Average Gas Consumption

[scubadada]

@Dan

Both SAC and RMV are adjusted for surface consumption using average depth/atm of your dive, right? Take a look at the calculations Average Gas Consumption

 

[Joneill]

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.

You’re doing more than fine

 

[Dan]

@Dan

Both SAC and RMV are adjusted for surface consumption using average depth/atm of your dive, right?

The SAC does. The RMV is just converting the psi/min (bar/min) to cfm (L/min) using the tank factor. So they use the same basis.

 

[scubadada]

The SAC does. The RMV is just converting the psi/min (bar/min) to cfm (L/min) using the tank factor. So they use the same basis.

So, both are surface consumption; SAC is pressure/time/atm and RMV is volume/time/atm, correct? Converting SAC to RMV only requires adjustment for the cylinder characteristics. SAC is cylinder dependent and RMV is cylinder independent. RMV is valuable for calculation of gas requirements using any cylinder(s) at any depth.

 

[Dan]

So, both are surface consumption; SAC is pressure/time/atm and RMV is volume/time/atm, correct? Converting SAC to RMV only requires adjustment for the cylinder characteristics. SAC is cylinder dependent and RMV is cylinder independent. RMV is valuable for calculation of gas requirements using any cylinder(s) at any depth.

Agree

 

[inquis]

Memorized values of what?
Stress is different than air consumption

Values of SAC translated from average RVM for the tank in use.
For instance when using a single AL80, if displayed SAC is over 19 psi/min (per atm), that is feedback of above average consumption for me. (That corresponds to an RMV of 0.5 cuft/min.)

Sure, stress is different, but certainly correlated (especially so at less than normal kicking effort). It's like having a perceptive buddy that asks, "Are you OK?"

 

[inquis]

but rather displayed psi/min or cuft/min AT DEPTH, so you can more easily see that you are using gas up faster if you are deep

With Gas Time Remaining available, I don't see the utility of non-normalized (depth dependent) air consumption.

 

[tursiops]

With Gas Time Remaining available, I don't see the utility of non-normalized (depth dependent) air consumption.

Agree. They should tell the diver the same thing.

 

[vatty]

I've some issue calculating my RMV, perhaps somebody can help me out.

My last dive night dive O/C (metrics)
Duration (min): 50
Depth (m): 16,1
Cylinder: Aluminum
Cylinder Volume (l): 12
Start Pressure (bar): 160
End Pressure (bar): 85
Average Depth (m): 9.8

For me it is easy to calculate my SAC, based on the following formula I learned.
(Start Pressure - End Pressure) * Cylinder Volume / (time * Pressure) = SAC

To make my calculation a little easier, I'll use an average depth of 10, so that my pressure is 2.
(160-85) * 12 / (50 * 2) = 9.0 SAC

What I understand from RMV, it is calculated based on:
(Start Pressure - End Pressure) / time * Cylinder Volume = RMV
(160-85) / 50 * 12 = 18.0 RMV

So then based on the first post:
0.6-0.69 cu ft/min = 17.0-19.5 l/min

I use 0.6-068 cu ft/min?

 

[tursiops]

I've some issue calculating my RMV, perhaps somebody can help me out.

My last dive night dive O/C (metrics)
Duration (min): 50
Depth (m): 16,1
Cylinder: Aluminum
Cylinder Volume (l): 12
Start Pressure (bar): 160
End Pressure (bar): 85
Average Depth (m): 9.8

For me it is easy to calculate my SAC, based on the following formulate I learned.
(Start Pressure - End Pressure) * Cylinder Volume / (time * Pressure) = SAC

To make my calculation a little easier, I'll use an average depth of 10, so that my pressure is 2.

(160-85) * 12 / (50 * 2) = 9.0 SAC

What I understand from RMV, it is calculated based on:
(Start Pressure - End Pressure) / time * Cylinder Volume = RMV
(160-85) / 50 * 12 = 18.0 RMV

So then based on the first post:
0.6-0.69 cu ft/min = 17.0-19.5 l/min

I use 0.6-068 cu ft/min?

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.