Why are we stuck at 3400PSI?

[scoobert]

I get that you can't dive on light tanks do to suddenly having a lift bag at the end of your dive.

But why can't we wrap steel tanks with carbon fiber and go to 5000PSI or 6,000PSI?

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I know cave divers go to 4000PSI already, without issues as far as I have seen.

 

[Subcooled]

Compressibility.
Pressure increases faster than the amount of gas.

4350 psi (300 bar) rated steel cylinders do exist and they are widely used in Europe, but #1 the pressure is 300 and #2 the contents are only 270 times normal pressure. 10% loss. When pressure increases, this relation becomes even worse.

I know that some composite cylinders can contain even higher pressures, for ever diminishing gains.

 

[scoobert]

Compressibility.
Pressure increases faster than the amount of gas.

4350 psi (300 bar) rated steel cylinders do exist and they are widely used in Europe, but #1 the pressure is 300 and #2 the contents are only 270 times normal pressure. 10% loss. When pressure increases, this relation becomes even worse.

I know that some composite cylinders can contain even higher pressures, for ever diminishing gains.

Is it linear?

 

[Cthippo]

As far as I can tell it's because divers aren't that fond of lightweight composite cylinders. 4500 and 5500 PSI cylinders are available and widely used in the fire service (I spent all day yesterday in one), but those are going to be very buoyant probably even when filled.

 

[Subcooled]

Is it linear?

No, it is very much not linear.
If I recall it correctly, it goes roughly like this (measured in atmospheres absolutes):
300 = 270 (-10%)
270 = 250 (-7.4%)
250 = 240 (-4%)
240 = 235 (-2%)

You know that squeezing becomes the harder the more you do it.

ps. these rough numbers need to be checked if you do more critical dive planning. These only act as rough guides.

 

[Scared Silly]

As far as I can tell it's because divers aren't that fond of lightweight composite cylinders. 4500 and 5500 PSI cylinders are available and widely used in the fire service (I spent all day yesterday in one), but those are going to be very buoyant probably even when filled.

Buoyant but also the issue of water intrusion, especially salt water. There is also a longevity issue.

 

[Cthippo]

Buoyant but also the issue of water intrusion, especially salt water. There is also a longevity issue.

The longevity issue may be solved. DOT has approved a company to recommission composite wrapped cylinders and recertify them for another 15 years. Yes, 30 years is less than the practically infinite lifespan of steel and AL, but it does change they calculus of the situation some.

 

[Akimbo]

Smaller tanks sound great but have proven not to be worth higher costs to the vast majority of divers. Lighter tanks are of little value if you have pay a lot more and have to add lead to reach neutral buoyancy.

The capacity of single tanks tends to hover around NDL limits. The relatively few divers interested in higher levels of reserve gas and making decompression dives just pack more inexpensive cylinders. That further kills the justification for manufacturers to tool up for smaller and lighter cylinders.

SCBA (firefighting mostly) cylinders are the exception to this market constraint. Unfortunately adding pressurized salt water to the equation makes their solution less attractive to divers. Marine grade Titanium alloys would be the obvious choice if material and manufacturing costs were not so prohibitive.

Many of us have daydreamed about super-high pressure gas cylinders but the physics of buoyancy change from the weight of gas is still a problem. That is one of the many reasons that rebreathers become a more practical option for deep and/or long duration dives.

 

[evandroairton]

Even rebreather divers with composite smal tanks have issues with covers tô not "unwrap" the fibers due to contact with ceiling, rocks, decks, etc.
Thr price tag is just one factor even in small volumes

 

[Scuba Scott]

1. SCUBA is a small market that is completely saturated with lower pressure tanks.
2. Diminishing returns on higher air pressure but higher cost to compress and more expensive compressors required
3. Where is the market to build tanks that most shops can't fill to capacity? Who wants to set up a compressor for higher pressure when there are no tanks available to be filled at that pressure?
4. These tanks are going to be expensive. Are you willing to pay extra for a tank you probably can't get filled to capacity?
5. The cost of everything goes up. Entire cascade systems become obsolete.
6. No more yoke regulators. Higher pressures would require DIN.
7. The switch from LP 2442 to HP 3000 was not nearly as dramatic and much of the same equipment was able to be used.
8. Selling air is barely profitable at current pressures.