tank size and pressure

[2airishuman]

I do not believe it is wise to advocate filling cylinders beyond the customary pressures for which they are intended. While this has been a common practice among cave divers, they are in a situation where they are balancing the risk of a cylinder rupture against other risks. They also are considering the ability to conduct dives that are not practicable without the extra air afforded by an overfill.

I'm trying to do some research on tanks, if I'm ever in the place to buy some. I'm interested in high pressure steel 133s, the weight of large tanks doesn't bother me. The information I've found on one website for Faber tanks says "these are the same as Faber low pressure 108, just in high pressure". Does anyone know if that means that for a 133 tank it's just plus rated for now, and eventually will be de-rated to low pressure (i.e. 108)? Thanks for any help

There are three facts to consider:

1. Current production Faber HP steels are special permit cylinders that are not eligible to be "plus rated." The ones sold in the USA and in other jurisdictions that copy the USA regulatory scheme are specified for 3442 PSI. At requalification (hydro) they will either pass, or not. If they pass they can continue to be filled to 3442 PSI.
   
   
2. 3AA cylinders with a "plus rating" (i.e. LP steels) do not "lose" the plus rating over time. It is vanishingly rare for a cylinder to pass hydro but fail the portion of the hydro that qualifies the cylinder for a "plus." Some hydro facilities do not "plus" rate cylinders because they are not trained to do so, and for some older cylinders the data from the manufacturer required to qualify for a "plus" is no longer available. Even if a cylinder has been, at some point, passed hydro without a "plus" for some reason, it can still get a "plus" at the next hydro as long as the facility performing the hydro has the necessary training and data to do so. Modern LP steels (last 20 years or so) have the necessary REE data stamped on the cylinder, so the problem with hydro shops not having the data should become less of a problem over time.
   
   
3. LP steel capacities are quoted based on the "plus" rating. Therefore, an LP108, rated 3AA2400, has approximately 108 cubic feet capacity at 2640 PSI (110% of 2400).

 

[RyanT]

108 / 2640 * 3442 = ???

It's 140. I think @tbone1004 said "roughly"

 

[tbone1004]

A 108 at 3442 is NOT 133CU'

Here's the math
108 / 2640 * 3442 = ???
I'm surprised TBone is wrong on something so commonly known.

Except I'm not. You forgot to put in the compressibility factor for gas at that pressure. We assume gas behaves linearly up until about 2000psi, but after that it starts getting weird. At 3442 psi, it has a compressibility factor of about 1.06. Your math says that 108 should hold 141cf, but in reality when pumped up to 3442 it actually holds around 133cf. Convenient in cave diving if you ignore that compressibility factor when gas planning because it adds an extra buffer in there, great if you ignore it for a fill station because you can charge for the extra "gas" that you pumped, but when comparing tanks it is not correct.
In this instance, both of those tanks have identical water volume at 17.0liters so if either a LP108 or a FX133 is filled to exactly the same pressure at exactly the same temperature, they will hold exactly the same volume of gas.

@bigolred tanks are required to pass many thousands of cycles to hydro pressure without failure of hydro *note not failure of the tank, it still has to pass a hydro test after rapid cycling up and down*. IIRC it's about 10,000? Sure those LP tanks get hydro'd at every fill, but even if you assume they are filled 2x/day, 365days a year, that's still several decades. I don't know of any tank that's failed hydro because of cave filling. Rental cylinders from the 70's still pass hydro when they go in, just saying.

@2airishuman all hydro shops are trained to put the + stamps on. Very few are motivated enough to look up the REE values, or they may not be available anymore like some of the PST tanks which is why the values are stamped on many of the newer tanks.

 

[Superlyte27]

EVERYTHING I've read about compressibility starts at 3600psi, 3442 in a 133 is not the same as 3442 in a 108.

 

[Superlyte27]

I don't believe they hold the same water volume. I one checked with LP85's vs hp100. They did not hold the same water. I haven't done this with 133'd and 108's. I would assume I'd get the same results as the 100's. Anyone have a chart with liquid volume of these two tanks?

 

[Superlyte27]

In the 90's when OMS was branding Faber (or vise versa) OMS Marketing literature read, "guaranteed 10,000 fills at 4000psi". I used to have the paperwork.

Of course the DOT threw a fit and OMS quit saying that.

At any rate, my tanks have been filled this way from the 90's and none of them have ever failed hydro, ever. And I have a LOT of tanks.

 

[tbone1004]

@Superlyte27
Per the manufacturer, they're both 17.0 liters. Since they quote to tenths of a liter and they're near identical external dimensions, it's a safe assumption that they're correct about the specs of their own products.
Blue Steel Scuba - Cylinder Specs

The LP85 is quoted as .77% bigger than the FX100 *13.0 vs 12.9 liters*, but it's close enough. Note that the new Faber FX100's are much bigger than PST or Worthingtons HP100's *the new Faber is about 1.5" taller than either the PST or Worthington and the same OD so the volume is going to be much larger.

Yes I know this is from wikipedia, but it's a chart taken out of an engineering book. 300K is room temp, and air has a compressibility factor of 1.03 at 200bar *note that 80/1.03 is 77.4*. 3% is quite a bit. 1.067% at 250bar/3600psi. Nitrox will be slightly different obviously, trimix more different, but air is what these are all quoted in anyway and the difference at least with nitrox is going to be less than the variance in our pressure gauges.
Compressibility factor - Wikipedia

 

[Superlyte27]

As far as I can tell, the math doesn't matter if they both have the same internal liquid volume.

 

[tbone1004]

As far as I can tell, the math doesn't matter if they both have the same internal liquid volume.

correct, and since they have the same liquid volume, they hold the same gas volume. You can choose to ignore compressibility and call a FX133 a 141 like we have been in cave country for decades upon decades *again, perfectly fine for gas planning in a cave if diving open circuit, but not for CCR bailout, and not when comparing the two tanks*. On OC it's fine because 141/3=47 so you assume you have 47cf, but your first third is only 44cf, and the second two thirds are closer to 49cf. On a CCR though, I think it's important to note for bailout because while 10cf of gas shouldn't matter, a lot of divers cut it close enough that it would.

 

[Storker]

We assume gas behaves linearly up until about 2000psi, but after that it starts getting weird. At 3442 psi, it has a compressibility factor of about 1.06.

Where did you get that number from? And which gas are you talking about? EDIT on your edit: Ah, OK. Wikipedia. And air.

If I use the van der Waals equation of state and the vdW constants listed by Wikipedia for O2, N2 and He, I get the following compressibility factors:

2640psi/182bar
EAN21: 0.97
EAN32: 0.96
TMX21/35: 1.03
TMX18/40: 1.05

3442psi/237bar
EAN21: 1.02
EAN32: 1.00
TMX21/35: 1.07
TMX18/40: 1.12