difference in weighting video rig??

[crpntr133]

I have done mainly fresh water dives and recently did a salt dive. In the fresh water my video rig is slightly negative. Before I did the salt dive I added 4lbs to my video rig just like I did for the rest of my gear. On the dive it was way negative.
So the question is, do you add weight to your rig when you switch from fresh to salt? If so how much? I know that there are several factors in this formula but there has got to be a general rule.

 

[HiDefPics]

There probably are way too many factors for a good general rule to really work, but what I seem to remember from OW classes is to add a factor of about 20% when moving from fresh to salt. To figure this out with your housing, you would have to know exactly how negative it is in fresh water. You can do this with a fish scale & access to a swimming pool or quarry. hang the rig from the scale in the water and see what it weighs. That should tell you how many pounds negative it is in fresh. And if I'm doing the math right - weight in fresh x .20 = lead needed in salt to make it neutral. Might not be exact, but would give you a starting place.

 

[Walter]

Weigh the rig including any lead, multiply that total by .026 and add that amount of lead.

 

[HiDefPics]

Just curious ... can you explain a bit about how/why this formula works? thanks.

 

[Walter]

Sure. When an object is neutrally buoyant, it weighs the same as an equal volune of water. Fresh water weighs 62.4 lbs/cubic foot. Salt water weighs 64 lbs/cubic ft. That's a difference of 1.6 lbs/cubic ft. 62.4*.026 = 1.622. 1.622 + 62.4 = 64.22.

 

[crpntr133]

There probably are way too many factors for a good general rule to really work, but what I seem to remember from OW classes is to add a factor of about 20% when moving from fresh to salt. To figure this out with your housing, you would have to know exactly how negative it is in fresh water. You can do this with a fish scale & access to a swimming pool or quarry. hang the rig from the scale in the water and see what it weighs. That should tell you how many pounds negative it is in fresh. And if I'm doing the math right - weight in fresh x .20 = lead needed in salt to make it neutral. Might not be exact, but would give you a starting place.

Thanks for the clearification Walter. With what info I have I would say that I was 2lb overweight.

Thanks guys.

 

[limeyx]

Thanks for the clearification Walter. With what info I have I would say that I was 2lb overweight.

Thanks guys.

20% is way too much. Unless you have a really big housing, I suspect you can almost certainly ignore the differences.

 

[crpntr133]

Not sure what a "really big housing" is. Mine is 9.5" front to back, 4 5/8" high, 5 3/8" wide + handles. I have seen smaller but also larger. No lights either.
I didn't take physics but I was thinking that water displacement would play a small factor in this. Since it is a smaller housing it would displace less thus not being as bouyant.

I emailed the housing company to see what they had to say.

 

[ronrosa]

I agree with Limeyx, the difference probably won't be noticeable.

 

[limeyx]

Not sure what a "really big housing" is. Mine is 9.5" front to back, 4 5/8" high, 5 3/8" wide + handles. I have seen smaller but also larger. No lights either.
I didn't take physics but I was thinking that water displacement would play a small factor in this. Since it is a smaller housing it would displace less thus not being as bouyant.

I emailed the housing company to see what they had to say.

My video housing weights 33-36 pounds dry weight (without lights).
something like 14x9x12 or something.

It is almost neutral (slightly neg) in salt water.

if I go to fresh I will leave the weighting alone initially and if it is too negative, I actually have 4 small (sub 1-pound) weights inside the housing I can remove (1 or 2 at a time), but definitely not 2-4 pounds.