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RMV stands for Respiratory Minute Volume. I don't see any other way to interpret that than as a measurement in volume per minute. E.g. cu-ft per minute. Expressing Respiratory Minute Volume as psi per min does not make sense (to me, of course), as psi is not a unit of volume.

That seems like the "right" answer for RMV (to me).

Yeah, I agree with Stuart.

This has always been something that bothered me, since it seems that critical language like this should have standard definitions. There SHOULD be a "right" answer. Yet I have seen the terms defined differently by different agencies. Here is how I think that the terms should be used:

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. In imperial units, cubic feet per minute.

SAC: Surface Air Consumption. This is how much gas a given diver consumes per minute at the surface. It is dependent only on the diver. In imperial units, cubic feet per minute.

SAC x depth in ATA = RMV

I don't know what to call PSI per minute. I guess it's a good thing to know for a particular dive when you are calculating turn pressures for yourself or something. But since it only makes sense in the context of a specific diver, a specific tank and a specific depth, it's not really a useful metric by itself, especially when planning team dives with different tanks, etc...

Baseline: A constant for a given tank, referred to as "K" above - Tank capacity (in CUF) divided by service pressure (PSI). Imperial untis are CUF/PSI

So the one number you really need to know is your SAC rate. Because with that, you can figure out how much gas you will use at any depth, and if you know your tank's baseline, then you can know how fast your SPG will drop.

I don't understand why this can't be standardized throughout the industry.
 
Nothing in "RMV" per se requires it to be at the surface. At least SAC says Surface! And nothing in SAC requires it to be a pressure. So SAC is more informative, and if used with units is unambiguous. RMV is ambiguous -- even with units -- without further explanation. Doctormike (above) is exactly correct: RMV is at the moment, at depth, and without knowing the depth cannot be related to a surface consumption rate.
 
. . .
I don't know what to call PSI per minute. . . .

RMP?
 
Nothing in "RMV" per se requires it to be at the surface. At least SAC says Surface!

RMV, SAC, SCR (Surface Consumption Rate) and SGC (Surface Gas Consumption) have been used interchangeably in the NAUI and SSI A/OW classes ive been involved with as resting air consumption rate at the surface. sometimes there's a SAC -> PSI, and RMV -> CFM differentiation, but it doesn't seem to be a standard. :shrug:

DCR (Depth Consumption Rate) seems to be rarely used as a standard term, but its definitely not ambiguous if you're talking about at surface or at depth. personally i use SCR/SAC and DCR in cubic feet/min, PSI per minute -> PSI per 5 minute only as an intermediate step in gas planning without a defining term.
 
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I mentioned in my original post that my personal view is sac= psi/min and rmv=cuft/min.
I guess my understanding of the way that the computer logs the consumption was skewed. I thought that it kept multiple data samples in memory rather than just using the overall time and volume used.
also, how is sac cylinder specific??? I understand that it is cylinder specific when performing the calculations to determine the rate, but how much I breathe is how much I breathe regardless of cylinder.
I am just now starting to get more involved with actual gas planning so forgive me if i am off base. I considered sac (psi/min) important for general calculations underwater. as an example say I have a 15 psi/min sac. if i am diving and want to achieve something in particular then i can generalize how much time i have to do whatever it is i am doing. so if i were to dive to say 70 ft i could guestimate using 45 psi/min. if i only wanted to burn 250 psi performing a certain task then i could then say i have about 5 and a half minutes to do so.
Is my thinking about this wrong?
 
I mentioned in my original post that my personal view is sac= psi/min and rmv=cuft/min.
I guess my understanding of the way that the computer logs the consumption was skewed. I thought that it kept multiple data samples in memory rather than just using the overall time and volume used.
also, how is sac cylinder specific??? I understand that it is cylinder specific when performing the calculations to determine the rate, but how much I breathe is how much I breathe regardless of cylinder.
I am just now starting to get more involved with actual gas planning so forgive me if i am off base. I considered sac (psi/min) important for general calculations underwater. as an example say I have a 15 psi/min sac. if i am diving and want to achieve something in particular then i can generalize how much time i have to do whatever it is i am doing. so if i were to dive to say 70 ft i could guestimate using 45 psi/min. if i only wanted to burn 250 psi performing a certain task then i could then say i have about 5 and a half minutes to do so.
Is my thinking about this wrong?

Again, this is why standard definitions are so important, as was the point of my last post.

SAC is cylinder specific IF you measure it in PSI per minute (as you said that you do). If you measure it in CUF per minute, then it isn't. So how much you breathe per minute is not affected by the size of the cylinder, but if you are using the PSI per minute for SAC then your SAC rate is clearly dependent on tank size.

Your SAC rate is 15 psi/min? Ok, lets assume an aluminum 80 with a service pressure of 3000. Baseline is 80/3000 = 0.027. So the 15 PSI that your SPG drops every minute on the surface is about 0.4 CUF (a good SAC rate!). Now breathe from an AL40 for a minute, and the SPG will drop 30 PSI, see..?

Just so you know, I have done a good bit of tech training from more than one agency, and I have never heard of anyone doing it like you describe. You know your SAC rate, you know your depth in ATA, so you know how many CUF you are using for each leg of the dive and ascent. The only time you need to convert these volumes into PSI changes is when you are figuring turn pressures in the planning stage, because you want a number that you can look for on your SPG without doing math in your head. But if you tell yourself that your SAC rate is 15 psi/min, and then you need to use a set of 119s instead of a set of 100s for some reason, that number is no longer accurate.
 
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.
 
I've always heard "SAC" used in the way it is used by subsurface. It seems natural and normal to me. RMV I have only heard about on scubaboard. Not to say that it isn't used in the real world by anyone, just that I've never heard anyone talking about RMV, only SAC. The descriptions that @doctormike posted are consistent with my understanding of the topic.

On a side note, I'm real glad this thread was posted. It lead to me doing some searching. I found some postings by Linus Torvalds that describe how subsurface accounts for variances in gas compressibility as pressures change. I like it even more now.

If only they would fix bluetooth support.. more and more new computers are using it.
 
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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.

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.
 
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