Air consumption?

[erikp]

Well, it's still 14.7 psi, but . . .

. . . the cubic foot per minute VOLUME has changed.

the K

I think you might be able to change the psi if you had a really long pipe and were able to exhale outside of the atmosphere? Maybe over the course of a few million years?

 

[MXGratefulDiver]

I rather enjoy this discourse, don't think it's pointless bickering. If I have offended anyone with "attitude," I apologize.

"Discourse" like this is why so many divers think gas management is hard.

From a "scientist" perspective, it's interesting. From a diver's perspective, it's pointless bickering ... because it has no relevence in how the terms are applied to scuba diving. Rather than allow those of us who could answer the OP's question and create that relevence for those who come here to learn these topics you persist in applying a nit-picking mentality that completely clouds the issue, and makes those who could otherwise learn something useful click away to another topic.

So what did you prove ... that you're somehow smarter than everyone else?

Whoop-de-bloody-do.

Scuba diving is not an exact science ... never was meant to be. Concepts matter ... because at least half of what keeps you safe is your mental approach to your dive. Gas management is one of the most important concepts that never gets talked about in most scuba classes.

Thanks for your participation in hijacking what could have been an important topic and turning it into a completely irrelevent "discourse".

... Bob (Grateful Diver)

 

[mts0628]

It really doesn't matter to me . . . I just like to argue sometimes!

Damn The Kraken.

 

[mts0628]

...Just because you read it in a book doesn't mean it's true...
I rather enjoy this discourse, don't think it's pointless bickering. If I have offended anyone with "attitude," I apologize.

Oh Doc Harry: where were you during my Biochemistry classes. I would have loved to toss that comment to Dr. P-E to account for my poor marks.

I also hope I have not offended anyone by my posts; if I have then I too am sorry.

"Discourse" like this is why so many divers think gas management is hard.

From a "scientist" perspective, it's interesting. From a diver's perspective, it's pointless bickering ... because it has no relevence in how the terms are applied to scuba diving. Rather than allow those of us who could answer the OP's question and create that relevence for those who come here to learn these topics you persist in applying a nit-picking mentality that completely clouds the issue, and makes those who could otherwise learn something useful click away to another topic.

So what did you prove ... that you're somehow smarter than everyone else?

Whoop-de-bloody-do.

Scuba diving is not an exact science ... never was meant to be. Concepts matter ... because at least half of what keeps you safe is your mental approach to your dive. Gas management is one of the most important concepts that never gets talked about in most scuba classes.

Thanks for your participation in hijacking what could have been an important topic and turning it into a completely irrelevent "discourse".

... Bob (Grateful Diver)

Yeah, on that last note, re-reading all that has been said, I kinda' feel like what I said and maybe how I said it was definitely overkill, especially from the OP's perspective.

So a couple of things learned by me, never challenge The Kraken and never bite a dangling carrot (or seaweed) offered by The Kraken D); more seriously, no matter how right we may think we are, that occasionally we must step back and evaluate if what we have to offer to an ongoing discussion will really benefit the original question (especially in the Basic Scuba Discussions), and finally not learned, but I apologize to the OP, Lorie, for what may seem like eye-glassing reading simply over units of measurement. Sorry Lorie. And for what it's worth, I think The Kraken's first post was pretty reasonable as far as volumes and should help you (but admittedly I didn't bother to check his calculations ).

With kindest regards and thanks to all,
Thomas

 

[Thalassamania]

With all due respect to the Kraken, Surface Air Consumption rate is reported in pounds per square inch per minute, while Respiratory Minute Volume is reported in cubic feet per minute. Substitute RMV for SAC in the above, and all is good. And as a good starting point, an average RMV is about 0.75 Ft³/minute.

Maybe in your world, but clearly not in everyone's. SAC for me is (and has been for half a century) Cubic Ft / min. The problem with PSI/min is that it is not a stand-alone figure, it is tank dependent.

 

[Thalassamania]

Hello Kraken:

My post was not to ruffle feathers, er scales, but to just correct the nomenclature and units of measurement.

So during your Class A physical, I no doubt believe that they reported your consumption of air in ft³/min. What they were reporting was the RMV. Where do you think those hoses were going to? As scientists, precision and accuracy would be paramount so having a measurable gas source attached to the hose would ensure that a proper SAC could be computed and then computing the cylinder constant, an RMV could be reported. This is the most obvious to me but there is probably another way.

Now I know little about aircraft life support, but I would imagine that the RMV would be handy to know if you were flying in a pressurized aircraft and you were required to be breathing off a tank of gas, probably oxygen. Now if you were a fat body or just out of shape, then they would not want to put you in an aircraft that you could suck a standard tank in to a vacuum because you were a gas hog, and thereby jeopardize the mission. I would also venture to say that the tanks on aircraft would be standard, with the exception of mission dependency (and not tailored to each pilot). Again, I know little about aircraft life support.

So, after one of my many Halo/Scuba physicals (where they don't bother with the treadmill and RMV- which I find kinda' odd, but I could never get a Doc to defer the rectal- which I found, "uncomfortable" to say the least), my first hardcore introduction to gas formulas and equations was at the Trumbo Point Naval Annex in Key West, were we had, I say diving aficionados to say the least, as instructors. Nomenclature was drilled into our heads as was a lot of other seemingly now useful information.

Years later, I had the opportunity to participate in scientific diver training- where there is no certification (for the watchful eye of one of my friends ) I was reintroduced to the concepts. Our bible was the NOAA Diving Manual, 4th ed., and Air Consumption Rates started on page 8-11. On page 8-14, there is a simple equation for converting the SAC to RMV. But the moral is, SAC is measured in psi/minute, whereas the RMV is measured and reported in ft³/minute. Evaluating the equation:

RMV=SAC*k, where k is reported as ft³/psi.​

Now let's look at how the measurements march out:

psi/minute*ft³/psi, the psi's in the equation reduce out, leaving ft³/minute- our RMV.​

So, SAC is independent of cylinder constants. It doesn't matter what size of cylinder you're breathing from, you can still compute your SAC [difference in psi/((depth in feet+33)/33). But, in order to compute your RMV, we need to know the cylinder constant, which can be whipped up in short order [cylinder volume/working pressure]. Then we report, "my RMV is 0.6ft³/minute", and mine really is, but I know I can do better.

And finally, NOAA actually has a neat .pdf that has most of the formulas that we could use. This is a bit more organized compared to what I have written, as I do not know if MS Equation Editor is compatible with posting. But I will say, just be thankful you don't have to read my handwriting!

With kind regards,
Thomas

The next edition of the NOAA Manual is in prep right now, I'll have to see who's doing that section and make sure that gets fixed.

 

[The Kraken]

I've enjoyed it too!!!

I generally try to answer the OP's question first, to the best of my ability, and then expand the discourse.

I'm sure the discussions regarding SAC and RMV will continue to pop up from time to time, but, hey, that's the nature of things.

In the end, plan your dive and dive your plan.

Safe dives . . . . . . . .
. . . safer ascents ! ! !

the K

 

[The Kraken]

But the major point here is that a new member has come to the board to ask a question that is very relevent to scuba diving, especially for those of a more advanced level.

I think this is a good thing and that we should welcome members with the greatest accord who come to us with such questions.

Never let us forget that we were, once upon a time (and some of us, long, long ago) were new divers.

Share the knowledge, but always endeavour to help educate the new diver in how to obtain or deduce that knowledge.

We are all still learning . . . at least I hope so.

the K

 

[Doc Harry]

Maybe in your world, but clearly not in everyone's. SAC for me is (and has been for half a century) Cubic Ft / min. The problem with PSI/min is that it is not a stand-alone figure, it is tank dependent.

You are indeed correct.

I think it is unfortunate that some people are confusing the issue by introducing terms such as RMV that have no practical application to scuba.

To the OP, the beauty of SAC is that incorporates all of your gas use, habits and cylinders into one portable, stand-alone number.

If you keep good records of your dives (starting PSI, ending PSI, cylinder and average depth) you can calculate an average SAC for that dive. Over a series of dives, you can then come up with an average SAC for gas planning purposes.

Again, sorry if I ruffled some feathers. But this is all about a very important concept of gas consumption and gas planning. It doesn't help the OP when people hijack the thread by introducing irrelevent concepts such as RMV.

 

[N² - O² diver]

Could someone explain my the "AL80" and "HP 100" and others to describe some measurments.
A have never heard it before.
Maybe because i do not speak or write englisch that often or it just not metric.

I'll see what the answers bring. And i have seen allot of questions and answers that i have never seen in my'n course ( in general ) , so defenitely a good preperation for the idc.

greetz