Halcyon Carbon Plate

[CuriousRambler]

All else being equal, an aluminum plate and a carbon plate (and a tungsten plate, and a stainless plate, and a uranium plate) will displace identical amounts of water. Displacement is purely based on volume of the object being submerged - mass and/or density have no impact on it. (Edited to add: this is assuming you're fully submerging a solid object. Once you have empty voids encapsulated in material, or if you're only partially submerged, the calculations start to change) So, assuming the plates are of identical design (thickness, profile, etc) displacement is the same. If displacement is identical, the buoyant force is technically identical (counter-intuitive, I know...hear me out) so you have to account for the mass of the plate. If the plate's mass is greater than the force of the buoyancy, it sinks (i.e. it "adds weight" to your kit), if the plate's mass is less than the force of buoyancy, it floats. To be neutrally buoyant, your total mass must equal your total buoyant force.

Buoyancy can be calculated by Force of Buoyancy= Submerged Volume x Fluid Density x Force of Gravity

Let's assume submerged volume = 1m^3 for all plates, since we're assuming they're identical design - this may vary in the real world, and obviously our plates are much much smaller than a cubic meter..just simplifying math for comparison's sake.

Density of sea water is roughly 1,025kg/m^3 - again, this number doesn't actually matter since you're diving different plates in the same water, we could use any number here.

Gravity is 9.81m/s^2, which is equal to 9.81newtons/kg, so:

Force of buoyancy for a given plate is going to be identical, at 1m^3*1025kg/m^3*9.81N/kg ==> 10,055.25 Newtons (again, we've got a BIG plate here)

Now we look at the density of the material, internet search shows carbon fiber ~1.55 to 1.70 g/cm^3, let's call that 1.6, or 1,600 kg/m^3. Aluminum is about 2700 kg/m^3. Back to our easy math, we've assumed 1m^3 plates:

CF "plate" is 1600kg (mass) * 9.81N/kg (gravity) = 15,696N (weight)
Al "plate" is 2700kg (mass) * 9.81N/kg (gravity) = 26,487N (weight)

Compare to buoyant force of 10,055.25N:

10,055.25 - 15,696 = 5,640.75N negative for CF
10,055.25 - 26,487 = 16,431.75N negative for Al

So the short answer is "carbon fiber is roughly 66% less negatively buoyant than aluminum." So, if you've found you need 1lb of lead to sink your aluminum plate, you'll probably need a bit less than another pound of lead for a carbon fiber plate.

All this is assuming my internet-provided density for CF is remotely correct; I'm not a materials guy and don't know how far off that number may be in reality. Also reasonably likely I botched my math somewhere..

 

[tbone1004]

I think the consensus is clear, and I'm in agreement:
You'll need an extra 1lb of lead with the carbon plate.

One thing to warn you of: Carbon Fiber and Salt Water are VERY bad together. @tbone1004 can explain it better than I can, as he works in composite fibers professionally, including as a design engineer.....but if the epoxy/resin fails AT ALL, the salt crystals can penetrate and damage the fiber integerity. I'd be cautious. If weight is of the utmost concern and you plan on diving salt water: there are the nylon ultralight backplates (Oxycheq used to make one, among others).

The math from @CuriousRambler holds with "general rules" of density where CF composites are roughly 50% of aluminum for density, which is roughly 30% of most steels. Give or take a few points here or there.
Now, one big negative to aluminum plates, particularly with doubles, is that the hardness of the aluminum is much less than that of the SS used in the all-thread and it will eventually round out the bolt holes. Not a huge issue, and this is over thousands of dives, but it is a thing.
Composites don't really use a "hardness" measurement because it's not really applicable, but either way it will wear against the allthread as well, so keep that in mind.

That wear point brings up where Victor was going. The matrix created by the fiber and the resin is very different than the matrix created in metallic alloys. The fibers themselves become encapsulated in the resin and theoretically the edges are all fully encapsulated. Provided that barrier remains uncompromised, the matrix is fully water proof. The issue because when the matrix is compromised. This occurs any time a hole is drilled, any time a crack forms, rubbing occurs that goes through the layer of resin, etc. Once that happens, water is going to naturally channel along the fibers, particularly to any air bubbles that were not taken out during the vacuum forming process. The water will eventually come out, if it's dry for long enough, but any particulate that entered with the water, salt molecules, minerals from caves, etc. will remain. If there is any motion in the fibers, which will happen as the plate naturally flexes during use on land, those salt crystals will start abrading the fibers and resin matrix from the inside out.

Does it matter? Not really on a backplate, but it really matters on something like a carbon fiber tank. You won't see any premature failures from that on a plate, but it's really not the best material to be using other than the sex-appeal or unless you REALLY need to take that 1lb off of your rig...

 

[saxman242]

I would be curious to see a properly designed cf backplate though. Something that actually oriented the fibers properly to pick up the loads, removed the material that wasn't doing anything, and properly reinforced the bolt hole locations(TiGr perhaps). Maybe one of these days I'll get bored and make one.

A 2x2 twill plate laid up in a mold made from an Al/Steel backplate isn't the way to do this "right". It is the way to make something expensive and pretty looking though.

 

[runsongas]

at that much effort to optimize the plate, a titanium ultralight pattern plate might be easier

 

[saxman242]

at that much effort to optimize the plate, a titanium ultralight pattern plate might be easier

True. Ti plate is a lot harder to machine and bend though. An out of autoclave carbon part could be done much more easily in a home shop environment.

Either is entirely too much effort when you can get light weight aluminum plates for so cheap, unless you're doing it just to do it.

 

[LanceRiley]

i was just asking on the carbon plate...

i dive with a 2 lb plate and a 1 lb sta. no weights. tropical.

I found out the Halcyon carbon plate is 1.1 lb with a 2 lb STA.... i thought this was the same thing hahhaah..

titanium on the other hand.. is expensive =( unless... somebody made titanium plates aside from halcyon?

 

[saxman242]

omebody made titanium plates aside from halcyon?

There's some chinese supplier that sells Ti plates for like $150 US an aliexpress. Who knows anything about them though.

Much like the halycon CF plates though, they aren't optimized for the material. They're just the typical Al/steel plate design cut out of a different material. A lighter plate could be made from either if someone took the time to make a plate. Hell, someone could probably make a much lighter aluminum plate too if they wanted to take the time to do the design work. Much of the current al plate design is just along for the ride and not contributing anything to strength

 

[tbone1004]

I would be curious to see a properly designed cf backplate though. Something that actually oriented the fibers properly to pick up the loads, removed the material that wasn't doing anything, and properly reinforced the bolt hole locations(TiGr perhaps). Maybe one of these days I'll get bored and make one.

A 2x2 twill plate laid up in a mold made from an Al/Steel backplate isn't the way to do this "right". It is the way to make something expensive and pretty looking though.

CF is never going to be an appropriate material for this application though. CF is engineered into products when they need to be some combination of incredibly stiff, thin, and/or light weight. None of that applies to literally anything we do in scuba diving. The difference between composite and machined aluminum for a backplate is a pound, that's it. Does a single pound EVER matter in scuba diving? The only time it could possibly ever matter is for flight, and even then, you'd get a better bang for your buck using an AL plate, and using AL d-rings instead of stainless steel. The cost differential to use even cheap composites are just not there for anything except the sex appeal. The Bonex scooters are the literal worst for it, and Scubaforce apparently actually had heat issues with normal canisters so they went to CF partially for sex appeal, but also to help shed scrubber heat. Fine. A backplate though? The decision for carbon fiber is 100% based on sales and marketing and 0% engineering.

For anyone looking at how insignificant backplate strength is, remember that DSS made backplates out of kydex with riveted SS inserts for the spine to prevent the bolts from wearing through and Dive Rite made ABS plates with no issues either.

 

[wetb4igetinthewater]

i was just asking on the carbon plate...

i dive with a 2 lb plate and a 1 lb sta. no weights. tropical.

I found out the Halcyon carbon plate is 1.1 lb with a 2 lb STA.... i thought this was the same thing hahhaah..

titanium on the other hand.. is expensive =( unless... somebody made titanium plates aside from halcyon?

Helium Dive Gear makes a really nice titanium backplate (I own one) Helium Dive - Оборудование для дайвинга из Титана

 

[saxman242]

A backplate though? The decision for carbon fiber is 100% based on sales and marketing and 0% engineering.

Oh, I don't think it's the right tool for the job, however, one could, in an exercise of overdoing it, design a backplate that took advantage of the material properties to the best of their ability. It'd be nice and light and a complete waste of time and effort. But at least you could milk it for everything it is worth.