difference in weighting video rig??

[crpntr133]

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.

Thanks for the info guys. The next time I dive salt I think I will leave the 4 lbs out. Maybe add a lb or two and adjust from there.

If anyone has more info or another point of view fire away.

 

[HiDefPics]

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

I really don't think it is too much. Maybe I wasn't clear in my post. What I mean is let's say it takes 2lbs of lead to make a housing neutral in fresh water. Then if you add 20% you'd have 2.4lbs of lead to make it neutral in salt water. Just as you're saying ... this is not a huge difference.

 

[crpntr133]

I really don't think it is too much. Maybe I wasn't clear in my post. What I mean is let's say it takes 2lbs of lead to make a housing neutral in fresh water. Then if you add 20% you'd have 2.4lbs of lead to make it neutral in salt water. Just as you're saying ... this is not a huge difference.

Wasn't totally sure on your post. I think we have it all cleared up, thanks.

 

[limeyx]

I really don't think it is too much. Maybe I wasn't clear in my post. What I mean is let's say it takes 2lbs of lead to make a housing neutral in fresh water. Then if you add 20% you'd have 2.4lbs of lead to make it neutral in salt water. Just as you're saying ... this is not a huge difference.

I thought you meant 20% of the dry weight. Maybe this way works out to same as 0.026 method?

 

[crpntr133]

It should be somewhere around 20% of the weight added to the rig. So I added 1 lb for fresh and would add 1.2 for salt. Not enough to deal with.

I emailed Ocean Images to see what they had to say about this and they say..26%. With my rig being slightly negative in fresh it should be close to neutral in salt.
HiDefPics way is possibly the long way home. What is happening by using the fish scales is you are seeing how negative the rig is. I guess if you let the rig pull up on the scales you could tell how positive as well.

 

[HiDefPics]

you got it. sorry i wasn't more clear regarding the use of the scale.

 

[Dan Gibson]

Or you can add a know weight to make it negative once again and then just subtract that known added weight. I have done this when an item is close to neutral or positive. Scales tend to give better readings in the middle range instead of at the ends.

What is happening by using the fish scales is you are seeing how negative the rig is. I guess if you let the rig pull up on the scales you could tell how positive as well.

 

[crpntr133]

Adding a weight is what I did to find neutral in fresh. Since I didn't know the formula for fresh to salt it did me no good. Now I know and won't bother with it.

 

[FredT]

When weighting housings for fresh vs salt, first get it neutral in fresh, then weigh the whole thing. Make a lead or solder pad that masses ~1/40th of the total housing and place it INSIDE the housing. Fresh water dive, no extra ballast. Salt water dive add the pad. Please note that the numbers are almost exactly what WWW gave earlier. Weight added outside the canister or housing also increases displacement, which complicates the buoyancy calculation.

FT

 

[crpntr133]

Sounds about right. Never heard the 1/40th of the total housing weight idea.
Fred, you specify to add the weight inside..why inside?