Weighting relative to backplate material

[Michael Guerrero]

I haven't seen this discussed before and started thinking about this as I'm getting ready to go dive somewhere warm next week. I got a pair of Dive Rite Stainless Steel XT Lite backplates for our upcoming trip for my wife and me. Our last time diving in warm water we either needed no weight, or in my wife's case, was overweighted with our usual steel backplates. Old backplates just under 6 lbs, lite backplates 2.5 lbs, same material (316 stainless). The lite backplate is thinner and has lots of cutouts. The old Dive Rite aluminum backplates weighed essentially the same as the new stainless ones, but are thicker and don't have all the cutouts and obviously are made of a less dense metal

On dry land, 2.5 lbs of lead, 2.5 lbs of stainless steel and 2.5 lbs of aluminum weigh the same amount. In the water, it's all about displacement, and an aluminum backplate will displace slightly more water than a steel backplate of the same weight. I'm guessing it's pretty insignificant. My question - if I take 3.5 lbs off my steel backplate (going from a 6 lb steel backplate to a 2.5 lb steel backplate), will 3.5 lbs more lead keep me at the same buoyancy? Or do I need a little more lead? And would need even a little more lead if I had gone with an aluminum backplate weighing the same as the lightweight steel backplate?

WTF are you talking about? Diving isn't supposed to be this hard. Get the engineer out of you and just go enjoy a vacation. If you need to add a pound here or there, big f'ing deal.

by the way, I'm totally jealous you get to go on a dive vacation and I don't!

Mike

 

[Bob DBF]

I always make my best educated guess before splashing, and then at 500# and 15' I hand off weight until I can hold the stop with an empty BC. I note the weight and rig in my notebook for later reference. I usually dive with 2 to 3# more weight should I want/have to hold a 10' stop and empty the tank. Nothing beats actual conditions for accuracy.




Bob
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"This is scuba board, where problems are imagined or overstated......and chests get thumped about what some would do about those "problems" "- PullMyFinger

 

[geoff w]

I always make my best educated guess before splashing, and then at 500# and 15' I hand off weight until I can hold the stop with an empty BC. I note the weight and rig in my notebook for later reference. I usually dive with 2 to 3# more weight should I want/have to hold a 10' stop and empty the tank. Nothing beats actual conditions for accuracy.

My best educated guess right now is 4 lbs, based on my last warm water diving with the same 3mm wetsuit, hood and globes, but with Al63s instead of the Al80s we'll be diving in Palau and a lighter steel backplate now. I can always add another pound between dives if I feel really light. We've never dived in current with reef hooks before, so it may be moot for some dives if they tell us to dive lighter or heavier.

At the end of the day, plus or minus 3-5 lbs is moot, because you can control that much buoyancy just with breathing. As I've progressed from wanting plenty of weight to stay down to the minimum needed to remain buoyant my diving enjoyment has gone up. I've gotten spoiled this year with only 2 lbs for So Cal diving since the water has been a little warmer than usual and I can dive with slightly lighter insulation

 

[eelnoraa]

Water=1.0
Salt water=1.026
Aluminum=depends but somewhere between 2.6 and 2.8
316 Stainless= 7.9
Lead=11.35

I quoted tbone here for the density number. The math is very simple: "weight in water' = metal dry weight * (metal density - water density)/metal density. Take 1 m^3 of solid (no hallow) AL for example, it is 2.7 tons. it disaplaces 1m^3 of salt water, which is 1.026 ton. So in salt water, this 1m^3 AL actually weight 2.7-1.026 = 1.574 tons.

So:
2lb AL plate in water weight 2*(2.7-1.026)/2.7 = 1.24lb
5lb SS plate in water weight 5*(7.9-1.026)/7.9 = 4.35lb
5lb lead in water weight 5*(11.3 - 1.026)/11.3 = 4.54lb

 

[tbone1004]

thanks for fixing that, I new the SS and Lead numbers looked too far off....

 

[sea_ledford]

But then you'll get there and the boat will be out of 2,3 and 4 lb weights so they'll just give you two 5s.

 

[dumpsterDiver]

I quoted tbone here for the density number. The math is very simple: "weight in water' = metal dry weight * (metal density - water density)/metal density. Take 1 m^3 of solid (no hallow) AL for example, it is 2.7 tons. it disaplaces 1m^3 of salt water, which is 1.026 ton. So in salt water, this 1m^3 AL actually weight 2.7-1.026 = 1.574 tons.

So:
2lb AL plate in water weight 2*(2.7-1.026)/2.7 = 1.24lb
5lb SS plate in water weight 5*(7.9-1.026)/7.9 = 4.35lb
5lb lead in water weight 5*(11.3 - 1.026)/11.3 = 4.54lb

2.7-1.026 = 1.574 .... Nope...

 

[eelnoraa]

sorry, typo, it is 1.674

 

[RJP]

I quoted tbone here for the density number. The math is very simple: "weight in water' = metal dry weight * (metal density - water density)/metal density. Take 1 m^3 of solid (no hallow) AL for example, it is 2.7 tons. it disaplaces 1m^3 of salt water, which is 1.026 ton. So in salt water, this 1m^3 AL actually weight 2.7-1.026 = 1.574 tons.

So:
2lb AL plate in water weight 2*(2.7-1.026)/2.7 = 1.24lb
5lb SS plate in water weight 5*(7.9-1.026)/7.9 = 4.35lb
5lb lead in water weight 5*(11.3 - 1.026)/11.3 = 4.54lb

sorry, typo, it is 1.674

As my grandfather used to say "Don't measure with a micrometer... if you're cutting with an axe."

:d

 

[geoff w]

Before I started this thread, I didn't give much thought to the material, other than aluminum will oxidize if the anodizing is scratched. Plus the Dive Rite lite stainless backplate does look cool and weighs about the same as an aluminum one. Now I have more reason to like the light stainless plate over aluminum - less lead needed!

I'm going to start empirical field studies to determine proper weighting with the lighter plate all next week in Palau and will report back on my findings.

Several dive trips ago, the dive operator didn't have anything smaller than 3 lb weights. So now I just throw in 8 lbs (4x1 lb, 2x2 lb) of soft weights in a carry on and don't worry about what they have.