Weighting relative to backplate material

[geoff w]

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?

 

[tbone1004]

all about specific gravity as it relates to water.

Water=1.0
Salt water=1.026

Aluminum=depends but somewhere between 2.6 and 2.8

316 Stainless= 7.9

Lead=11.35

So since it is all about volume not weight, and stainless steel is less dense than lead, removing 2.5lbs of stainless steel is about 1.75lbs of lead.

Factoring aluminum into the mix, 2.5lbs of aluminum and about 0.85lbs of steel displace the same amount of water. So lots of maths involved, but wouldn't worry about it at all. You're not going to be perfect because no ones weighting is ever perfect, but the general assumption with the AL plate is that is not counted in weighting scenarios and SS and lead are treated about the same.

 

[CT Sean]

It doesn't matter because the AL and SS plate are pretty much the same size and therefore are displacing the same amount of water.

AL plates weighs 2.5 lbs and displaces, say 1 lb of water so is -1.5 lbs
SS plate weights 6 lbs and displaces the same 1 lb of water so it's -5 lbs
still a 3.5 lb difference between the two

SS vs lightweight SS is a little bit of a different concept in that the differece is the amount of water displaced but you get the same end result.

 

[decompression]

Boy do I wish "balanced rig" was taught to all divers.

Simply, add up all your neg items (plate, regs etc) and all the pos items (exposure suit, maybe tank etc). Then account for the weight of your gas. Ensure you have and equal amount of pos and neg. The wing will offset the weight of gas. 5lbs of air in tank so 5lbs of air in wing. Throughout the dive you will be neutral.

So, if you were neg with a SS and changed to a lighter AL plate you've removed 3.5 lbs of neg you'll be less neg. You'll still need to do a buoyancy check.

 

[tbone1004]

he was asking about the specific gravity differences of the material because while 2.5lbs of all 3 are equal on land, they are not equal in water.


Water=1.0
Salt water=1.026

Aluminum=depends but somewhere between 2.6 and 2.8

316 Stainless= 7.9

Lead=11.35

2.5lbs of lead weighs 11.35x of the equivalent volume of water.
2.5lbs of Aluminum weigh 2.7x
2.5lbs of 316 SS are 7.9x

So, assuming you have a 2.5lb aluminum backplate, it is only displacing about 1lb of fresh water for easy math. The same weight SS plate will displace 3lbs of water, and same weight of lead will displace 4.5lbs of water. Obviously the lead will be comparatively smaller volume wise.

So Mr. Decompression, if we are talking about balanced rigs, and this is most certainly not taught to us by any agency, we have to factor in how much weight is being displaced in water vs. on land. Like I said above, we tend to assume SS and Lead weigh the same in water, and it's obviously not true, but since we are dealing with a minimum of 5lb swing in the weight of the tank, talking about a 1-2lb difference in buoyancy is pretty negligible and certainly within the limits of having a balanced rig *which fwiw can only properly happen on a rebreather in a drysuit....*, any OC diving in a drysuit is still subject to being grossly overweighted for the majority of the dive, and anything in a wetsuit is just a joke to try to get "balanced". No tech diver I know has a properly balanced rig because the weight of twinsets or sidemount cylinders plus all of the misc stuff we have to carry always makes us overweighted. There is a difference in getting close enough to call it good, and nitpicking. Assuming SS and Lead weight the same in water that they do on land is one of those "it's close enough" moments since we are talking about the difference of a pound.

 

[TSandM]

You are overthinking this. The differences between displacement of aluminum and steel in the very small volume items you're considering is smaller than the variation in actual weight of scuba weights. You want your weighting accurate to within a couple of pounds, not a couple of ounces.

 

[geoff w]

Now my head IS spinning!

Boy do I wish "balanced rig" was taught to all divers.

Simply, add up all your neg items (plate, regs etc) and all the pos items (exposure suit, maybe tank etc). Then account for the weight of your gas. Ensure you have and equal amount of pos and neg. The wing will offset the weight of gas. 5lbs of air in tank so 5lbs of air in wing. Throughout the dive you will be neutral.

So, if you were neg with a SS and changed to a lighter AL plate you've removed 3.5 lbs of neg you'll be less neg. You'll still need to do a buoyancy check.

A 2.5 lb stainless backplate and a 2.5 lb aluminum backplate displace different volumes of water, and will have different weight under water, even if they weight the same dry. I think adding up all the positive and negative weighted items is next to impossible, except for rough ballpark approximations

he was asking about the specific gravity differences of the material because while 2.5lbs of all 3 are equal on land, they are not equal in water.

Water=1.0
Salt water=1.026

Aluminum=depends but somewhere between 2.6 and 2.8

316 Stainless= 7.9

Lead=11.35

2.5lbs of lead weighs 11.35x of the equivalent volume of water.
2.5lbs of Aluminum weigh 2.7x
2.5lbs of 316 SS are 7.9x

So, assuming you have a 2.5lb aluminum backplate, it is only displacing about 1lb of fresh water for easy math. The same weight SS plate will displace 3lbs of water, and same weight of lead will displace 4.5lbs of water. Obviously the lead will be comparatively smaller volume wise.

So Mr. Decompression, if we are talking about balanced rigs, and this is most certainly not taught to us by any agency, we have to factor in how much weight is being displaced in water vs. on land. Like I said above, we tend to assume SS and Lead weigh the same in water, and it's obviously not true, but since we are dealing with a minimum of 5lb swing in the weight of the tank, talking about a 1-2lb difference in buoyancy is pretty negligible and certainly within the limits of having a balanced rig *which fwiw can only properly happen on a rebreather in a drysuit....*, any OC diving in a drysuit is still subject to being grossly overweighted for the majority of the dive, and anything in a wetsuit is just a joke to try to get "balanced". No tech diver I know has a properly balanced rig because the weight of twinsets or sidemount cylinders plus all of the misc stuff we have to carry always makes us overweighted. There is a difference in getting close enough to call it good, and nitpicking. Assuming SS and Lead weight the same in water that they do on land is one of those "it's close enough" moments since we are talking about the difference of a pound.

I have a pretty good idea where I'm going to be at weight wise in Palau - lose 3.5 lbs of steel in the backplate, simple webbing harness instead of Dive Rite Transplate, and diving Al80s in Palau instead of the little Al63s we dived in the Bahamas a few years ago (1.7 lbs more buoyant when empty). I'll need 4 to 5 lbs of lead more than before. In any case, I'll be close to start with and can try taking weight off after a day or so (unless we get lots of surge)

---------- Post added November 4th, 2014 at 11:59 AM ----------

You are overthinking this. The differences between displacement of aluminum and steel in the very small volume items you're considering is smaller than the variation in actual weight of scuba weights. You want your weighting accurate to within a couple of pounds, not a couple of ounces.

But it's so hard to think of anything except a dive vacation now as I count down the days!

I realize the variation is small, just surprised that it hasn't been discussed before, considering some of the other mind numbing topics here at times. The bottom line for me is taking off 3.5 lbs of steel is more than offset by 3.5 lbs of (additional) lead. Dive operators rarely ever have 1 lb weights, so this helped me decide to bring some 1 lbs soft weights for my wife and me

 

[tbone1004]

I guess the only way to do it would be in a pool with a luggage scale, but again the volumes of water displaced are so small it isn't worth worrying about, like Lynne said, it's about pounds, not ounces.

you have it right, extra 2lbs to factor AL80 from AL63, 1.8 but close enough.
Lose 3.5lbs from the plate, probably 0.5lbs from the shoulder pads of the transplate, so add 3lbs there, throw an extra 4lbs on and you'll be close enough, put two threes on if you have trouble on the first dive, or just go with two 3's knowing you're going to be a pound or two overweighted. It's close enough, and a far cry from the "balanced rig" where it is going against most recreational instructors overweighting their students by 5-10 or more pounds from the get go.

 

[geoff w]

Now that I have that figured out, I just need to figure out how much saltier the water will be, since the air and water are warmer than So Cal, more surface evaporation, hence I'll be more buoyant!

 

[CT Sean]

Not sure if you're serious, but if so, you are talking about at most a couple tenths of a percent difference in total weight (you and all your gear plus lead).

If you go by this Wikipedia image, you are going from 33 parts per thousand to somewhere between 34 and 35. That's 1/4 to 1/2 lb for a 250 lb diver (diver plus gear). It's not enough to think about.