What is average surface air consumption?

[Ken abucs]

Tuning up your gear can make a huge difference. My BCD felt like it was flopping around, between the added drag, and the lack of confidence it instilled I was running a RMV of around 1.05 over 4 dives. After I added a clip on the chest, and a crotch strap, I felt a lot more comfortable in the water, and my RMV dropped to .83.

That's a 21% improvement. I just need one more improvement like that, and I'll be really happy.

BTW,

I'm diving wet and cold, 6'3", 245lbs

Come on all you lurkers, don't let the people with excellent numbers skew this thread. Let's here how much air real people are using, and what you've found that helped.

 

[pir8]

i don't think so. RMV is not the same as SAC expressed in cubic feet per minute.

while SAC expressed in cubic feet DEPENDS for calculation
on how many cubic feet of air you started with, SAC expressed in cubic
feet as a final result expresses the amount of cubic feet per second used,
and that does not depend on the cylinders you will user thereafter.

in other words, to CALCULATE SAC in cubic feet to begin with, you need to
factor in the cubic feet you used (and that means factoring in your tank
volume and pressure). however, once you figure that out, you can then
express SAC in cubic feet and that is an absolute measure that will not
change depending on what tank you use.

if your SAC in cubic feet is .76 per minute, you will use .76 cubic feet per minute
whether you have an AL 80 or a steel 95 on your back.

hope that makes sense

Unless changing tank/ equipment causes a change in your SAC rate.

 

[H2Andy]

Unless changing tank/ equipment causes a change in your SAC rate.

my point was that SAC expressed in cubic feet and RMV are different things

when i say my SAC is .72 cfm, that is my SAC, not my RMV. it has nothing
to do with my SAC changing due to environmental or equipment factors.

 

[Vayu]

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.

 

[Cave Diver]

my point was that SAC expressed in cubic feet and RMV are different things

when i say my SAC is .72 cfm, that is my SAC, not my RMV. it has nothing
to do with my SAC changing due to environmental or equipment factors.

SAC is expressed in PSIG per minute.

RMV is expressed in cubic feet per minute.

They are two different measurements that are often wrongly used interchangeably.

 

[Rick Murchison]

my point was that SAC expressed in cubic feet and RMV are different things

Hmmm.... aside from RMV being (usually) expressed in LPM, vice CFM, they're the same thing. If I'm at 100 ft with a RMV of .5CFM, I'll be consuming .5CFM at 4 ATM, the same mass as 2CFM measured at the surface, which, when divided by 4 ATM, yields a SAC Rate (or SCR if you prefer) of .5CFM.
There is no practical difference in the two; for gas planning purposes they are the same thing
Tank size is irrelevant to consumption rate.
Rick

 

[Rick Murchison]

SAC is expressed in PSIG per minute.

RMV is expressed in cubic feet per minute.

They are two different measurements that are often wrongly used interchangeably.

Ah... if you measure SAC Rate in psi/min (which is why I prefer the less ambiguous term SCR, which most of us understand to be expressed in CFM, and I prefer it over RMV, which is properly expressed in LPM - useful if you are using metric but my little pea brain was raised on imperial) the the number will vary with tank size, sure... no matter what you use or what you call it you're going to need to do some conversions in your planning if you're using different size tanks.
As for me, if you'd like to express your SAC Rate in CFM or LPM or psi/min based on a specific tank size, have at it. Any will do, as math is an equal opportunity servant.
Rick

 

[Cave Diver]

Hmmm.... aside from RMV being (usually) expressed in LPM, vice CFM, they're the same thing. If I'm at 100 ft with a RMV of .5CFM, I'll be consuming .5CFM at 4 ATM, the same mass as 2CFM measured at the surface, which, when divided by 4 ATM, yields a SAC Rate (or SCR if you prefer) of .5CFM.
There is no practical difference in the two; for gas planning purposes they are the same thing
Tank size is irrelevant to consumption rate.
Rick

Our training differs...

I was taught that SCR/SAC is calculated as follows: (Total Gas Consumed)/(Depth in ATA) x Time = xxx psig per minute used.

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

If RMV and SAC are the same thing, how can you use SAC as part of the calculation for determining RMV?

 

[Cave Diver]

Ah... if you measure SAC Rate in psi/min (which is why I prefer the less ambiguous term SCR, which most of us understand to be expressed in CFM, and I prefer it over RMV, which is properly expressed in LPM - useful if you are using metric but my little pea brain was raised on imperial) the the number will vary with tank size, sure... no matter what you use or what you call it you're going to need to do some conversions in your planning if you're using different size tanks.
As for me, if you'd like to express your SAC Rate in CFM or LPM or psi/min based on a specific tank size, have at it. Any will do, as math is an equal opportunity servant.
Rick

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.

 

[H2Andy]

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

well, i calculate my SAC based on real dives ... i just dont' pull it off thin air

do you know what your SAC is? how do you do gas planning?