Subsurface

[tursiops]

i think that i have thought about it to where i confused myself somewhat. Ok i understand that breathing a particular volume will register different pressures on different tanks. 5cuft of air is going to be different psi in an 80 vs my double 117's. still, i think that knowing the sac for the tank would be nice so that you can just multiply the rate against the atmosphere you are in so that you could generalize time and pressure use.

Nothing wrong with that....just be sure you use the tank-specific psi/min number, whatever it is called!

 

[GlennL]

You can do that. Take your SAC rate (measured in cuf/min), divide it by the tank's baseline, then multiply that by how many atmospheres you are diving at. This will give you your psi/minute at depth.

I think we are starting to come together some. Doing the same thing just different ways. The way I was thinking involves knowing tank specific sac rates. I guess the "standard" way would involve knowing the tanks baseline (more math involved though).
Six in one hand a half dozen in the other.

 

[doctormike]

I think we are starting to come together some. Doing the same thing just different ways. The way I was thinking involves knowing tank specific sac rates. I guess the "standard" way would involve knowing the tanks baseline (more math involved though).
Six in one hand a half dozen in the other.

Yeah, if you always use the same tank, then I guess it doesn't matter. The problem would be that learning it that way may be a problem if someday you get a different tank, or if you ever advance your dive training.

 

[GlennL]

Yeah but aren't the calculations going to change anyway because changing tanks changes its baseline anyway? Not trying to be difficult, just trying to learn.
And part of what started my inquiry about subsurface is that I noticed oceanlog lists both sac as psi/min and rmv as cuft/min

 

[stuartv]

RMV: Respiratory Minute Volume. This is the volume of gas that a given diver actually moves in and out of their lungs in a minute at any point in the dive. It is dependent on the diver and on depth.

No, it is not dependent on depth.

Nothing in "RMV" per se requires it to be at the surface.... RMV is ambiguous

Right. Nothing requires it to be at the surface - because it doesn't matter what depth you are at. Therefore, it is completely unambiguous.

If I breathe in and out 6 times in 1 minute and that results in me cycling 0.5 cu-ft of air through my lungs, then I am going to cycle 0.5 cu-ft of air whether I am sitting at my desk or whether I am hovering above a reef at 33' depth. Either way, my RMV is 0.5 cu-ft/min. Depth doesn't matter and the measurement is completely unambiguous.

The only thing that depth matters for is if you measure the RMV at one pressure and you want to figure out what volume of gas that would translate to at a different pressure.

For example, if you cycle through 0.5 cu-ft of air in 1 minute while you are at 33' depth and you want to know how much gas you have left in your tank. If it was a full 100 cu-ft tank, that means the gas inside would occupy a volume of 100 cu-ft if allowed to expand until it was at 1 ATA. At 33', the ambient pressure is 2 ATA, so the gas in your tank would only expand to 50 cu-ft of volume. You used 0.5 cu-ft, so you just used 1%. You have 49.5 cu-ft left at your current depth. If you ascend to the surface, you would have 99 cu-ft left.

Depth determines the gas pressure. The pressure, the gas composition, and the volume determine the mass. So, the mass of the gas you breathe is dependent on the depth. The volume is not dependent on depth. Thus, Respiratory Minute Volume does not depend on depth - only time and volume (lung tidal volume and breathing rate) - and is unambiguous.

OTOH, SAC is very ambiguous because the number of psi per minute is complete meaningless without also stating the cylinder water volume and working pressure - or the cylinder's surface air volume - you are using.

Anyway, IIRC, my TDI manuals defined RMV and SAC the way I use them.

Also, if there are two common terms used, where one is for volume per minute and the other is pressure per minute, and one of the terms is unambiguous that it refers to volume per minute (Respiratory Minute Volume), and the other is ambiguous regarding whether it is a measurement of pressure or volume, then it seems to me to make the most sense to use the unambiguous term for what it says it is and take the other term to mean the other thing. But, that's just me and obviously not everyone agrees. I do wish I could find a sensible explanation of a reason to do it some other way, though (where, "because that's how I was taught," would not be considered a sensible explanation).

 

[stuartv]

Yeah, if you always use the same tank, then I guess it doesn't matter. The problem would be that learning it that way may be a problem if someday you get a different tank, or if you ever advance your dive training.

The only real use for SAC, that I know of, is estimating how long until get from pressure A to pressure B.

To do that, you have to know your tank-specific SAC, as well as the ambient pressure.

If you use RMV to do the same thing, you have to know your RMV, which is the same no matter what tank you are using, and the ambient pressure.

I find it easier (for me) to just know my RMV and use it against any tank I happen to be using for calculating run times. Doing math underwater is always (for me) going to be a process of approximations and rounding - not working anything out to 3 significant figures - so it's not that hard. E.g. An AL80 is really 77 cu-ft, so 1/3 of a tank is roughly 25 cu-ft. 1/3 of tank is also 1000psi. So, 500 psi is roughly 12.5 cu-ft. Math based on a good RMV of 0.5 is also pretty easy. And if I want to estimate based on a stressed RMV, 1.0 is a fair number to use and easy to work with. And, if I want a more realistic actual number (based on an RMV of 0.7), I do the math for 1.0 and then take 3/4 of the result. Or add 1/3 (multiply times 4/3) depending on what I am calculating.

 

[Lorenzoid]

. . .
If I breathe in and out 6 times in 1 minute and that results in me cycling 0.5 cu-ft of air through my lungs, then I am going to cycle 0.5 cu-ft of air whether I am sitting at my desk or whether I am hovering above a reef at 33' depth.

If you breathe in and out 6 times in 1 minute and that results in you cycling 0.5 cf of air through your lungs at your desk, then doesn't that mean you would cycle 1.0 cf of air through your lungs at 33' depth?

 

[stuartv]

If you read my post, you see the standard definitions that I use, so if your definition of RMV is gas consumption at the surface, then I guess it wouldn't be.

Respiratory minute volume is a medical term. It means the volume of gas that you move in and out of your lungs in a minute. How do you figure that is not dependent on depth?

I explained that. My lungs hold the same volume no matter what depth I am at. If I'm at 33' depth and I cycle 0.5 cu-ft of 2ATA air, it's still only 0.5 cu-ft (at 2ATA).

 

[stuartv]

If you breathe in and out 6 times in 1 minute and that results in you cycling 0.5 cf of air through your lungs at your desk, then doesn't that mean you would cycle 1.0 cf of air through your lungs at 33' depth?

It means you would cycle 1.0 of surface-corrected air volume. It's still only 0.5 cu-ft of air at 33'.

 

[doctormike]

If I breathe in and out 6 times in 1 minute and that results in me cycling 0.5 cu-ft of air through my lungs, then I am going to cycle 0.5 cu-ft of air whether I am sitting at my desk or whether I am hovering above a reef at 33' depth. Either way, my RMV is 0.5 cu-ft/min.

I think that the point of confusion is that the term CUF that we use to measure gas volumes in the imperial system is defined as a physical volume at sea level ambient pressure. So an AL80, which is clearly smaller than 80 cubic feet, is said to contain 80 CUF because when you fill it to service pressure, that amount of gas molecules would expand to fill an 80 CUF space at the surface. So when we calculate gas consumption at depth, we account for that by keeping track of how much of the gas is used with a volume that only makes sense in the context of sea level. Since we can't measure molecules of gas that are being consumed, that's how the calculations are done, as far as I can tell.

When I plan a dive and multi-deco tells me that I need 60 CUF of bailout, that's a meaningful percentage of my tank pressure that I can use in dive planning. I do see what Stuart is saying - the physical volume of your lungs doesn't change at depth. But since we aren't measuring molecules, we do the equivalent of measuring volumes by using language that is based on physical volume at the surface. Again, a different way of conceptualizing the same concept.