Average Gas Consumption

[vatty]

Not quite.
SAC has the units of bars/minute at 1 ATM.
So your SAC is (165-85)/(50*2)=0.8 bars/min, which is 11.6 psi/min in imperial units.

Your AL tank holds 12 l at one bar (not really, but that is what you are using) so the conversion to RMV is based on
bars/min x liters/bar: 0.8x12=9.6 l/min, which is 0.34 cuft/min in imperial units.

You don't use much gas.

Okai, interesting.

So to write down in my notes:

• SAC = (Start Pressure - End Pressure) / (time * Pressure)
• RMV = SAC * Cylinder Volume(l)

In my case:

• SAC; (160 - 85) / (50 * 2) = 0.75 bars/min (10.8 psi/min)
• RMV; 0.75 * 12 = 9.0 l/min (0.31 cuft/min)

Thanks!

EDIT:

Ah, my original formula actually already calculated the liters I used per minute:
(160 - 85) * 12 / (50 * 2) = 9.0 l/min (0.31 cuft/min)
Cause if I use @tursiops numbers:
(165 - 85) * 12 / (50 * 2) = 9.6 l/min (0.34 cuft/min)

I went back into my course book of SSI and found the formula, but that is indicated as SAC, as shown here:

 

[scubadada]

Hi @vatty

Good example of confusing definitions/conventions. The SSI SAC would generally be called RMV

As has been pointed out many times previously, metric is easier than imperial to use. You have bar/min/atm for your SAC. Multiplying by cylinder volume is actually l/bar so the RMV comes out l/min/atm. The tank factor is more confusing in imperial due to the way we define cylinder volume, gas capacity at working pressure. We have to divide the gas volume in cu ft by the working pressure in psi to get the units to work out and get the RMV in cu ft/min/atm

It has always helped me immensely to do unit/dimensional analysis when working out math problems. It helped me immensely in a couple of quarters of upper division physical chemistry, it makes scuba a breeze

 

[rob.mwpropane]

I'm lucky, my average RMV over the last 1669 dives is 0.36 +/- 0.04 cu ft/min (mean +/- std dev) I attribute my low gas consumption to having been a competitive swimmer from the age of 4 through high school. I don't really know the answer. I did learn an effective breathing pattern for scuba. My gas consumption has always been on the low side, 1970 to now. I am not svelte, 5'10", about 180-5 lbs. I have exercised about 6 days/week for my entire adult life. I'll be 68 years old next week, I'm doing just fine.

Actual picture of @scubadada;

(Jk)

 

[scubadada]

Actual picture of @scubadada;

(Jk)



View attachment 701709

The hair is the same color, that's about it

 

[Dan]

Okai, interesting.

So to write down in my notes:

• SAC = (Start Pressure - End Pressure) / (time * Pressure)
• RMV = SAC * Cylinder Volume(l)

In my case:

• SAC; (160 - 85) / (50 * 2) = 0.75 bars/min (10.8 psi/min)
• RMV; 0.75 * 12 = 9.0 l/min (0.31 cuft/min)

Thanks!

EDIT:

Ah, my original formula actually already calculated the liters I used per minute:
(160 - 85) * 12 / (50 * 2) = 9.0 l/min (0.31 cuft/min)
Cause if I use @tursiops numbers:
(165 - 85) * 12 / (50 * 2) = 9.6 l/min (0.34 cuft/min)

I went back into my course book of SSI and found the formula, but that is indicated as SAC, as shown here:
View attachment 701695

Unit math:
SAC = AC x V / (t x P)
L / min = bar x L / (min x bar)

You can see on the right hand side (RHS) of the equation, the unit ‘bars’ would cancel each other out (bar / bar) and left with L / min which is the unit on the left hand side (LHS) of the equation.

So, always include the unit of the variable in the equation when you want to make sure the equation is expressed correctly.

Same approach with the unit conversion:
9.0 L / min = 9.0 L / min x 0.0353 cft / L
The ‘Ls’ on the RHS of the equation cancel each other leaving with cft / min or cfm
9.0 L / min = 0.32 cft / min = 0.32 cfm

 

[Centrals]

Average gas consumption?
Depends on several issues and the difference is huge.
Min is around 6L per min and max was about 20L when kicking against current and dived with twin and deco bottles(x2).
In rec dive it is the ndl which is the deciding factor for me rather than amount of gas in my 11L Al Tank.

 

[tursiops]

In rec dive it is the ndl which is the deciding factor for me rather than amount of gas in my 11L Al Tank.

Common for deeper dives, but not for shallower dives.

 

[scubadada]

Average gas consumption?
Depends on several issues and the difference is huge.
Min is around 6L per min and max was about 20L when kicking against current and dived with twin and deco bottles(x2).
In rec dive it is the ndl which is the deciding factor for me rather than amount of gas in my 11L Al Tank.

Hi @Centrals

Thanks for bringing this up, it has been briefly discussed in this thread on previous occasions.

That's why the poll question is, "What is your average RMV?" This takes into account all the various conditions you dive in, warm, clear water with little current to cold murky water with brisk current, and everything in between. I think nearly everyone has assumed RMVs for entire dives rather than for a portion.

I have shared my own data. I have RMVs for 1669 dives over the last 11 1/2 years. My average RMV is 0.36 with a std dev of 0.04 cu ft/min. The range is 0.26-0.63 cu ft/min. It is interesting that the variation is actually so small. A little over 2/3 of my dives have an RMV of 0.32-0.40 and 95% are between 0.28 and 0.44. Knowing the average, and the variation around the average, allows one to examine dives that fall at the borders or outside of the usual values. Sometimes there are good explanations, though not always. When I am cold, my RMV increases significantly, perhaps as much, or more, than that due to exertion. Cold often tends to apply to a higher percentage of a dive than brisk exertion.

 

[scubadada]

...In rec dive it is the ndl which is the deciding factor for me rather than amount of gas in my 11L Al Tank.

Common for deeper dives, but not for shallower dives.

A little knowledge is a dangerous thing. I do a lot of diving in SE Florida, a little over 1200 dives over a dozen years. Most of the diving is between about 50 to 110 feet. Nitrox 32 and 36 are both readily available.

If you know your NDLs and your gas consumption, it's pretty easy to figure out if your dives will be gas or NDL limited for no stop dives. Of course, repetitive dives will have shorter NDLs. So, using an AL80, on a 1st clean dive, up to a depth of 70 feet, I'm gas limited. At 100 feet or deeper, I'm NDL limited. At 80 feet, it's very close between gas and NDL at EAN32 and I'm gas limited with EAN36. At 90 feet, I'm NDL limited with EAN32 and it's a tossup between gas and NDL with EAN36.

This kind of information is useful in dive planning. Some charter boats have a rough run time limit, many prohibit deco on general charters.

 

[Subcooled]

Average RMV or SAC is only usefull for monitoring personal development and for team gas planning. It's not well suited for global comparisons.

It is affected by build and experience but also by temperature, suit, currents, task loading (e.g. are you swimming and how fast or just floating around). My wetsuit SAC can be 0.46 cuft/min but with a drysuit it is easily between 0.64 and 0.7. When doing physical labour my air consumption can further double - because of the work of course.