Beyond blind rote air management

[MXGratefulDiver]

Couple of problems with the article.

Actually it can't be described in psi because that varies with the tank capacity and working pressure. Volume is constant.

Nope. SAC is volume of air breathed at surface at rest per minute. RMV is the actual volume of air breathed per minute on a single dive. It is specific to that dive and varies depending on a rather large number of factors.

Here again you are confusing SAC with RMV. SAC is a number and is unrelated to air breathed during a dive. SAC is volume of air breathed at surface at rest per minute. SAC usually remains nearly the same for a diver over time, again varying somewhat with fitness and a number of other factors. However it doesn't vary nearly as much as RMV over a number of dives. RMV is dive related. SRMV (raw, average, and deep) are values that are calculated from RMV and depth (for average and deep) describing what the air used during the dive would equate to had it been breathed at the surface.

Sorry to be pedantic - but there is way more than enough confusion relating to SAC, RMV, and SRMV (Surface Respiratory Mninute Volume).

Actually what you've done is take a couple sentences out of context for the purpose of creating an argument about them ... and all it proves is that on ScubaBoard some people will find a way to create an argument out of anything.

FWIW - that article's made the rounds, pretty much worldwide, for about a decade now. So far you're the only person I've ever heard express any confusion about the terminology or how it's been used ...

... Bob (Grateful Diver)

---------- Post added April 16th, 2014 at 07:52 AM ----------

Actually, the fact is that a lot of people use the terms in different ways. You may have been taught one use of the term; others have been taught another. In the Dive Planning course I wrote, I talk about this confusion. I point out that SAC rate has come to mean different things because of the way it is used. A lot of people use SAC to mean what other people mean by RMV.

If a diver is always using the same sized tank, it is a lot easier for that diver to think of SAC in terms of PSI. A large percentage of divers have never even seen anything but an AL 80. There is nothing wrong with them thinking of SAC in PSI, as long as they know it will be different should they ever try a different sized tank.

The article explains that SAC, as an expression in PSI, is dependent on the size of the cylinder ... it also gives you a very simple formula that uses both tank size and working pressure to convert from SAC to RMV.

An important concept to consider is that looking at consumption in terms of pressure is useful for tracking actual vs predicted consumption during the dive ... since the only real-time feedback mechanism available to you is your pressure gauge.

Looking at the same consumption rate as an expression of volume (cubic feet per minute) is useful for dive planning, since you know the rated volume of your cylinder (assuming it's filled to its working pressure). Knowing your baseline, or tank factor, is useful for determining how much gas you actually have prior to the dive if it's not at working pressure ... as well as how much gas you actually used during the dive.

This is why it's important to establish terms for gas consumption rate as both pressure per minute and volume per minute, because they're used for different purposes ... the latter for dive planning, the former for tracking real-time use during the dive. What you call them is rather irrelevent, and I've heard many different terms used, both by agencies and individuals. That is why I specified definitions for the way I was using them in the article.

... Bob (Grateful Diver)

 

[Kharon]

Actually what you've done is take a couple sentences out of context for the purpose of creating an argument about them ...

No, actually I put my faith in published, reliable sources (Chapter 2 of the SDI Solo Diving Manual) rather than stuff posted on the internet where we all know "they can't put anything on the internet that isn't true". Nor did I take anything out of context. Your definitions do not agree with the established standard definitions. You are certainly entitled to your own opinion - but not your own facts.

 

[nimoh]

No, actually I put my faith in published, reliable sources (Chapter 2 of the SDI Solo Diving Manual) rather than stuff posted on the internet where we all know "they can't put anything on the internet that isn't true".

I prefer to read articles and then think for myself. I happen to like Bob's article on gas management ever since I read it more than a year ago. I would also argue that Bob is a reliable source (based largely on Scubaboard conversations), so what does a publisher add to an article that Bob is missing?

It seems to me that you are under the assumption that since untrue things can be posted on the internet, then everything on the internet is untrue (although this is probably an overstatement on my part, but I think you get the point).

I do agree with you that there is a lot of bad information on the internet, but there is a lot of good information too. Information coming from reliable sources does get more credibility but I think the quality of the information can stand on its own.

 

[Redshift]

No, actually I put my faith in published, reliable sources (Chapter 2 of the SDI Solo Diving Manual) rather than stuff posted on the internet where we all know "they can't put anything on the internet that isn't true". Nor did I take anything out of context. Your definitions do not agree with the established standard definitions. You are certainly entitled to your own opinion - but not your own facts.

I agree with you that the definition of SAC is in volume. That's because the atmospheric pressure doesn't change when we breathe, but if we breathe from a cylinder on the surface, then the air consumption can very well be measured in pressure. Why not? And if instead of "putting your faith" on what you read, you'd actually think and go through the explanation, you'd see that Bob's exercise makes sense and can simplify things.

And while this way of calculating gas consumption has its downsides (being dependent on cylinder volume and the possible initial confusion to those used to thinking in volume), the traditional way also has its problems and, in my view, can even be considered to be wrong. The problem is that volume is not a property of a gas, it's a property of the rigid container holding the gas. So when we say that a 10l cylinder filled at 200bar has 2000l of gas, it doesn't, it hasn't expanded, has it? It still has 10l of gas. And when we breathe from it, it will always have 10l of gas. What we do is we imagine the volume that gas would occupy if transferred to an expandable container at sea level. And when convert our sea-level air consumption to that at a certain dept by multiplying by pressure, again we are not really breathing that volume, that's the volume of an imaginary container at the surface (at least my lungs don't expand to allow me to breathe that much underwater...). We then subtract from the total "imaginary" volume in the cylinder and convert to pressure because what we have is a pressure gauge. Bob changed the reference point from being a volume of an imaginary container at sea level to the pressure of a specific cylinder. It's not hard to follow...
If we'd want to be really correct and not dependent on reference pressures or cylinder sizes, then we'd have to think in particles (or moles), but I have a feeling no one would want to do that

 

[mathauck0814]

No, actually I put my faith in published, reliable sources (Chapter 2 of the SDI Solo Diving Manual) rather than stuff posted on the internet where we all know "they can't put anything on the internet that isn't true". Nor did I take anything out of context. Your definitions do not agree with the established standard definitions. You are certainly entitled to your own opinion - but not your own facts.

So IANTD is an agency similar to SDI/TDI and their published works are aligned with Bob's explanation. Can you scan or link the pages that you have suggesting SAC is expressed in cubic feet rather than psig/min? Uploaded two pages from the IANTD Deep Diver Instructor manual concerning SAC/RMV calculation.

 

[MXGratefulDiver]

No, actually I put my faith in published, reliable sources (Chapter 2 of the SDI Solo Diving Manual) rather than stuff posted on the internet where we all know "they can't put anything on the internet that isn't true". Nor did I take anything out of context. Your definitions do not agree with the established standard definitions. You are certainly entitled to your own opinion - but not your own facts.

What you're failing to understand is that there are no established, standardized definitions for those terms ... which is why I clarified how I'm using them.

I'd encourage you to try reading more than one source (as I have) to get a better understanding of what I'm trying to tell you. Try looking at material from IANTD, NSS-CDS, NAUI, GUE ... even some of the older standards like Jeppesen's Advanced Sport Diver Manual.

There are no standard definitions ... and I'm not attempting to create them. I'm only clarifying how I'm using the terms in order to discuss the concepts talked about in the article.

I really do like the fact that you brought this topic up ... way too many people don't think about it, because it's notoriously deficient in most recreational dive training. However, I'd encourage you to not take a single source at face value, or assume that its use of terminology is in any way "industry standard". The scuba diving industry has few standards ... particularly with respect to gas management protocols. That's why I created that article in the first place.

What terminology you use doesn't matter once you're underwater ... but the application of the principles and concepts of managing your gas supply properly certainly do. If you're comfortable with the terms as you're using them, that's great. But realize that there's way more sources of information out there than the SDI Solo Diving Manual, and they don't all use those terms in anything resembling a consistent manner ... which is why I felt it necessary to define how I was using them.

So let me just put this out there as food for thought ... when you're diving, how to you determine how much air you've used and how much you have remaining? Do you mentally calculate how much volume you have in your cylinder at a given time? Or do you simply look at your pressure gauge? Give some thought to why that question matters ...

... Bob (Grateful Diver)

 

[scjoe]

No, actually I put my faith in published, reliable sources (Chapter 2 of the SDI Solo Diving Manual) rather than stuff posted on the internet where we all know "they can't put anything on the internet that isn't true". Nor did I take anything out of context. Your definitions do not agree with the established standard definitions. You are certainly entitled to your own opinion - but not your own facts.

The term RMV is a physiology term and in that context has nothing to do with diving and always means surface consumption. SAC on the other hand is a diving term. The use of the RMV term by the scuba world and dive training agencies has more to do with trying to sound sophisticated than to communicate clearly.

 

[kwinter]

What the hell are we talking about here? Semantics? RMV or SAC? Call it macaroni if you like. It's volume per unit of time that's important for every diver to know. CFM or LPM at 1 ATA and then adjusted for depth/pressure. If you want to convert it to pressure per time, that's fine too. Every diver should be able to do that math. And figure gas volume and time for a dive. Or backup/bailout. And if they can't, then it is a failure in their dive education.


Please pardon any typos. Sent from my iPhone using Tapatalk

 

[MXGratefulDiver]

... does DAN qualify as a reliable source?

Alert Diver | Estimating Your Air Consumption

... what about the NOAA?

http://www.ndc.noaa.gov/pdfs/AirRqmtFormulas.pdf

... Bob (Grateful Diver)

 

[Basking Ridge Diver]

I have an Oceanic Veo 2.0 and use the Oceanlog 2.x to download my dives. Inside the dive log if you fill in your cylinder size, start and ending pressure - it kicks out a SAC Rate cu/ft per min for each dive. I dont care which acronym is used SAC or RMV - the calculation is what matters to get my average over a series of dives which I then use to come up with my dive plan. The SDI Solo Diving Manual - uses both SAC and RMV in Chapter 2 (page 32) - SAC is a constant and represents gas used at rest on the surface. RMV is a variable and represents gas needed for specific dive.