How is “visibility” defined?

[carldarl]

My last dive I saw a huge concrete block with my eyes the same time I saw it with my forehead. I measured the viz as "crappy."

 

[String]

My last dive I saw a huge concrete block with my eyes the same time I saw it with my forehead. I measured the viz as "crappy."

Im still lying to the others and claiming my finding the thing we were looking for on last dive was superb navigation at night and in sub 2ft visibility.

In reality i was just swimming around having given up all hope of finding it when i found it..... With my head

 

[CIBDiving]

Very subjective unless you carry a secchi disk with you

Oh yeah! that removes the subjectivity allright

Quote http://www.mlswa.org/secchi.htm :

1. The same person should be taking all readings since sharpness of vision varies from person to person.
2. The reading should be taken on the same day of the week, or at least not more than one day before or after the same day of the week.
3. It is preferable that the measurement be taken between 10:00 a.m. and 4:00 p.m. so that the light rays from the sky are at a similar angle each time the reading is taken.
4. Avoid taking the measurement when the lake is choppy or rough.
5. The Secchi disk measurement should be taken at the deepest part of the lake. This may be determined by viewing a bathymetric map or using a depth gage.
6. After anchoring the boat at the predetermined site, take the reading on the shady side of the boat.
7. The reading should be taken at the same location each week. In order to guarantee a sampling at the same location in the lake, a buoy may be permanently set at the site. If it is not possible to place a buoy at the site, line up two objects on the shore some distance apart (one at the shoreline) and line up two other objects on the shore and at right angles to the plane of the first two, anchor your boat and take the reading. Mark the shoreline objects and the intersect points on your map of the lake so that you will be able to find the same site the following week. If it is necessary that a substitute take the reading later in the season, they will be able to find the same location.
end quote

 

[String]

If you're on about various methods for determining the vis theres a very obvious one.

Take a plastic measuring tape down with you

We did similar by agreeing that when you could no longer determine the outline of a rocky outcrop you'd call that the edge of vis. We used one of our backup dmsb reels and tied a knot in it to mark the length. Got back onboard the boat and measured it.

Useful exercise in seeing how accurate our guesstimates were if nothing else.

 

[PAdiver93]

What's viz got to do with it....who needs viz. Feeeel your way. That sudden surprise is the best, just make sure your have enough in you to deal with it. Just kidding! That's about right, I actually look from the boat over and if I can see the bottom, I look at the depth sounder on the boat and there you go wa la! Viz. Peace

That's great - but using your method estimate the viz for my last dive. Depth 75, at the surface water was brown, after u get into the water u could not see your fins. About 30 ft down I was finally able to make out an outline(!) of my buddy - he was about 3 ft away from me. another 20 ft and I was able to see the bottom and make out some of the object (identify the fish that was about 10-15 ft from me). No ambient light whatsoever.


And would the Secchi wheel work?

 

[archman]

Quote http://www.mlswa.org/secchi.htm :

1. The same person should be taking all readings since sharpness of vision varies from person to person.
2. The reading should be taken on the same day of the week, or at least not more than one day before or after the same day of the week.
3. It is preferable that the measurement be taken between 10:00 a.m. and 4:00 p.m. so that the light rays from the sky are at a similar angle each time the reading is taken.
4. Avoid taking the measurement when the lake is choppy or rough.
5. The Secchi disk measurement should be taken at the deepest part of the lake. This may be determined by viewing a bathymetric map or using a depth gage.
6. After anchoring the boat at the predetermined site, take the reading on the shady side of the boat.
7. The reading should be taken at the same location each week. In order to guarantee a sampling at the same location in the lake, a buoy may be permanently set at the site. If it is not possible to place a buoy at the site, line up two objects on the shore some distance apart (one at the shoreline) and line up two other objects on the shore and at right angles to the plane of the first two, anchor your boat and take the reading. Mark the shoreline objects and the intersect points on your map of the lake so that you will be able to find the same site the following week. If it is necessary that a substitute take the reading later in the season, they will be able to find the same location.
end quote

Except for #1, #2 and #5, this is pretty standard protocol. A lot of new people forget about the shade thing, however. For a quick and dirty measure, one can axe all but #3 and #4 and the value should be pretty accurate. Just not very precise.

I think almost everyone that has serious long-term water clarity monitoring projects have switched to photometers and/or turbidometers. Some of these can be set up remotely for continous use.

I like that Michigan Lakes website. The most thorough I've ever read.

 

[tedtim]

When I was flying we had limits of 1/2 mile flight visibilty in the training area. I asked my co-pilot what he thought the visibility was. If he said 1/4 mile and I had 1/4 mile, then we had 1/2 mile and were good to go.

I use to Excellent. It is all subjective. I love it when guys tell me the vis was 30 feet. Makes me laugh. I ask them if they are sure it is 30 feet or if it is really 33 feet.

 

[TSandM]

It's all subjective. The dive staff at the place in Australia we've been diving the last few days kept apologizing for the viz being only 15 to 20 meters. At home, I'm a happy camper if the viz is 15 FEET.

 

[mccabejc]

Here's a poll I took a while back to see how folks defined viz. Seems like most folks call viz the point at which you can see an outline.

http://www.scubaboard.com/showthread.php?t=83847&highlight=visibility

 

[caymaniac]

This viz thing is tricky. Diving in Lake Michigan, I've had really good viz of 20ft. in the last twenty years maybe twice, often it is about 10ft., and plenty of times 3 to 4 ft. While diving in Bonaire I estimated 60 to 80 and in the Caymans about the same although I've had 100 plenty of times...but it is in all just estimated because I never had a ruler with me.
I will say that I do remember the day of the best viz, it was Easter morning on Little Cayman 1992. When we pulled up to connect to the bouy, the DM said "it doesn't get any better than this" , I estimated at least 200ft. on that day.
While in Australia they state that the viz at Osprey is 100 ft., I don't think it was, more like 60 to 80 and then on the Ribbons more like 30 to 40ft. Most times that we've been to the Keys the viz was 30 to 40, but have had 60 on occasion. It's all really a guess, I really don't want to spend the time measuring it out so I think almost everyone just...guesstimates!