Slightly Positive or Slightly Negative Buoyancy

[Teamcasa]

I’m using a Ocean Images housing with two Nocturnal Lights SL50’s. On my last dive, the first dive with the new lights, I found the whole unit very negative and difficult to hold for a steady shot. I am currently building a new arm system that I plan on having the ability to adjust the lights and the buoyancy. I was using LOCline with no success.

My question is do you experienced videographers like your systems positive, negative or neutral? I was also thinking of putting on a small bag that would allow me to inflate and float the system on the surface.

Dave

 

[thomjinx]

I prefer to have mine just ever so slightly positive. I can always get to the surface, I can't always get to the bottom. (safely)

 

[sinistar]

My current setup (no lighting yet) is just ever so slightly positive. Very nice after diving 3-4 times a day for a week - no sore hands or wrists. Very easy to hold the camera one handed also.

I would avoid a bag/bladder if possible - just one more thing to manage across depth changes. And I thought I read something once about using some type of closed cell foam (or something that had fixed bouyance characteristics across depth - like the housing itself). However, not sure of this alternate approach is too big to dive and/or travel with when it has to counteract the added lights and batteries?

 

[Teamcasa]

My idea for a buoyancy compensator would be a PCV tube that could be adjusted for perfect buoyancy. As for the bag, it would only be deployed to float the camera rig on the surface.

Dave

 

[ronrosa]

My current setup (no lighting yet) is just ever so slightly positive. Very nice after diving 3-4 times a day for a week - no sore hands or wrists. Very easy to hold the camera one handed also.

I would avoid a bag/bladder if possible - just one more thing to manage across depth changes. And I thought I read something once about using some type of closed cell foam (or something that had fixed bouyance characteristics across depth - like the housing itself). However, not sure of this alternate approach is too big to dive and/or travel with when it has to counteract the added lights and batteries?

I use closed cell foam to make my housing neutral, but removable when I want to set my housing in the sand to film creatures down low.

Here is a pic of a buoyancy collar I made for a very negative w/a lens.

closed cell foam collar

I also made individual blocks that can be added or removed from the housing secured with rubber bands or surgical tubing.

 

[sinistar]

ronrosa, I think that is exactly what I had read about before! Now, I'm curious where you get that type of foam and if it soft enough to act as additional packing material (for something important) when traveling.

Teamcasa, I was thinking about the PVC idea. So please check the numbers and theory as I did this very quickly.

1 cubic inch of fresh water => 0.0361 pounds
...or...
28 cubic inches of fresh water ~> 1 pound

And let's say a person uses 2" PVC (I simplified to a inside diameter is 2" and zero wall thickness to make the math easier). Then the length of tube required to hold/displace 28 cubic inches of volume (or roughly 1LB) would be:


Volume(cylinder) = PI * Radius^2 * Length
... or ...
28 ~> PI * Radius^2 * Length
... or ...
Length ~> 28 / (PI * Radius^2)
... or ...
Length ~> 28 / ( 3.1415 * 1" * 1")
... or ...
Length ~> 8.9"

Now, obviously we're more likely to be talking about saltwater (so we'll need to displace a bit more volume). And I'm really not sure if the 2" PVC pipe has a 2" inside or outside diameter. And of course the pipe weights something. And finally, it's been a while since I've done a displacement problem so perhaps I've really went south someplace???

However if the numbers are even close, it would call for about 9" long of closed PVC pipe per pound of equipment being displaced. So if a diver needed to counter 5 Lbs of lights and batteries they would need several feet of this PVC (or several shorter tubes bundles together).

And now I'm really curious, was that a valid displacement calculation?

 

[ronrosa]

You can't use this foam as packing material. It's too hard.

I got the idea from an excellent "DIY" article. It lists suppliers and detailed instructions.

DIY closed cell foam buoyancy collar

Making a buoyancy collar is a little involved because you want it to fit snuck around the lens. Blocks are easy. I have several 6" long x 1" square blocks to fine tune housing and/or light buoyancy.

 

[drbill]

I've kept mine slightly positive for the past 3 years (ever since my second light housing flooded)... I prefer it floats to the surface if I lose my grip on it.

However, I just purchased a new HD camcorder and will be buying HID lights as well so I'll have to start experimenting with bouyancy options to get it where I want it to be.