What is average surface air consumption?

[waterbearer]

Nope, you guys don't want to hear about mine.

 

[Silver_Diver]

Male
19
6'
155 lbs
3/5mm wetsuit and hoodie

I usually compute my SAC in PSI/min, and I average about 1:50 on 2500 PSI at 30 ft. so a SAC of about .31. My dive buddy usually ends with about 1-200 PSI more than me so about .29. But we tend to stay down much longer than our other dive buddies so I'd say it's uncommon. It seems more like .7 is typical.
When I started my SAC was more like .45.

 

[H2Andy]

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.

no, because what he is calling RMV is then used to calculate SAC on the second
step of his equation.

if RMV were the same as SAC, then why would he use RMV to calculate SAC?

take a look at his equation

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.

exactly.

well, looks like we're on the same page after all.

 

[Atticus]

Andy - I think what he's saying is that RMV and SAC (with associated data) are two ways of representing the same measurement. Given the tank size and working pressure, and either RMV or SAC you can convert to the other.

I was taught that SAC is expressed in PSI and a given number is relevent only for a particular tank (ie: in order to understand a SAC number you also have to understand the tank size and service pressure). If you have that info then you can convert it to the "more generic" RMV number and make comparisons between different types of tanks.

I figure you already know this and are just busting folks' chops. Either that or this is like the question on the LSAT about the opposite of "hot" being "not hot" where "cold" is the wrong answer.

~Merlin

no, because what he is calling RMV is then used to calculate SAC on the second
step of his equation.

if RMV were the same as SAC, then why would he use RMV to calculate SAC?

take a look at his equation




exactly.

well, looks like we're on the same page after all.

 

[H2Andy]

I figure you already know this and are just busting folks' chops.

i must say that i apologize if it came across as though i was busting anyone's chops.

that is not my intent at all. i assure you, that is not what i want to accomplish.


it seemed to me that the definition of RMV given *is not the same concept* as
SAC, whether given in psi or cfm.

SAC (whether expressed in psi or cfm) is the actual consumption rate and has
already been converted from a tank-dependent to a tank-independent figure.

RMV *as given by Cave DIver* seemed to be dependent on the type of tank used,
and thus, was only the first step in calculating SAC.

that's all i am saying. i think he agreed with me that RMV and SAC were not the
same thing, as some have contended.

i'm getting confused again

 

[Rick Murchison]

SAC (whether expressed in psi or cfm) is the actual consumption rate and has already been converted from a tank-dependent to a tank-independent figure. ... i'm getting confused again

Yes, you are.
There are two ways to express consumption.
(1) You can express it as a volumetric consumption rate, which is independent of tank size and for this thread we have been using CFM. (When it comes to "RMV" that is a Scuba adaptation of its usual units which are LPM, but it's still a volumetric consumption rate). If you say "My 'X' is .5 CFM" you can substitute "SAC Rate," "SCR," or "RMV" for "X" and it all means the same thing. This is the equivalent of MPG in cars. The size of the tank doesn't matter, you get the same MPG under the same driving conditions.
(2) You can express it in the units you use to gauge your fuel supply, in our case psi/min, but in other parts of the world you may run into bar/min. This number is tank specific, but once you've decided on a tank size it's the one you need because it's expressed in the units you have on your gas gauge. If you say "My 'X' is 25 psi/min" you can substitute "SAC Rate" or "SCR" or even "RMV" (though that would be very odd and an improper use of the term, as the "V" refers specifically to volume) and it all means the same thing - and is tank dependent. The automotive equivalent is "I get 121 miles on a quarter tank of gas," a tank dependent number but the one you need when you're figuring out how far you can go on a particular gauge reading.
---
As I have said, my preference is to express "SAC Rate" in psi/min, and "SCR" in CFM. I personally don't use "RMV" in my language because it really is more properly expressed in LPM and would confuse anyone with a respiratory medical background if you're talking "CFM" and "RMV" together. However, this thread and your confusion make the point of why "RMV" was grabbed as a term that would be less ambiguous than "SCR" in trying to get people to talk apples-to-apples when discussing gas consumption and planning.
Rick

 

[Charlie99]

It's pretty simple.

Don't use any SCR SAC RMV or XYZ numbers without having units attached, and nobody gets confused.

BTW, one way of looking at SAC/RMV is that the actual unit is "cubic feet per minute PER ATA." Normally, since the definition of SAC and RMV assume 1ata, the "per ata" is left off, but it really is there whether or not explicitly called out.

 

[Rick Murchison]

It's pretty simple.

Don't use any SCR SAC RMV or XYZ numbers without having units attached, and nobody gets confused.

BTW, one way of looking at SAC/RMV is that the actual unit is "cubic feet per minute PER ATA." Normally, since the definition of SAC and RMV assume 1ata, the "per ata" is left off, but it really is there whether or not explicitly called out.

No... you're trying to make it more complicated than it is again. A volumetric consumption rate is at ambient pressure. What changes with pressure is the mass in each volumetric unit, and therefore the percentage of gas taken from any given tank with each unit of volume.
If my volumetric consumption is .5CFM at the surface it is still .5CFM at 4 ATA. But the pressure drop from the tank will be 4 times as great because there is 4 times as much gas in the same volume.
Rick

 

[Charlie99]

No... you're trying to make it more complicated than it is again. .......If my volumetric consumption is .5CFM at the surface it is still .5CFM at 4 ATA. But the pressure drop from the tank will be 4 times as great because there is 4 times as much gas in the same volume.
Rick

Just going back more towards first principles.

2.0 cubic feet per minute at 4 ata is a consumption rate of 0.5 cfm/ata. No mutually agreed upon definitions required.

Or going the other way, 0.5cfm/ata is also the consumption rate when using 1.5cfm at 3ata. (1.5cfm/3ata = 0.5cfm/ata).

My experience in many different fields has been that carefully carrying along units in calculations is a quick and effective way to detect errors in formulae.

 

[GreenDiverDown]

Hold on folks...Let's wait up for the guys who'll say that comparing SAC rates is bogus.

I don't know if it is bogus but having reviewed this thread, it seems pretty meaningless to me.

... and what's with calculating SACs to the hundreths... like that is meaningful in someway?