Dumb down true air Capacity!!!

[ScubaJCBS]

I have searched the forum over for a good way to figure the true air capacity of a scuba cylinder. I am attempting to figure this out, I am not a rocket scientist and don't clam to be. Can someone please assist me with the calculations. Please don't site laws without explanation. One of the main problems I'm finding is with a cylinder that is in liter water volume. ie Lp50 is 7.8L but most of the formulas I find are either straight up metric liters to bar or inches to psi. Thank you in advance.

 

[tbone1004]

True capacity is volume x pressure x z-factor.
Best way is to use water volume in liters and pressure in bar then convert to imperial.

Why do you need to find exact capacity? Most are very close

 

[ScubaJCBS]

Biggest thing is figuring what the differences is on Lp at different pressures

 

[Whitrzac]

cross multiply and divide.

50------- x
2640psi 3200psi

50x3200 /2640 = 60.6cf

 

[loosenit2]

a simple, field expedient method to figure out your working pressure for any tank is to use tank factors.

Tank Factor = Rated Volume/Rated Pressure
EX: LP 95 TF = 95/2640 = 3.5

That means you have 3.5 cuft for every 100 psi in your tank. In the field the calculations are very easy then. You take the first 2 digits of your pressure and multiply by 3.5 and you get your tank volume.

ex 2000 psi in LP 95 = 20*3.5 = 70cu ft.

The calculations are more accurate if you don't round the tank factor (as I did) but often result in numbers that are harder to do in your head. Rounding to even numbers gets you close enough for planning factors and there are generally enough other factors that change (temp of tank, errors in SPG etc) that make rounding a good enough solution.

 

[boulderjohn]

a simple, field expedient method to figure out your working pressure for any tank is to use tank factors.

Tank Factor = Rated Volume/Rated Pressure
EX: LP 95 TF = 95/2640 = 3.5

That means you have 3.5 cuft for every 100 psi in your tank. In the field the calculations are very easy then. You take the first 2 digits of your pressure and multiply by 3.5 and you get your tank volume.

ex 2000 psi in LP 95 = 20*3.5 = 70cu ft.

The calculations are more accurate if you don't round the tank factor (as I did) but often result in numbers that are harder to do in your head. Rounding to even numbers gets you close enough for planning factors and there are generally enough other factors that change (temp of tank, errors in SPG etc) that make rounding a good enough solution.

I was just typing out a similar message when this appeared.

For an LP 50, it works out to 0.0189. Multiple the tank pressure by that, and you have its gas volume. For ease in estimating, use 0.02 and know it is a trifle high. For greater ease, multiply by 2 and move the decimal point two spaces.

2 * 3200 = 6400.

You can estimate that you have a tad less than 64 cubic feet.

 

[Gareth J]

True capacity is volume x pressure x z-factor.
Best way is to use water volume in liters and pressure in bar then convert to imperial.

Why do you need to find exact capacity? Most are very close

@TBone is correct, working in metric units is easier and quicker. The issue is the need to make conversions for local requirements.

Available Gas
Cylinder volume x cylinder fill pressure = Amount of available Gas.

10 (litre) x 210 (bar) = 2100 litres of Gas

Usable Gas
Using rule of thirds

(2100 / 3 ) 2 = 1400 litres (leaving a reserve of 700 litres)

How long will it last?

Surface breathing rate [1] x Absolute pressure [2] = gas consumed at depth
25 (litres/min) x 4 (30m) = 100 litres/min at 30m

1400/100 = 14 minutes

Most modern computers can be switched between imperial and metric units.
With the increase in air integrated computers, switching to metric is much easier now than in the past.

Being British - we use a mix of Imperial and metric in normal life anyway (at least my generation does).
The issue for you is how well you equate the metric units.

--------------------------------------------------------------------------------------------
[1] 25 litres/minute is a conservative rate, normally used for example calculations.

[2] Every 10m of depth increases pressure by 1 bar (gauge)
10m = 1 bar (gauge) = 2 bar (absolute)
20m = 2 bar (gauge) = 3 bar (absolute)
20m = 3 bar (gauge) = 4 bar (absolute)

[3] 14.7 psi = 1 bar ---- for ease 15 psi = 1 bar.
[4] 33ft = 10m

 

[boulderjohn]

A quick note on precision in calculating tank volumes--Charles Law is very much in effect.

I do a lot of diving in a huge sinkhole in New Mexico. The water temperature is around 60° F all year. When we dive in the summer, we arrive in the morning and begin to set up.

• In the summer, the air temperature may surpass 100° F, and our tank pressures rise rapidly, but we know we will have a lot less pressure once we have been in the water long enough for the tanks to cool to 60°.
• In the winter, we may be diving with air temperatures in the 30s, although it is usually warmer. Our tank pressures will increase significantly once we are in the water.

 

[tursiops]

I have searched the forum over for a good way to figure the true air capacity of a scuba cylinder. I am attempting to figure this out, I am not a rocket scientist and don't clam to be. Can someone please assist me with the calculations. Please don't site laws without explanation. One of the main problems I'm finding is with a cylinder that is in liter water volume. ie Lp50 is 7.8L but most of the formulas I find are either straight up metric liters to bar or inches to psi. Thank you in advance.

Every answer you've gotten here is correct, for certain purposes, but may not be what you are actually asking. Can you be more specific about what you want to know? For example, do you want to know how much air a cylinder holds when fully pressurized, or when partly pressurized, or perhaps when empty (since you mention water volume)? When you say "true" do you mean "not approximate" or "taking into account that air is not a perfect gas" or what? Maybe you jsut want to know how many cubic feet equivalence is a "7.8 liter" cylinder? What DO you want to know?

 

[ScubaJCBS]

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.