Dumb down true air Capacity!!!

[tursiops]

I have Lp50s, OMS Lp98's and am getting a set of Lp85s. I fill my own tanks just trying to decide what the advantage of one tank pressure is over the other and just want to be able to do the math.

Then forget all that stuff with z-factors and tank factors.....
Just use ratios.
At 2640 (2400+10%) psi, your tanks are at rated capacity, i.e. 50, 85, and 98 cuft.
At 1000 psi, your 50 has 1000/2640 * 50 = about 19 cuft.
At 3200 psi, your 50 has 3200/2640 *50 = about 60 cuft.
Replace 50 in those calculations with 85 to get what the 85's hold.
Replace 98 in those calculations with 98 to get what the 98s hold.
the general formula is P/2640 * Size = Volume, where in the examples P=1000 or 3200, and Size = 50 or 85 or 98.
Does that work for you?

 

[ScubaJCBS]

Ding Ding! Yes I know the really detailed stuff has a place but for a redneck from NC. I need simple for what I'm doing this works.

 

[rx7diver]

I always find things like "z-factors", "tank factors", etc., confusing and unnecessary. If you're wanting to learn the "advantage of one tank pressure over the other", then all you need to do is work with relative proportions: V1 / P1 = V2 / P2.

1. For example, consider a Faber LP 85 (which is rated at 81.1 cu ft @ 2,640 psig). By relative proportions, we have: 81.1 cu ft / 2,640 psig = V2 / P2.

2. For a second example, consider a Sherwood Genesis/PST HP 100 (which is rated at 102 cu ft @ 3,500 psig). By relative proportions, we have: 102 cu ft / 3,500 psig = V2 / P2.

Now, suppose you want to know the "advantage/disadvantage" of filling each cylinder to 3,000 psig (instead of to its rated pressure of 2,640 psig and 3,500 psig, respectively). Then simply plug P2 = 3,000 psig into each of the two expressions above, and solve for V2 to yield the amount of (free) gas each cylinder would hold if filled to 3,000 psig. Your two answers will be in cubic feet.

I hope this helps. (I hope I haven't misunderstand what you are hoping to learn.)

KISS.

rx7diver

 

[tbone1004]

@rx7diver except that’s not entirely accurate because of the z factor. The hp100 holds about 5% more gas than your calculation shows. It’s not important when diving thirds because the first third is smaller than the last one but it’s very important when calculating available volume.

If you do the math with a Faber lp85 and a fx100 you’ll get the same values that you did in your example. The problem with that is they actually hold the same amount of gas and you’d never know that without the z factor. Logic like yours is what gets people into the misinformation that a lp104 is bigger than a hp130 and spreads like wildfire.

 

[mderrick]

As simple as I could make it and still retain a modicum of accuracy....

Calculating SCUBA Cylinder Capacity | Dive Gear Express®

Calculating SCUBA Cylinder Capacity Or why does my Aluminum 80 SCUBA tank only hold 77 cubic feet of air? ©2018-2020 by Mark Derrick, Gas Blending Instructor at Dive Gear Express ...

www.divegearexpress.com

 

[ScubaJCBS]

As simple as I could make it and still retain a modicum of accuracy....

Calculating SCUBA Cylinder Capacity | Dive Gear Express®

Calculating SCUBA Cylinder Capacity Or why does my Aluminum 80 SCUBA tank only hold 77 cubic feet of air? ©2018-2020 by Mark Derrick, Gas Blending Instructor at Dive Gear Express ...

www.divegearexpress.com

Yeah I read that in an attempt to figure it out but I'm more touch it feel it learner. That site is a wealth of knowledge for sure. Is that you? I will probably try to get my wife to hold my hand through that information. She calls me her man child anyways might as well make her earn it!

 

[inquis]

I use the ratios described above, but also start with the table at the DGX site that includes the initial impact of the z-factor. For example, an LP85 only has 81.1 cuft. Scale this up to 3000 psi by multiplying by 3000/2640. (Remember which is on top by knowing you want a number bigger than 1 -- more pressure has more gas, right!)

(Side note: this will be off because the z-factor at 3000 psi is sightly different than that at 2640 psi, but this is typically ignored.)

 

[mderrick]

Yeah I read that in an attempt to figure it out but I'm more touch it feel it learner. That site is a wealth of knowledge for sure. Is that you? I will probably try to get my wife to hold my hand through that information. She calls me her man child anyways might as well make her earn it!

You can start by using the formulas to duplicate the answers in the tables at the end. Once you have done one or two common cylinders the light bulb will go on and you'll realize it's pretty straightforward.

 

[ScubaJCBS]

Roger that will work on it thank you a lot.

 

[Angelo Farina]

I always make these calculation in my brains, as they are truly simple.
On the tank you usually see engraved the capacity (in liters - that at least for Faber and Luxfer tanks I own).
You multiply by the pressure (in bar) and you get the total volume of air.
Example: my Faber steel tank is 15.0 liters, if I fill it at 200 bars I get 3000 liters.
OK, something less, due to air imperfect compressibility.
The Z-factor of air (at 300K, that's ambient temperature) is as follows:

150 bar 1.007

200 bar 1.033

250 bar 1.067

300 bar 1.109

So at 150 bar the imperfect compressibility is negligible. At 200 bar, you "loose" 3.3% of your capacity, so instead of 3000 liters I really have 2900.
If I overfill at 250 bars, then the nominal capacity would be 15x250=3750 liters, but the Z-factor diminishes it by 6.7%, hence I have only 3500 liters.