Marine store.

I've never bothered to use it for estimating viz, but you certainly could.

 

[Thalassamania]

Out in the middle of the Atlantic Gyre I've seen visibility in the 300 to 400 foot range. It's freaky to be able to see the entire hull of your mothership at a great distance.

 

[matt_unique]

We were just chatting about this in the NE Lobstah Divers forum. The most reliable, non-scientific way in my opinion is to measure verticle vis. If I can see the surface of the water I look at my computer. We know horizontal vis is not the same as verticle vis, but for diver to diver reports it's quite useful.

--Matt

 

[Divrdug]

Regardless of how far the visibility, some people are just horrible at judging distance. That's why I like the depth below the boat then minus some, because it lets put your depth guage to use. As an Archer I have become quite proficient at estimating distance over land out to almost 2 hundred yards and I'm amazed at how poorly most people do when it comes to guessing distance. Now put them under water and the distortion throws them off even more.

 

[Tortuga James]

Ken, remember that the visibility in a pool is determined by how good the filtration system is and how many people have recently been in the pool.
If the pool has been closed overnight with no one in it and it was filtered the whole time, chances are the vis is spectacular. As soon as people get in and start splashing around, now you have all sorts of great stuff in the water--bodily oils, skin flakes, etc,

etc, etc...I was hoping it was just the amber lenses in my mask

 

[donacheson]

In perfectly gin clear water, like bottled water clear, how far could you see? I think 100' would be the max. I understand depth and light absorption play a role.

I find that there is a huge discrepancy in people's estimation of what the viz is on any given day. What would you say the end of the visibility is determined by? If the viz was 40', could you see a diver at 45'? I usually will estimate the viz to be the distance that I can see clearly. So if I estimate the viz to be 20', I usually can make out objects and divers at 30-35', but fuzzy.

I started thinking about this in the pool yesterday. One length is 25 yards (75 feet) and I could not make out the markings on the far side of the pool horizontally (With a mask on, of course). When I hear that the viz is 100', I find that hard to believe.

I am very used to diving limited viz inshore water, and I find that when others think the viz is awful, I think it is pretty good.

In the atmosphere four factors are important (and I'm pretty sure the same ones apply in water): the properties of the medium (air or water), the target, its background, and the observer. Meteorologists have standardized the measurement of atmospheric visibility in two ways. If observed by a human, the observer is supposed to chose a certain type of target - a dark object against the horizon sky by day and a light showing against a dark background by night. If observed instrumentally, the extinction coefficient of the atmosphere is measured and the visibility calculated assuming a black target against a white background by day and a "standard" observer with a threshold of contrast of (about) 0.05 or a 25 Watt incandescent bulb against a dark background by night and a "standard" observer's threshold of illuminance whose value I don't recall.

As another responder has suggested, oceanographers use a Secchi disk with black and white segments painted on it as a substitute for a black target against a white background, dropping it until it's just recognizable. I've seen salt water extinction coefficient data taken in deep water off the coast of Hawaii which suggests that the visibility there must be at least 300 feet.

Personally, I try to judge underwater visibility based on something of a known dimension - length of a wreck, the dive boat hull length, the length of a line training off the stern of a dive boat, etc.

 

[Xman]

In New Zealand we have a fresh water spring which apparently has the clearest spring water in the world. It has been measured as having 63 metres vis.

See here:
http://en.wikipedia.org/wiki/Te_Waikoropupu_Springs

 

[Thalassamania]

The standard way oif determining visibility is with a Succi Disk or a transmissometer.

 

[mobeeno]

As a videographer, I define visibility in a slightly different way... it is the distance at which I begin to lose clarity in a recorded image. Therefore my estimates tend to be on the low end, but I usually state how I judge it if a non-imaging diver asks. Another way I define visibility is the greatestest distance I can clearly identify a great white shark approaching me!

Well said