SAC Rate Changes with Depth

[EireDiver606]

I calm and relax further I am from the surface.

You are a true pressure seeker.

 

[Kharon]

You have your units bass-ackwards at psi/cf -->should be cubic-feet-per-psi: "cf/psi".

Yep - caught me. I had the fraction upside down. Doesn't change the fact that psi, without a lot more information tells you nothing and even with that information makes for much more difficult comparisons.

If you use 1500psi from an Al80 (3000psi) on a half hour dive you have used 38.7 cf with this tank.

If you use 1500 from a lp 80 (2400psi) on a half hour dive you have used 45.5cf with this tank.

So saying you used 1500psi over a half hour or a SAC/RMV of 50psi is meaningless without the volume and working pressure. Adding this informationmakes comparing the rate confusing, difficult, and completely non-intuitive.

An the other hand, saying a SAC/RMV of 1.29 cf/min in the first case and 1.52 cf/min in the second is directly comparable and intuitively easy to understand.

And damn, I got sucked in again. I have to ignore this thread.

 

[Kevrumbo]

Yep - caught me. I had the fraction upside down. Doesn't change the fact that psi, without a lot more information tells you nothing and even with that information makes for much more difficult comparisons.

If you use 1500psi from an Al80 (3000psi) on a half hour dive you have used 38.7 cf with this tank.

If you use 1500 from a lp 80 (2400psi) on a half hour dive you have used 45.5cf with this tank.

So saying you used 1500psi over a half hour or a SAC/RMV of 50psi is meaningless without the volume and working pressure. Adding this informationmakes comparing the rate confusing, difficult, and completely non-intuitive.

An the other hand, saying a SAC/RMV of 1.29 cf/min in the first case and 1.52 cf/min in the second is directly comparable and intuitively easy to understand.

And damn, I got sucked in again. I have to ignore this thread.

@Kharon , sorry to point out that you totally misunderstand the concept because you don't know how to apply the information, and so you dismiss it as difficult, confusing and completely non-intuitive.

RMV is an arbitrary constant across all cylinder size ratings -you pick a particular RMV (for example warm water drift dive "nominal: 11 liters/min per ATA", "stressed exertion: 22 liters/min per ATA", or "panic emergency: 30 liters/min per ATA"), and analyze consumption for the volume of gas and tank size at depth that you take on your dive. Since your SPG reads in pressure units, it's more useful to convert RMV to a pressure SAC rate, and then use that value to calculate a depth consumption rate "DCR" at the total ambient pressure you are exposed to for the cylinder size that you are using.

So choosing RMV of "nominal 11 liters/min per ATA", and dividing it by the cylinder size rating of an AL80 which is 11 liters/bar, you get a pressure SAC rate of 1 bar/min per ATA:

Once again here is the math: 11 liters/min per ATA ÷ 11 liters/bar = 1 bar/min per ATA.

Choosing RMV "nominal" again, but dividing it this time by a different tank such as a LP Steel 80 which has a cylinder size rating of 12 liters/bar, you get a pressure SAC rate of:

11 liters/min per ATA ÷ 12 liters/bar = 0.9 bar/min per ATA.

For a hypothetical planned depth of 10 ATA (90msw!), the DCR for the AL80 will be:
1 bar/min per ATA x 10 ATA = 10 bar/min;
The DCR for the Steel LP80 will be:
0.9 bar/min per ATA x 10 ATA = 9 bar/min.

In 10min, the depth consumption delta for the AL80 will be:
10 bar/min x 10min = 100 bar;
The depth consumption delta for the Steel LP80 will be:
9 bar/min x 10min = 90 bar.

So if you started out with near rated service pressure of the AL80, and the Steel LP80 -->roughly 200 bar and 180 bar respectively, you can see that after 10min at 10 ATA only half-tank now remains in both cylinders. [Again that's assuming you maintain the same DCR over those ten minutes as you lose consciousness due to the extreme narcosis breathing Air at 90msw:300fsw]

This is the correct analysis -with the math- of the misunderstood point that you @Kharon were trying to make. . .

 

[Bob DBF]

An the other hand, saying a SAC/RMV of 1.29 cf/min in the first case and 1.52 cf/min in the second is directly comparable and intuitively easy to understand.

One more step, there is no way to relate your RMV of X cf/min to how much gas you are consuming out of your particular tank unless you can relate that to your SPG in psi, since that is the only precise measure of gas in your tank.

Making a dive plan with turn pressures is using that information, although with my diving it needn't be as precise as "metric wall 'o numbers" Kevrumbo , because it's just fun NDL diving, but the principle is the same.


Bob

 

[nolatom]

Or you could just drift dive, so you never have to 'turn" the dive, just watch your pressure gauge and the clock is incidental (assuming your nitrogen ticks are still in the green). So's the current, which is typically the thing we fail to estimate correctly when planning an out and back dive, or a cross-current dive.

Yup, I'm an English major ;-)

 

[Kharon]

One more step, there is no way to relate your RMV of X cf/min to how much gas you are consuming out of your particular tank unless you can relate that to your SPG in psi, since that is the only precise measure of gas in your tank.

Making a dive plan with turn pressures is using that information, although with my diving it needn't be as precise as "metric wall 'o numbers" Kevrumbo , because it's just fun NDL diving, but the principle is the same.

Bob

OK - WOW - I think I see where the problem lies. Let me try to be perfectly clear.

You are absolutely correct. SAC/RMV/cf per min is pretty much useless in dive planning. It is too difficult to even try to think in those terms. PSI is definitely the number to use in keeping track of how much air you have in your tank during a dive.

That's not what SAC/RMV is for. They are used to compare the rate of air consumption after the dive is over - from dive to dive for a diver, or between divers for the same dive, or for different divers on different dives. They have no use during a dive. They are a number that normalizes the rate of consumption post dive.

In other words, PSI and SAC/RMV serve completely different purposes. I think what I am seeing in this thread is people trying to justify using one or the other for both objectives or perhaps only considering one objective or the other.

Perhaps people (during the dive) look at their tank pressure and then the dive time do a division and say "I'm using air at N psi/min so I have X minutes of dive time left". I can't imagine doing that.

When my air gets to the turn point I head back. I really couldn't care less how many minutes are on the clock. Nor do I ever try to estimate how much dive time I have left. I only care about how much air I have left. When the clock runs out the dive is over. That works for the diving I do. Perhaps other dive styles require you to know how much time you have left. Not my kind of diving.

I do use SAC to look at my rate of air consumption across different dives. It allows me to compare air consumption for dives in cold water and thick neoprene to dives in warm water with no insulation. I can see if on a particular dive my rate of consumption was high and think about why.

Since I dive 27cf 2640psi, 30cf 3000psi, 50cf 2640psi, and 80cf 3000psi tanks depending on the dive purpose, psi/minute is useless in comparing dives after the fact. It's only useful to me during dives. Different numbers - completely different purposes.

 

[boat sju]

. . . it's like mud wrestling with a pig isn't it?

 

[Kharon]

. . . it's like mud wrestling with a pig isn't it?

Yep - a greased one.

 

[Kevrumbo]

. . .
Perhaps people (during the dive) look at their tank pressure and then the dive time do a division and say "I'm using air at N psi/min so I have X minutes of dive time left". I can't imagine doing that.

When my air gets to the turn point I head back. I really couldn't care less how many minutes are on the clock. Nor do I ever try to estimate how much dive time I have left. I only care about how much air I have left. When the clock runs out the dive is over. That works for the diving I do. Perhaps other dive styles require you to know how much time you have left. Not my kind of diving.
. . .

No, that's not the the proper method nor the rationale for using RMV and pressure SAC calculations for simple recreational NDL dive planning.

I have a exertion RMV of 22 liters/min per ATA, 11L Aluminium 80 tank with a 200bar starting pressure, and consume 30 bar in five minutes at 18msw depth in temperate 15° C waters diving the Kelp Forest off Catalina Island into a current:

Five minutes elapsed time and my SPG should read 170bar -unclip & check it! It does. . .

Another five minutes for total Ten minutes elapsed time and my SPG should read 140bar -check! It does.

Fifteen minutes elapsed time and my SPG should read 110bar -check! It does.

Twenty minutes elapsed time and my SPG should read 80bar -check! It does,
and then start a multi-level ascent going back with the current to the shallows and vicinity of the Diveboat at 9msw & eventual safety stop. . . surface with 32 bar remaining and Thirty-Five minutes Total Dive Time.

@Kharon -It's not that hard to perform the simple iterative subtraction above ~30 bar every five minute interval at 18msw~ during the dive and "predict" within 5 bar what the SPG will show remaining, all easily derived and applied from RMV and pressure SAC rate.

 

[boulderjohn]

As usual, this is turning into a battle between whose definitions are better than others. “Here is my definition of SAC, and it is better than your definition of SAC because it is the one I was taught.” Different people use different definitions, and it is easy to use the one they are talking about when they talk about it.

By the way, for those who say getting your SAC rate in psi is worthless because of the possibility they will change tank sizes, I had more than 400 dives under my belt before I saw my first steel tank. I had seen and used nothing but Al 80s before that. Knowing my SAC/RMV/SCR/whatever in PSI would have been very practical.