Check my thinking on steel vs. alum buoyancy (not just weight)

[schuylerclark]

I'd appreciate it if someone more experienced than myself could check my thinking and make sure I have this right. In comparing a ~2 lb aluminum backplate to a ~2 lb steel backplate (the Dive Rite "lite" model), it seems to me that the steel would still be more negatively buoyant than the aluminum. Assuming they're the same weight, and since steel is typically ~3x denser than aluminum, it must also have less volume (hence the cutouts in the plate), thereby displacing less water and being more negatively buoyant, right?

If I had both plates on hand, I'm sure I could measure the volumes and calculate the difference but since I don't and I'm confined to a thought experiment, am I thinking of this correctly? If I am, then my follow up question would be is this buoyancy difference significant enough to reduce the additional weight needed for diving or is the real world effect negligible?

 

[lowwall]

Yes.
Yes.
Significant for same size objects, but only about a pound [edit: half pound, see below] for your example of a standard size AL plate versus a skeletonized or dogbone SS plate.

 

[inquis]

Based on density, the buoyancy of stainless steel / aluminum is 87% / 63% of the dry weight (mass). For these two plates, each having a 2 lb mass, the steel buoyancy is about 0.5 lb more negative.

 

[DivingLane]

Dang, science hurts my head. I would have told you 2lbs is 2lbs, you're crazy, but in the back of my head, I knew density and volume had something to say.

 

[lowwall]

@inquis nailed it.

Density of aluminum: 2.7 g/cc (cubic centimeter)
Density of 304 stainless steel: 7.9 g/cc

Let's make it slightly easier on ourselves and assume both plates are 1 kilogram or 1000 grams (2.2 pounds) and we are talking about freshwater which has a density of 1g/cc.

So we have:
1000/2.7 = 370cc of aluminum
1000-370 = 630 grams negative buoyancy for AL plate

1000/7.9 = 127cc of steel
1009-127 = 863 grams negative buoyancy for SS plate

863 - 630 = 233 grams difference or .514 pounds

 

[Big Dan K]

Works out to about 9 ounces of extra lead, if you account for the buoyancy of lead.

 

[schuylerclark]

Thanks y'all for the confirmation and I appreciate the explanations - physics isn't my strong suit so I'm glad for the help.

 

[Big Dan K]

They are more expensive, but you might check out the Helium Dive titanium backplates also. I am a physics guy with OCD and spent way too much time evaluating BP weights, both in the suitcase and in the water, plus estimating the strength of the plates. The Helium Dive plates seemed to be the sweet spot. Love the ability to mount trim weights on the plate, rather than the cam bands too.

 

[sea_ledford]

Thought experiments are all fine, but this reminds me of "measure with a micrometer, mark with chalk, cut with an axe."

Any difference is going to be hidden and overcome by the volume and weight of boltsnaps and snap shackles. Why don't we ever have these discussions on the 2-3" of overlapping neoprene at the end of wetsuit legs where it covers the boots? Probably has more of an effect.

Don't lose the forest for the trees.

 

[inquis]

Any difference is going to be hidden and overcome by the volume and weight of boltsnaps and snap shackles.

Not hidden at all when those snaps & shackles are present in both cases. In this particular case, the difference isn't a big deal, but the OP didn't know that to start with -- thus the question. Other changes (AL to steel tanks for instance) can be significant. Not everyone has the luxury of jumping in the water to see the difference, especially when contemplating a new equipment purchase.