Find the Buoyancy of your DSLR Rig

Hi all! I've just got my complete S&S DX-D80 rig in the water over at our swimming pool (fresh H2O). TTL III, YS-110 x 2, 60mm lens and ULCS regular 8" buoyancy arms x 4. The thing is still negative! I don't know by how much. Not really surprising, but still more neg. than I want. Now, I can exchange the regular buoyancy arms for the "ultra" buoyancy arms as long as they don't hit salt.

So, my question: The spring scales used for weighing fish are not really accurate enough to give me negative weight in ounces. Is there another place I can purchase a more accurate scale tht will give me buoyant weight down into the ounces range?

I'm going to feel really funny with a pair of 12" "ultra" buoyancy arms for macro!? That's one BIG "W"!

hi pat,

why not just weight it on land and then measure how much water it displaces. with these two information you can calculate the exact wheight for salt (1 liter saltwater=1,03 Kilo) and freshwater ( 1 liter freshwater=1Kilo).

so for example if it weights 3,6 kilo and displaces 2,9 liters, than it will be 0,7 negative in freshwater.

hope i remember that well ... it's been a while

serge

How do you measure the amount of water it displaces ?

How do you measure the amount of water it displaces ?

I think the fish scale is easier, but here is how to measure the volume of water displaced.

Partially fill a bucket with water. Mark the level with a pen. Completely submerge the housing. Mark the new level. Then you can compute the volume from the difference in levels and the dimensions of the the bucket. If you don't want to do math, you can measure the volume by removing the housing and adding water to the bucket until the level gets back to the high mark. The volume of water you add is the volume of the housing.

Very, very similar to Steve's technique........here's another way........at least for those of us who rely on inches and feet and pounds..........and don't fish .........

and taken one step further......to determine the submerged weight.....whether positive, negative, or neutral.

start with a pot of known diameter ( my wife has a deep, straight sided kitchen pot used for boiling large amounts of water that measures 13" across).

place a ruler on the inside of the pot.........and fill with water........enough to cover either the entire rig or possibly just single components that make up the underwater camera rig ( imho the preferred method in order to determine exactly what parts of the rig are more or less buoyant).......for me filling the pot to the 7" marker worked for my purposes.

next place the rig (or component) into the pot of water and note the increase in water level as denoted on the ruler.

for example let's say the water level increased by 1"........to the 8" line....

now let's introduce an equation to determine volume of a cylinder (the kitchen pot in this example is cylindrical).......in order to find how much water was displaced.

Volume = Length times Pi times radius squared

Volume is the unknown in this equation. But we do know the Length (or height) of the cylinder, which is 1" (the increase in height of water as denoted by the ruler when the object was introduced into the pot). Pi = 3.14. And the radius of a 13" diameter pot is 6.5. If we square 6.5 (6.5x6.5) we get 42.25.

So V (volume) in cubic inches (i'm using inches in all my calculations above) would equal:

V=L*Pi*r²

V=1*3.14*42.25

V=132.665 cubic inches

since each cubic inch of salt water weighs @ 0.037 pounds.....then you could multiply 0.037 times 132.665 to find out the weight of the water being displaced (in pounds):

0.037*132.665=4.9 pounds

If the object that was placed into the water also weighed 4.9 pounds (dry) then the object would be considered neutrally buoyant (and would have floated in the pot of water)

If the object had a dry weight of more than 4.9 pounds, it would have rested on the bottom of the pot of water......and if it weighed less than 4.9 pounds the object would have floated.

hth,

b

Steve, you said you prefer the fish scale method. $5-20 US, but you can't measure ounces on any scale I've found at West Marine or Boater's World. Everything measures in 2 pound increments, unless you spent $80, in which case it's getting a little ridiculous. What scale would you use?

I could also hang it from a letter scale above the swimming pool?

Of course, I could do the high school math and fing the weight of water displaced subtracted from the dry weight to find buoyancy, but that seems like so much trouble to go through. Couldn't I buy something? That would be sooooo much more fun!

Steve, you said you prefer the fish scale method. $5-20 US, but you can't measure ounces on any scale I've found at West Marine or Boater's World. Everything measures in 2 pound increments, unless you spent $80, in which case it's getting a little ridiculous. What scale would you use?

I don't own this scale, but check out http://www.amazon.com/Taylor-34205104TJ-IN...4953&sr=8-3

It is a 20lb scale with oz increments, which should be fine for weighing the rig in water. Out of water, use your bathroom scale.

Interesting. Thanks!

Direct measurement is always better than by doing a calculation - especially for lazy people like us underwater photographers! :-) I vote for the fish scale - you can even get a digital fish scale now-adays.

Cheers

James

The thing I realized about "buoyancy" arms is that they aren't really that buoyant. They are just a little bit positive which means they really aren't contributing much to being negative, but don't really help with much positive lift.

Even though there have been numerous posts about methods for increasing the buoyancy of a rig I use the high density foam swim tubes that you buy at WalMart slipped over my extension arms. I've used them down to 120 feet and have been very happy with my $4 solution. I originally tried foam pipe insulation but it was too easily compressed, even at 40 feet and I had to wrap tape around them because of the split that is precut into them.

I slipped the tubes over the arms and it only took me 1 dive with a knife to trim the foam to the point where the housing (D200 in a Subal) was where I wanted it buoyancy wise.

If that would work for you I can send a picture of the setup.

Andy

If that would work for you I can send a picture of the setup.

 

Andy

Hello Andy,

we're all interested...

can you post a picture of the setup?

...Even though there have been numerous posts about methods for increasing the buoyancy of a rig I use the high density foam swim tubes that you buy at WalMart slipped over my extension arms. I've used them down to 120 feet and have been very happy with my $4 solution...

 

Andy

I can't believe that there is anything associated with underwater photography that ONLY costs $4. This is great! I can't wait to see it.

-Jim

I can't believe that there is anything associated with underwater photography that ONLY costs $4. This is great! I can't wait to see it.

 

-Jim

Actually, it was less than $4 as the foam tube was about 5 feet long and I only used half of it. It seems to be relatively impervious to salt water. They have been on 40 or 50 dives and look the same as when I put them on.

I will post when I get home this evening. (which may be tomorrow in your part of the world)

Find a container slightly bigger than your camera setup.

Put camera in container.

Put container on scale.

Fill with water until overflow and note the weight.

Take the camera out.

Fill with water until overflow and note the weight again.

The difference in the two measurements is how much your camera is negative.

EDIT: I just read the rest of the thread more carefully and noticed Steve had a similar idea. This way you don't need to make any volume measurement. You only have to read the scale. You can get .1 lb precision with a $30 scale.

http://www.amazon.com/Weight-Watchers-Glas...049&sr=8-22

Ryan Canon suggested that I do a negative weight total of my system with a Rapala scale from Wal Mart (Ryan has lots of great and practical ideas and advice - thanks!!). I did this and he did too.

We did the weights separate and the weights were close.

My weights were also close as far as the strobe weight to Backscatter's recent article.

I did a post last November or early December with pics of how I weighed my setup in a pool

here is the link: http://wetpixel.com/forums/index.php?showt...5&hl=rapala

and then I checked my figures by weighing piecemeal in a bucket of water. Gave me an idea of how negative it was actually. Only problem with the scale was with weights under 3 oz and those I confirmed by combining it with a known weight quantity I already had.

Steve (Photobeat) also helped me with the weighing and we were careful to let the camera settle and just get the buoyant weight underwater and not any in-water movement.

For buoyancy I use the Patima BCs (that Sam(shchae) got for me - thanks!) one on each regular 8" ULCS arm and corecell foam as needed for a collar (that Ryan Canon made for me - thanks!) on the 105vr heavy lens. I also use a little corecell on 2 of the arms. I'm very happy with this setup as it is not bulky and when I come up to very shallow water I can easily dump buoyancy (very important). Yes the BCs compress a bit but not enough for me to need to adjust them much even at 140 feet - but that was with wide angle and remember I have a little bit of non-compressible foam on there too. If I went that deep with the 105vr lens I'd have to pump the BCs with a little air hose I have and of course always release some air on ascent.

In DavidGs manatee post here there is a pic of me with the setup u/w w/BCs and 16mm wide angle Subal d200 ND20 ( I'm trying to keep an inquisitive manatee away from the dome port - they won't hurt it but I wouldn't want to find out what might happen if they pushed on it too hard as they can do out of affection and curiosity, etc):

http://wetpixel.com/forums/index.php?showt...&hl=manatee

The d80 shouldn't be as negative as the d200, maybe 9oz or so less?

Good Luck, Carol

sorry carol, what is the patima BC?

thanks, serge

EDIT: AHHH ... sorry. i just found the link

Is this a thread about Archimedes and the dangers of a college education?

More appositely, I'd like to report a problem that became apparent as I struggled towards neutral bouyancy: I found that the port started to exert a bouyant moment about the center of bouyancy (the arms usually exert thier own moment slightly in front of the center of bouyancy, too), rotating the lens upwards. This was so annoying that I've settled for a slightly negative rig, which is much more comfortable to use.

Tim

B)