Using "fuller" tanks.

[Storker]

steel cylinders (12l and 15l) are by far the most prevalent here.

Just curious: Both in 200 bar rating, or do you use 232 bar 12s?

If you could get a 15l or 18l tank the same weight as a 12l steel then that might have mileage

But then you'd risk getting a tank that was positively buoyant when empty, especially for the 18s. Didn't you say that you don't want that?




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[rx7diver]

Just ran through some very quick calculations. Assumptions:

1. Cylinder is 25" tall, including a neck that's 1". (So, without the neck, the cylinder is 24" tall.)
2. Cylinder has an OD of 6"
3. Bottom of cylinder is a semi-sphere that has a radius of 3"
4. Top of cylinder is a semi-sphere that has a radius of 3"
5. Side, bottom, and top of cylinder are infinitely thin.

Then:

1. Cylinder will displace ~0.36 cu ft of water, and
2. this amount of water weighs about 23 lbs, and
3. this "ideal" tank has a capacity of ~80 cu ft at a service pressure of ~3,300 psig.

Shrink the inside of the cylinder (so the top, bottom, and walls have some thickness--like a *real* cylinder), and the service pressure will necessarily increase (if the capacity is to remain at ~80 cu ft), but should remain well below 4,500 psig (300 Br). Add a couple of lbs for the valve.

Looks like the cylinder (without valve) will need to weigh ~21lbs. So, my preferred cylinder (25" tall, 6" OD, less than 25# weight with valve, neutral in water when empty, able to be filled from today's 4,500 psig cascade banks) doesn't seem too far-fetched after all, I would think!

NOTE 1: These calculations are conservative. For example, I used the density (0.9980 g/mL) of *pure* water rather than of sea water. So my preferred cylinder can actually weigh a bit more than 21 lbs (but not more than 23 lbs to meet my design criterion).

NOTE 2: If I assume insanely thick 1/2-inch top, bottom, and walls, then the working pressure of this cylinder would need to be 4,850 psig. Of course, the top, bottom, and walls would never be this thick. Probably.

Safe Diving,

rx7diver

P.S. I'll check these quick calculations after I return home.

 

[Diving Dubai]

Just curious: Both in 200 bar rating, or do you use 232 bar 12s?

Steels get filled to 232-240 bar, Those using Ali normally get 210 at LDS (other commercial ops go to 200) Of course when your tank has been sitting on the dive deck in the summer sun (day time highs 45-50c night time lows 34-37cfor 24 hours your spg normally reads nearer 250

---------- Post added June 7th, 2015 at 10:07 AM ----------

But then you'd risk getting a tank that was positively buoyant when empty, especially for the 18s. Didn't you say that you don't want that?

I meant if these cylinders had same weight and buoyancy of a 12l steel. My bad - typing late at night.

 

[Patoux01]

Those using Ali normally get 210 at LDS

There are aluminium tanks at 240 bar, it's worth checking if you don't want an angry customer . (since you're supposed to check the tank before filling anyway)

 

[Diving Dubai]

Just ran through some very quick calculations. Assumptions:

1. Cylinder is 25" tall, including a neck that's 1". (So, without the neck, the cylinder is 24" tall.)
2. Cylinder has an OD of 6"
3. Bottom of cylinder is a semi-sphere that has a radius of 3"
4. Top of cylinder is a semi-sphere that has a radius of 3"
5. Side, bottom, and top of cylinder are infinitely thin.

Then:

1. Cylinder will displace ~0.36 cu ft of water, and
2. this amount of water weighs about 23 lbs, and
3. this "ideal" tank has a capacity of ~80 cu ft at a service pressure of ~3,300 psig.

Shrink the inside of the cylinder (so the top, bottom, and walls have some thickness--like a *real* cylinder), and the service pressure will necessarily increase (if the capacity is to remain at ~80 cu ft), but should remain well below 4,500 psig (300 Br). Add a couple of lbs for the valve.

Looks like the cylinder (without valve) will need to weigh ~21lbs. So, my preferred cylinder (25" tall, 6" OD, less than 25# weight with valve, neutral in water when empty, able to be filled from today's 4,500 psig cascade banks) doesn't seem too far-fetched after all, I would think!

NOTE 1: These calculations are conservative. For example, I used the density (0.9980 g/mL) of *pure* water rather than of sea water. So my preferred cylinder can actually weigh a bit more than 21 lbs (but not more than 23 lbs to meet my design criterion).

NOTE 2: If I assume insanely thick 1/2-inch top, bottom, and walls, then the working pressure of this cylinder would need to be 4,850 psig. Of course, the top, bottom, and walls would never be this thick. Probably.

Safe Diving,

rx7diver

P.S. I'll check these quick calculations after I return home.

Forgive me, and it could because my head is full of cold, I don't get your design. Are you saying you want an Ali tank of smaller size but because it's higher pressure then it holds 80cf? There is a 12l short (dumpy) which has the OD of a 15l (around 8"?) and is about the height you suggest although they weigh 13kg - 28lb

For interest here is a picture of an S80 which has been sectioned that I have lying about (don't ask)

View attachment 210036


The side walls are just over 1/2" thick the radius over 1"

I was mulling this over some more last night. You could press a thinner wall, although as you get thinner then wall thickness deviations become more likely. You could then insert an internal frame of longitudinal stiffeners bonded internally to the ID of the cylinder. Somehow form the top (thats 1 issue if you don't want a join.)

The issue here is, that because the cylinder has a thinner wall section its more likely to deform - the point load of a cam faster is quite high - this is the same reason carbon isn't resilient

A cylinder with dents all over it isn't good.

Aluminum cylinders aren't the greatest but they off set their bad weight to volume ratio and buoyancy characteristic with their reliance to saltwater and cheapness.

Again you could possibly make steel cylinders slightly thinner than they already are while maintaining the pressure, except the gains in weight loss would be offset by a higher price (harder to manufacture).

In order to justify a higher price (it should be accepted that a new design would cost more) there needs to be significant gains.

Once again - the mass market (commercial dive ops) are happy with S80 and they satisfy the needs of the vast majority of divers (perhaps because they don't know better) so a different cylinder would have small market appeal to some rec divers - potentially divers who use twinsets and who want more gas with lower weight

 

[Storker]

Steels get filled to 232-240 bar

Thanks for the info.

I meant if these cylinders had same weight and buoyancy of a 12l steel.

That'd be a mite difficult, no?

A 12L displaces some twelve-plus-change liters of water and would need to weigh a little more than 12kg to be neutral. (Reference point: My 200 bar 15L weighs some 14kg without the valve and is close to neutral when empty.) An 18L displaces some eighteen-plus-change liters of water and would need to weigh a little more than 18kg to be neutral. It's just not physically possible to have a 15L or 18L with the same weight and buoyancy of a 12L. They'd have to weigh more to have similar buoyancy characteristics.


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[Diving Dubai]

Thanks for the info.




That'd be a mite difficult, no?

A 12L displaces some twelve-plus-change liters of water and would need to weigh a little more than 12kg to be neutral. (Reference point: My 200 bar 15L weighs some 14kg without the valve and is close to neutral when empty.) An 18L displaces some eighteen-plus-change liters of water and would need to weigh a little more than 18kg to be neutral. It's just not physically possible to have a 15L or 18L with the same weight and buoyancy of a 12L. They'd have to weigh more to have similar buoyancy characteristics.

Perhaps I'm not being clear in my intentions.

Lets take 2 cylinders, an Al S80, and a Steel HP80 (these are the two I have got data for)

The dry weight of the S80 is quoted to be 31lbs or 14kg and it can be filled to 3000psi or 207 bar. It has a buoyancy swing from -2lbs (-0.9kg) full to +4lbs (+1.8kg) empty
The dry weight of the HP80 is quoted to be 28lbs or 12kg and it can be filled to 3500psi or 240 bar. It has a buoyancy swing from -9.lbs (-4kg) full to +3lbs (1.3kg) empty

So for a similar empty volume the steel weighs less, both have a similar buoyancy shift of 6lbs (2.7kg) but the steel stays negative and of course holds more gas. Both have the same internal volume , but the steel will have a smaller external diameter because the wall thickness is less but because of the extra pressure holds 10% more gas when filled

If we put a Steel HP120 into the mix which is around 15l? Then we get the following: dry weight of 39lbs or 17kg and it can be filled to 3500psi or 240 bar. It has a buoyancy swing from -10lbs (-4.5kg) full to +1.3lbs (0.5kg) empty

A 15l holds 40% more gas (if my calculations are correct HP 80 = 82cuft gas which is 2321l which if divided by 240 = 9.6l) but its dry weight is only 30% greater.

So what if we can make a cylinder out of "fantasium" it could in theory have the same dry weight as a 12l steel if it were stronger and have similar buoyancy characteristics i.e. remains negative at the end of the dive if it had the correct density.

Of course I could be talking gibberish as I am on cold and flu drugs at the moment - but in my head it makes sense

 

[danvolker]

Perhaps I'm not being clear in my intentions.

Lets take 2 cylinders, an Al S80, and a Steel HP80 (these are the two I have got data for)

The dry weight of the S80 is quoted to be 31lbs or 14kg and it can be filled to 3000psi or 207 bar. It has a buoyancy swing from -2lbs (-0.9kg) full to +4lbs (+1.8kg) empty
The dry weight of the HP80 is quoted to be 28lbs or 12kg and it can be filled to 3500psi or 240 bar. It has a buoyancy swing from -9.lbs (-4kg) full to +3lbs (1.3kg) empty

So for a similar empty volume the steel weighs less, both have a similar buoyancy shift of 6lbs (2.7kg) but the steel stays negative and of course holds more gas. Both have the same internal volume , but the steel will have a smaller external diameter because the wall thickness is less but because of the extra pressure holds 10% more gas when filled

If we put a Steel HP120 into the mix which is around 15l? Then we get the following: dry weight of 39lbs or 17kg and it can be filled to 3500psi or 240 bar. It has a buoyancy swing from -10lbs (-4.5kg) full to +1.3lbs (0.5kg) empty

A 15l holds 40% more gas (if my calculations are correct HP 80 = 82cuft gas which is 2321l which if divided by 240 = 9.6l) but its dry weight is only 30% greater.

So what if we can make a cylinder out of "fantasium" it could in theory have the same dry weight as a 12l steel if it were stronger and have similar buoyancy characteristics i.e. remains negative at the end of the dive if it had the correct density.

Of course I could be talking gibberish as I am on cold and flu drugs at the moment - but in my head it makes sense

If it was "fantasium", you would expect it...want it, to be either dead neutral at the end of the dive, or slightly positive.
Purposely diving tanks which remain negative means far more reliance on elevator technology of the BC or wing--which is a convolution in thinking.
The ultimate gets us to lower drag when swimming, no need at all for a wing or BC because no buoyancy shift occurs and we are using exposure suits with slick ultra low drag surface material, and with a non-buoyant insulated fabric combined with electrically heated underwear layer ( such as Thermalution Heated Undersuit -70M (Shortsleeve) ) or even more evolved wetsuit undergarment.

None of us should really want a bc or wing--it just slows you down and adds huge bulk to the kit.
So the next order of business, is finding a method of compensating for changes in tank buoyancy as it goes from full to near empty.
This could be chemical reaction --on the order of how a sperm whale changes buoyancy near the end if it's dive by CO2 changing the buoyancy of the wax like substance in it's large head. We could have a layer of "something"...unobtanium, that can line the exposure suit, and as the scuba tank loses pressure throughout the dive, the suit can get just slightly negative..... Best would be NO stinking BC!!!

 

[rx7diver]

Forgive me, and it could because my head is full of cold, I don't get your design. Are you saying you want an Ali tank of smaller size but because it's higher pressure then it holds 80cf?

Well, I would leave the choice of metal to the metallurgists and design engineers. But I was imagining a steel alloy (recall, significant weight is shaved, and strength gained, when the steel my PST HP 80 is made of, is used rather than the venerable 3AA chromoly? steel) or a titanium alloy. My idea is that the tank walls would be much thinner (not thicker); walls of steel scuba cylinders are typically thinner than walls of aluminum cylinders, I understand, AOTBE. I worked my example in my "Note 2" with 1/2" sides only as an extreme example. The tank I imagine would have much thinner walls.

I just dug out my 1987 Scubapro catalog. In that catalog, Scubapro offered their "71.4 cubic foot Slim Tank" (presumably manufactured by Faber). Its specifications are very close to the specs I listed above. Its specs:

1. Volume: 71.4 cu ft at 3000+10% psig
2. Length: 25.39 +/- 0.20 inches
3. OD: 6.00 +/- 0.05 inches
4. Weight: 26.0#
5. Material: 4130, heat treated (DOT 3AA approved)

So, imho, this Scubapro cylinder is *almost* the "preferred" cylinder I spec-ed. Problem is, it is too heavy, relatively. Because it weighs 26.0# (rather than the 21# that I spec-ed), its buoyancy is -2.3# empty (rather than the neutral empty buoyancy that I spec-ed).

Maybe using another, newer steel alloy (or a titanium alloy), rather than 3AA, would make this old Scubapro tank closer to the preferred cylinder I spec-ed, above.

This has been fun!

Safe Diving,

rx7diver

 

[Diving Dubai]

you would expect it...want it, to be either dead neutral at the end of the dive, or slightly positive.
Purposely diving tanks which remain negative means far more reliance on elevator technology of the BC or wing--which is a convolution in thinking.

Our opinions differ on that, I prefer to be ever so slightly negative at 50 bar. Although I rarely get that low. In anycase the buoyancy can be generally controlled by breathing and or just a touch of gas in the BC/wing.

I did think about the possibilities of changing tank buoyancy - but that would require a larger tank and something else to go wrong. And in reality what benefit will it achieve?

I suppose you could start streamlining gear - give surfaces the plate technology replicated from sharkskin and used in the banned sporting swim suits - but given generally in scuba we try not to move too fast - what gain would streamlining achieve in the real world?

All kind of a sideline to the original post of how to get more gas for similar sized tanks?