Diminishing Returns and Gas Calcs for Cave Filled LP Tanks?

[UCFKnightDiver]

Hey all,

I am interested in understanding how to better calculate the volume of gas in my cave filled LP108s. This seems particularly important when diving with a team with HP tanks. The simple calculation to do this is 108/2400*3600*2 which gives you 324cuft of gas. However, as I understand it, in the metric system, both HP130s and LP108s would be considered a 16L (or maybe it was 17L) tank and is measured by the amount of liquid the tank would hold rather than the volume of gas at a specified pressure. If the internal volume of an HP130 and LP108 is truly the same, then this would then equate to about 260-270ish cuft with the LP108s filled to 3600psi, which is a good bit less than 324cuft in the simple method above. What am I missing here, is there some reduction or additional factor I need to apply to the equation to account for diminishing returns?

 

[tmassey]

Your 108’s are not 108 at 2400 psi; they are 108 at 2640, that is, with the 10% overfill.

108 / 2640 * 3600 * 2 = 294.

Of course, everybody else calls those 104s, not 108… that makes the math 283, which is really darn close to the metric measurements.

3600 psi = 248bar * 16l = 3968l = 140 ft3 * 2 = 280 ft3.

(Note: 16 L may be incorrect here. I used that from the content of the first post, but looking up the chart these may actually be 17 L tanks. The math is correct for 16L and the point holds either way.)

For a much more detailed breakdown: Calculating SCUBA Cylinder Capacity | Dive Gear Express®

 

[UCFKnightDiver]

Your 108’s are not 108 at 2400 psi; they are 108 at 2640, that is, with the 10% overfill.

108 / 2640 * 3600 * 2 = 294.

Of course, everybody else calls those 104s, not 108… that makes the math 283, which is really darn close to the metric measurements.

My Faber tanks have 108cuft at 2400(psi) stamped on the neck.

 

[tmassey]

My Faber tanks have 108cuft at 2400(psi) stamped on the neck.

No doubt. 2400 is the working pressure. If the Hydro has a plus, they’re allowed a 10% overfill. That’s all defined in federal law and regulations.

What Faber calls them is not defined by federal law and regulations. Faber can call them whatever they want. It doesn’t change the physics. Calling them 108’s is a “marketing name“. It doesn’t change the fact that they hold what they hold.

And again, all of the LP tank “Imperial capacity“ (or marketing capacity!) is defined assuming the 10% overfill. When they sell them to you, they can get the 10% overfill. So it’s not disingenuous to market them by that. But LP 85s are only 85 ft.³ at 2640. LP50’s are only 50 ft.³ at 2640. Even though the tank is stamped with a federally mandated rated pressure of 2400.

ETA: to make it clear, not all DOT 3AA tanks have a rated pressure of 2400. Older LP 72s are 2250, if I remember correctly, and there are a series of “medium pressure“ 3AA tanks that are rated at 3000 psi. None of it changes the fact that all of those tanks are marketed with a volume that assumes a 10% overfill. Every single one.

Another ETA: Tank capacities are vague, pretty much across-the-board. When they say a tank holds 16 L of water, it’s very unlikely that it’s going to hold 16.00 L. in fact, the specifications clearly describes those as “nominal“; it was never designed to be exactly that size. So all of the math is going to be somewhat off.

Yet another ETA: It gets worse. At least with most of the steel tanks, their marketing capacity is pretty close to the actual capacity. The farthest off is those 104’s marketed as 108‘s. Of course, as you very well may know, aluminum 80s don’t hold 80 ft.³. For the most part, they hold 77 ft.³ or so. Yeah, they happen to have stamped into them an “80”… But they can never hold that much within spec. Again, a marketing name.

 

[Manatee Diver]

I was told to use a tank factor of 8 for all 10X tanks.

 

[UCFKnightDiver]

No doubt. 2400 is the working pressure. If the Hydro has a plus, they’re allowed a 10% overfill. That’s all defined in federal law and regulations.

What Faber calls them is not defined by federal law and regulations. Faber can call them whatever they want. It doesn’t change the physics. Calling them 108’s is a “marketing name“. It doesn’t change the fact that they hold what they hold.

And again, all of the LP tank “Imperial capacity“ (or marketing capacity!) is defined assuming the 10% overfill. When they sell them to you, they can get the 10% overfill. So it’s not disingenuous to market them by that. But LP 85s are only 85 ft.³ at 2640. LP50’s are only 50 ft.³ at 2640. Even though the tank is stamped with a federally mandated rated pressure of 2400.

ETA: to make it clear, not all DOT 3AA tanks have a rated pressure of 2400. Older LP 72s are 2250, if I remember correctly, and there are a series of “medium pressure“ 3AA tanks that are rated at 3000 psi. None of it changes the fact that all of those tanks are marketed with a volume that assumes a 10% overfill. Every single one.

Appreciate the info! Even with the updated calculation, I still think the calculation may be 20-30 cuft higher than the actual volume of gas if LP108s/104s and HP130s truly contain the same internal volume.

 

[tmassey]

They don’t. They never did.

See the charts here: Updated SCUBA tank specifications list -- in PDF and spreadsheet

Even tanks that are marketed as being the same size do not have the same water volume. I used 16 L from your first post, but the charts I linked to shows that they might be 17 L.

Just know two things: one, the simple things that people say about scuba tanks are almost always wrong. When people say “108‘s are the same as 130s”, that is pretty much universally wrong. If you look at the two tanks next to each other, you can see that they’re not quite the same size and shape.

Two, the way that they are wrong tends to be in a way that doesn’t matter. Usually, what they’re really saying is “they’re close enough that it doesn’t matter.” Of course how much matters is up to each person, but if you’re truly worried about a variation of 20 ft.³, you’ve probably cut your gas reserves way, way too close.

And the point that @manateediver makes about tank factors really reinforces that. In cave diving, to keep the math simple, we use a simple, small number to be able to know the capacity of a tank, called a tank factor. It’s simply how many cubic feet it holds per 100 psi. We then divide our tank pressure by 100 psi to give us a nice simple two digit number, then multiply that number by the tank factor, and we know how much gas we have.

Tank factors are rounded to the nearest half digit. It’s not exactly high resolution! So trying to figure out if one tank has 15 ft.³ more than another just doesn’t matter: we’re going to hack off that little bit of difference with our tank factors anyway.

 

[tbone1004]

My Faber tanks have 108cuft at 2400(psi) stamped on the neck.

post pictures of that, because I can all but guarantee that they don't.

If you're filling LP bottles to cave pressure add a factor of 1.06 and you'll be close enough

108*3600/2640=147/1.06=138

Either way though, water volume doesn't lie and your 108's are 17L on the inside. 17L=.6cf. 3600psi=248bar. .6*248=148.8. 148/1.06=139 so that's close enough.
Note, if you run that calculation for the Faber FX133's which are the same water volume, and do it at 3442psi, you get 134cf which is close enough.
At 2640, the Z factor is 1.008, and .6*182/1.008=108.

I'd call the bottles 135cf for gas planning since it's divisible by 3 for on paper gas planning for penetration distances and times.

On the validity of 16L for those, Faber calls the bottles out in tenths of liters with claims of 12.9 for FX100's, 7.8 for LP50's, etc. When that is combined with the math above getting close to the nominal values, I'd say that's good enough to believe. If you're comparing bottles from Faber, using the chart on their website and they say the water volumes are the same, I'd believe it.
LP95=FX117
LP108=FX133
LP120=FX149
LP85~FX100 *I'd argue 12.9L vs 13L is close enough to say that you're not going to choose one or the other over a 1% difference in water volume.
Note that this doesn't apply when crossing brands, particularly with the 100's that have historically been all over the place on dimensions.

 

[UCFKnightDiver]

@tbone1004 I'm pretty sure these are pics of the service pressure and tank volume on my Faber 108s

 

[tmassey]

I can show you photographs of my aluminum 80s that have “S80” and “3AL-3000” stamped in them. Doesn’t change the fact that they hold 77.4 ft.³. Not 80.

The claims on most packages are exaggerations. Why should scuba tanks be any different?