Subsurface
- Thread starter GlennL
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The volume of gas per minute (note in English and science we say volume per minute, not minute volume which should mean minute times volume) is assumed to be constant, regardless if depth. However, the amount of gas (in mol) or mass of gas (kg) consumed varies with depth. The volume of your lungs stays the same.
Respiratory minute volume sounds like garbled up words. "Respiratory volume per minute" would at least make that clear. But I'll stick with gas consumption rate.
You don't find RMV to be useful?
I think it's a pretty useful concept to recognize that an HP100, which has 100 cu-ft of gas at the surface, can only give you 50 cu-ft of gas at 33'. Every diver should understand that, regardless of any discussion of RMV or SAC or SCR or anything else.
And if you recognize that your HP100 only has 50 cu-ft of gas at 33', than an RMV of 0.5 cu-ft/min is easy to apply to determine how long you can breathe that tank at 33'.
All we're really talking about is the concept of where you apply the ambient pressure factor in your calculations. Do you multiply RMV times pressure? Or do you divide tank surface volume by pressure? You get the same answer, either way.
But, it is clearly understood that Imperial tank volumes are given in surface-corrected units. An HP100 obviously does not have an internal volume of 100 cu-ft. That number is obviously "corrected to 1 ATA". It is also obvious that your actual lung internal volume does not change at depth. So, why mix concepts around? Why try to make RMV be a "surface-corrected" concept when it is not? Why not keep the "surface-corrected" concept strictly tied to Imperial tank measurements. Especially since it doesn't even apply to metric tank measurements, which are given in water volume and working pressure. RMV is the volume you cycle in 1 minute. Period. Your lungs don't change size, so that volume is the same whether the gas is at 1 ATA or 2 ATA or some other pressure. If you're trying to take an RMV and an amount of time and work to find some measurement that applies to an Imperial tank (e.g. time until the tank is empty), then apply the ambient pressure there (to the amount of gas in the tank), since the tank volume (of an Imperial tank) is inherently a surface-corrected measurement anyway.
No, I mean I don't find the concept of using the fact that lung volume remains constant to define the term "RMV" to be useful. One can call what you call "RMV" whatever one likes. I was taught to call it Surface Consumption Rate (SCR). The calculations are the same. The calculations are useful.
If I define RMV to be the physical volume you cycle in one minute (thus, depth-independent) and tell you mine is 0.5 cu-ft per minute, how would that make that number (or the concept or definition) any less useful to you than if I told you it is 0.5 cu-ft per minute at 1ATA?
I don't think it does make it any less useful. Thus, adding "at 1ATA" (i.e. adding "surface correction") to the concept is extra clutter for no purpose - other than to make it harder for new divers to understand.
You are conceptualizing gas planning in a way that I never have before. That's not to say that it's wrong, it's just not what I learned in TDI. Always happy to learn new things, but this seems to make it more complex and difficult to do.
My SAC rate is 0.5
I dive to 100 feet (4 ATA), stay for 10 minutes
Then I stay at 66 feet (3 ATA) for 10 minutes
Then I'm at 33 feet (2 ATA) for 30 minutes
My gas usage is (4 x 10 x 0.5) + (3 x 10 x 0.5) + (2 x 30 x 0.5) = 20 + 15 + 30 = 65 CUF.
I have an AL100. OK? Yup, with rule of thirds, that's OK.
If you do it the other way, you are calculating the tank volume at every leg? Which then changes as the tank is used? By doing it this way, using a tank content measure that is scaled to a standard ambient pressure (surface), you can just add up the volumes.
Carry on.
when wrestling with a pig in the mud, you have to realize, at some point of time, that the pig just might be enjoying itself.....
OP
GlennL
Contributor
Whoooo buddy... my brain hurts. Lol. Anyway
Let me see if I got this right.
Stuart, you are saying to divide tank volume by atmosphere? So at 100ft a 100cuft tank is going to have 25cuft. An rmv of .5cuft would give 50 minutes of usage?
Doctormike is saying that at 100 ft you would take your rmv and multiply by atmosphere and then divide a the given volume by that number to find time? 100/(.5x4)= 50 minutes.
Same thing just different concepts. I think most people think in the latter terms.
Am I tracking?
Let me see if I got this right.
Stuart, you are saying to divide tank volume by atmosphere? So at 100ft a 100cuft tank is going to have 25cuft. An rmv of .5cuft would give 50 minutes of usage?
Doctormike is saying that at 100 ft you would take your rmv and multiply by atmosphere and then divide a the given volume by that number to find time? 100/(.5x4)= 50 minutes.
Same thing just different concepts. I think most people think in the latter terms.
Am I tracking?
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