Average Gas Consumption

[Blackcrusader]

@rsingler has previously told us what the lower limit of RMV is, but I cannot find it. I believe this was from ventilating patients under anesthesia. Perhaps he can explain that to us again.

I believe it is around 5l/min.

 

[ATJ]

I've mentioned it before but I'll mention it again as Dirk has raised it again. Not accounting for the non-linear compression of gas is a big issue, especially for me as I usually use 300bar tanks (filled to ~300 bar).

Below are the data for my last 10 dives. The first two columns are from Subsurface using the appropriate calculations for compressibility. The third column are the data for the same dives but using a linear calculation. F120 is a 300bar 12L tanks and F100 is a 232bar 12.2L tank. The biggest difference is the 5th dive in the list. My starting pressure was 306 and ending pressure was 161bar.

 

[dirkhh]

Not accounting for the non-linear compression of gas is a big issue, especially for me as I usually use 300bar tanks (filled to ~300 bar).

Yeah. Not the 3% difference that some people think this might make. More like 25%.
Thanks for sharing those data.

 

[scubadada]

Hi @dirkhh

If you use the true gas capacity of a cylinder rather than the ideal gas capacity, is the calculated RMV more accurate?

 

[dirkhh]

Hi @dirkhh

If you use the true gas capacity of a cylinder rather than the ideal gas capacity, is the calculated RMV more accurate?

I wonder if that's a trick question...
Assuming you want the RMV to actually reflect, you know, reality, the only way to do that that I am aware of is to use the true gas capacity (adjusted for the gas you are actually breathing, as different gasses have different compressibility) in your cylinder at the beginning and end of your dive. In both cases calculating the actual gas content of the cylinder at the given pressure. The difference is the amount of gas that you breathed on the dive. And as the examples a few posts up have shown, that difference can be quite significant.

And, btw, this is why the often suggested "psi per minute" definition of gas consumption during a dive is odd to me. Because depending on the current pressure in your cylinder, the amount of gas that is implied by a one psi drop in pressure is different...

 

[scubadada]

Hi @dirkhh

I appreciate the information you provide.

If you were diving an AL80 filled with air to 3000 psi and used 2500 psi over 60 minutes, at an average depth of 60 feet, what would Subsurface calculate as the RMV?

Doing the manual calculation twice, first with the true air capacity, and the second with the ideal air capacity, I get RMVs of 0.383 and 0.395 cubic feet per minute.

I, almost exclusively, dive rental AL80s, and am wondering how far off my RMV calculations are.

Most appreciatively, Craig

 

[GJC]

Hi @dirkhh

I appreciate the information you provide.

If you were diving an AL80 filled with air to 3000 psi and used 2500 psi over 60 minutes, at an average depth of 60 feet, what would Subsurface calculate as the RMV?

Doing the manual calculation twice, first with the true air capacity, and the second with the ideal air capacity, I get RMVs of 0.383 and 0.395 cubic feet per minute.

I, almost exclusively, dive rental AL80s, and am wondering how far off my RMV calculations are.

Most appreciatively, Craig

And since you are looking for a very high level of precision, are you using 80 cu ft tank capacity for your calculation or 77.6?

Calculating SCUBA Cylinder Capacity | Dive Gear Express®

 

[scubadada]

And since you are looking for a very high level of precision, are you using 80 cu ft tank capacity for your calculation or 77.6?

Calculating SCUBA Cylinder Capacity | Dive Gear Express®

My two calculations were with 77.6 and 80.1 cu. ft.

 

[dirkhh]

My two calculations were with 77.6 and 80.1 cu. ft.

Actually, since all of these values are silly. What's the wet volume of the cylinder?
But for this specific example, the error is indeed rather small - assuming an 11.1l tank your SAC (in Subsurface terms) would be .38 cuft/min (we only provide reasonable accuracy to 2 digits - your pressure readings aren't at 3 digit precision...

 

[Divin'Papaw]

While this discussion is certainly interesting in reality SAC/RMV is primarily a planning tool and you just need enough accuracy to know if you should plan your gas needs based upon 0.40, 0.50, 0.60, 0.70, etc.

I calculate my gas needs off of 0.50 average and 0.80 max (super stressed). It has served me well so far.