The Kraken
He Who Glows in the Dark Waters (ADVISOR)
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OK, still waiting for an answer . . . .
the K
the K
Air consumption?
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While mts0628 is technically correct, let's do keep in mind that our audience here is divers ... not engineering students.
When I teach the seminar that the article you referenced was written for, I am careful to explain that in real world terms, SAC and RMV are simply two different ways of expressing the same value. They are both needful for gas management, and the terms applied to each are arbitrarily assigned more to prevent confusion than out of any actual difference in value. What you call RMV (in CF/M) is useful for dive planning because we know the volume of our cylinder, while what you call SAC (in PSI/M) is useful for dive execution because our SPG is the only real feedback we'll have available to keep track our actual consumption.
In point of fact, using PSI/M as a starting point for calculating how much gas you will need for a dive (the original question, remember?) only adds needless steps to the calculation ... because it is dependent on the size of the cylinder you're using. And, in fact, using it for gas calculation won't tell you how MUCH gas you'll need for the dive ... it will only tell you how much pressure you will need in a specific size cylinder.
In other words, 25 psi/minute will mean something very different for a low pressure 95 than the same 25 psi/minute will mean for a standard AL80 ... because the volume of gas represented by a psi of pressure in each cylinder will be very different.
Bottom line ... if you want to know how much gas you'll consume for a given dive profile (the original question, remember?), it's better to calculate in volume, since the volume of the cylinder you're using is a known quantity. You CAN, of course, perform the same calculation for PSI/minute ... but you'll first have to work out the CF/PSI conversion for the cylinder you're planning to use.
Personally, I prefer simpler ...
... Bob (Grateful Diver)
nepolean77
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I do not do the maths as it depends on the tank you can dive that long on one tank so just watch your oxygen meter no need to make diving complicated its what scares people off.
where did I put that damn oxygen meter?
Not entirely correct..... And I don't care what the NOAA manual says.
True, both SAC and RMV have the same units (volume/unit time). So it can be said that since both reflect volume per unit time (e.g., cubic feet per minute), they "are simply two different ways of expressing the same value."
BUT....Technically speaking, RMV and SAC are not interchangable terms. SAC and RMV may share the same final unit (volume/unit time), but their derivations are entirely different.
RMV is simply the volume of gas passed through the lungs in a minute (hence, respiratory minute volume, RMV). RMV is usually measured in a closed circuit and is a direct measurement of respiratory volume.
SAC, on the other hand, is the pressure drop of a cylinder over a period of time, at depth, on either a closed or open circuit, corrected to 1 atmosphere of pressure, corrected for time to one minute, and corrected for the cylinder volume and rated pressure that changes the numerator from pressure to volume.
One reason RMV and SAC are technically not interchangable is because pressure drops due to BCD inflation, drysuit inflation and regulator leaks may included in the SAC calculation, but there are no such unaccounted losses in RMV measurements.
In other words, SAC calculations may or may not include gas consumed in functions other than respiration. This is a distinctly different beast than RMV.
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English is a very contextual language.
Since we're responding to a question that was posted on a scuba diving forum, one can safely assume that the context of the question pertains to how divers use the terminology ... not how medical or engineering students would use it.
The author of this thread asked a very simple question ... "How much air would you use on a dive at say 30' for 40 mins? " Technical explanations of the nomenclature don't answer the question, but instead just confuse the issue.
From a diving perspective, SAC and RMV are, in fact, interchangeable. Otherwise you'd have no practical way to make the measurement (i.e. by reading the pressure drop on your SPG over a period of time), or apply it to gas planning (i.e. answering the question "do I have enough air available to make this dive") ... since one is expressed in pressure and the other in volume. The conversion is rather simple, and is described clearly in the article muddiver linked to previously ...
RMV = SAC x baseline, where
baseline = cylinder volume (in CF)/cylinder working pressure (in psi)
As I said, I prefer simple.
One thing I've learned from writing technical manuals for the past 32 years is that you must always ... ALWAYS ... consider your audience. Otherwise you'll end up presenting a lot of technically accurate, but completely useless, information. That's what's happening in this thread. You guys are so hung up on nomenclature that ... for all intents and purposes ... you've forgotten to answer the OP's question. For all intents and purposes, you're arguing about the best way to derive "miles per gallon", when all the user wants to know is how much gas he needs for the trip.
No wonder so many people think this stuff is complicated ...
... Bob (Grateful Diver)
Yes, I agree totally! Language is contextual. That's why it is important to use SAC and not RMV in scuba.
Yes, so many people make it complicated by teaching terminology and methods that don't really apply to scuba.
The OP's question was a good one - because it brings up the whole topic of communicating gas consumption in a common language - SAC and cubic feet (or cubic liters), not RMV or PSI.
Harry
vladimir
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Since the thread has devolved into pointless nitpicking, what's a cubic liter?
The average here is 14.7 psi, or 1 ATM, or 1 BAR with slight variations for barrometric pressure caused by weather, the 1200 foot above sea level altitude I am at, the air handeling system kicking on, etc......
Ok, now I kind of agree with you. What you followed up with makes more sense.
Since the thread has devolved into pointless nitpicking, what's a cubic liter?
The amount of beer in a liquid form that the standard issue Marine, either American or British, will comsume between long winded speaches about their personal exploits around the globe.
mts0628
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Hey Kraken:
Geez, I was not ignoring you. I am sure this will be good as antsy as you seem but sure, I'll bite.
So, sitting on the couch at my computer, I am breathing at atmospheric pressure, 14.7 psi. Now I am patiently waiting for the bang.
So, Doc Harry, first let me ask, are you a DMO or a Pulmonologist? And even if so, please cite references so that I may be enlightened, especially on the order of units for SAC and RMV. Don't worry about the availability of references, as I have a pretty useful university library and medical school (with a library) about 15 minutes away.
Now I do agree with some of your descriptions. In medicine and as you stated, "RMV is simply the volume of gas passed through the lungs in a minute ". No doubt, and more specifically the tidal volume multiplied by breaths per minute is the RMV. In scuba diving, RMV is equal to the SAC multiplied by the cylinder constant.
Now you said this: "SAC, on the other hand, is the pressure drop of a cylinder over a period of time, at depth, on either a closed or open circuit, corrected to 1 atmosphere of pressure, corrected for time to one minute, and corrected for the cylinder volume and rated pressure that changes the numerator from pressure to volume." I agree with the underlined but then as you continue, you incorporate the cylinder constant into your definition so that the units of SAC and RMV are in fact the same. I think this is where we differ but where I have cited the NOAA Diving Manual, I inferred by you that this is not a valid reference [I assume on this point for now]. Also, and I think the most important for me, is to see a reference to SAC [from a textbook], and as it relates to scuba diving, measured in cfm.
And finally- I think, on these points, "One reason RMV and SAC are technically not interchangable is because pressure drops due to BCD inflation, drysuit inflation and regulator leaks may included in the SAC calculation, but there are no such unaccounted losses in RMV measurements.
In other words, SAC calculations may or may not include gas consumed in functions other than respiration. This is a distinctly different beast than RMV."
I agree with this, with the caveat that the RMV and SAC do not have the same units. I mostly agree that if the RMV was calculated by tidal volume multiplied by breaths per minute it maybe more accurate; in contrast to computing the SAC and then multiplying it by k, but during the activity, shouldn't all activity be performed at depth and a specific time so that extraneous gas use is not an issue. But even if not, wouldn't gas use not pertaining to respiratory function but included into the calculations, yield a more real world RMV?
Whew!
With kind regards,
Thomas
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