What is average surface air consumption?

[H2Andy]

Tank size is irrelevant to consumption rate.
Rick

well, tank size is very relevant to CALCULATING SAC expressed in cubic feet per minute
and not at all for SAC expressed in psi

i disagree with Cave Diver that saying my SAC is .76 cfm is the same as saying my RMV is .76 cfm, at least how he defines it in his cited posts

stop for a minute:

how do you get to SAC expressed in cubic feet per minute?

how do you get to RMV?

it's different, isn't it?

that's because they're different concepts.

RMV = SAC/(Rated Cylinder Working Pressure/Rated Cylinder Volume) = xxx cft per minute used.

what if you are calculating SAC using a cylinder boosted beyond its working
pressure?

what if you are calculating SAC using an RMV based on a pressure that is more
or less than the working pressure?

dont' you see how RMV as you define it is a theoretical number only, and it depends on a cylinder being full to its exact working pressure being used?



at any rate, when i say my SAC is .72 cubic feet per minute, that is my SAC,
not my RMV. different things. that means in that particular dive, i used .72
cubic feet per minute, and i would have used that no matter what tank i
had been wearing. but to CALCULATE how much gas i used in cubic feet
per minute, i have to factor in what type of tank and what starting pressure
i had, or the figure is meaningless.

 

[Rick Murchison]

Looks like we were posting at the same time which clears up the discrepancies we were having.

I was taught SAC and SCR are the same thing: Surface Air Consumption Rate.
Some just abbreviate that as SAC, some SCR and a few use SACR.

Well.... SCR (Surface Consumption Rate) is the term we used instead of SAC Rate for CFM expressions for gas planning when we started diving things other than 72's and 80's, and needed a better apples-to-apples comparison. That was oh, probably about 17 years ago for me.
RMV - a medical term of long standing and expressed in Litres/min - has (in my experience) been only recently adopted by the Scuba community - and the CFM useage for RMV is, to my knowledge, a unique Scuba adaptation of the original term.
So, I'll stick with SCR as the "best" term for CFM, but I understand what you mean when you use "SAC Rate," whether expressed in psi/min and tank size or CFM, "SCR," whether expressed in CFM or psi/min and tank size, or "RMV," whether expressed in LPM or CFM. Now if you start using "RMV" and psi/min together you might get a raised eyebrow from me, but otherwise we can communicate effectively and do accurate gas planning just fine
Rick

 

[Cave Diver]

I'd love to see how actually being in the water would change some of these numbers posted.

The RMV of .53 that I posted originally was calculated on a dive I did 4 years ago when I first took a deco procedures class, warm caribbean 86* water, 3 mil shorty wetstuit, single aluminum tank

My calculated RMV when I dove the Oriskany last week was .53, 76* water temp, drysuit, steel doubles.

My breathing rate has stayed pretty consistent when I'm actually in the water.

 

[Rick Murchison]

Any volumetric consumption rate (CFM or LPM) is a measure of how much volume you breathe per unit time at ambient pressure. That volume doesn't change (to any significant degree) with changes in ambient pressure. But the density of the gas you're breathing does change, so the number of molecules you use per breath does change, and so the psi per minute changes with depth... and with tank size... but not the volumetric consumption rate. If I express SCR (or SAC Rate) in CFM then I have made the conversion from a tank dependent rate (psi/min) to a volumetric consumption rate, and it is the same rate regardless of tank size, regardless of depth, and regardless of whether I call it RMV, SCR or SAC Rate. Cave Diver is absolutely correct in his statement.
Rick

well, tank size is very relevant to CALCULATING SAC expressed in cubic feet per minute
and not at all for SAC expressed in psi

i disagree with Cave Diver that saying my SAC is .76 cfm is the same as saying my RMV is .76 cfm, at least how he defines it in his cited posts

stop for a minute:

how do you get to SAC expressed in cubic feet per minute?

how do you get to RMV?

it's different, isn't it?

that's because they're different concepts.



what if you are calculating SAC using a cylinder boosted beyond its working
pressure?

what if you are calculating SAC using an RMV based on a pressure that is more
or less than the working pressure?

dont' you see how RMV as you define it is a theoretical number only, and it depends on a cylinder being full to its exact working pressure being used?



at any rate, when i say my SAC is .72 cubic feet per minute, that is my SAC,
not my RMV. different things. that means in that particular dive, i used .72
cubic feet per minute, and i would have used that no matter what tank i
had been wearing. but to CALCULATE how much gas i used in cubic feet
per minute, i have to factor in what type of tank and what starting pressure
i had, or the figure is meaningless.

 

[Cave Diver]

well, tank size is very relevant to CALCULATING SAC expressed in cubic feet per minute
and not at all for SAC expressed in psi

i disagree with Cave Diver that saying my SAC is .76 cfm is the same as saying my RMV is .76 cfm, at least how he defines it in his cited posts

stop for a minute:

how do you get to SAC expressed in cubic feet per minute?

I don't. As I stated numerous times, the definition I was taught of SAC is expressed in PSIG. I use RMV for CFT per min.

how do you get to RMV?

it's different, isn't it?

that's because they're different concepts.

I already provided the calculation for RMV in previous post.

Yes it's different from SAC, see previous post.

Yes, they are different concepts, see previous post.

what if you are calculating SAC using a cylinder boosted beyond its working
pressure?

what if you are calculating SAC using an RMV based on a pressure that is more
or less than the working pressure?

dont' you see how RMV as you define it is a theoretical number only, and it depends on a cylinder being full to its exact working pressure being used?

It doesnt matter if it's boosted beyond working pressure. It doesnt matter if it's filled to less than working pressure. Try the math I provided and see for yourself.

Now, I don't see it as being a theoretical number. Try the math for yourself. If you still don't understand what I'm saying, I'll come back and post numerous examples until we can both come to an understanding.

 

[Cave Diver]

Now if you start using "RMV" and psi/min together you might get a raised eyebrow from me, but otherwise we can communicate effectively and do accurate gas planning just fine
Rick

I'd dive with you anytime Rick!

BTW, my terminolgy came from IANTD advanced nitrox class taken about 4 years ago. Definitions may have changed during that time, but the concept is the same.

 

[Rick Murchison]

This is all fine and well when on the internet. Most of you know diving online is alot different than real diving. I'd love to see how actually being in the water would change some of these numbers posted.

My breathing rate has stayed pretty consistent when I'm actually in the water.

Not mine...

Well, lesseee here....
Looking at my log, I see recent rates ranging from 0.33 CFM up to 1.61 CFM.
I'm sure I could easily go higher under stress or a workload; I doubt I could get much lower, even asleep
For planning purposes I use 0.55 for the bottom and 0.45 for deco.
Rick
Oh, y'all are using body size & such...
Male
Old (But not as old as Smedley)
Fat
Ugly

I suggest many folks ain't gettin' much variety in their diving; far too little excitement
That 1.61 dive was a real experience...
Rick

 

[Cave Diver]

Not mine...

I suggest many folks ain't gettin' much variety in their diving; far too little excitement
That 1.61 dive was a real experience...
Rick

I recently had one at 1.77, but let's not talk about that... :11:

 

[waterbearer]

Still talking about each others' SACs, are we?

 

[Cave Diver]

Still talking about each others' SACs, are we?

Yeah, anything you care to contribute?