Understanding Gas Management

I was just reading a post by TS&M on another thread and I wanted to be sure I understand her example of gas management scenarios. I haven't even started training yet but I like to learn before I'm taught so that I can be sure to retain as much information and ask questions while I'm learning. Sooooo if you'll indulge me please...

...gas management at the simple recreational dive level basically breaks down into three steps.

First, determine your "rock bottom" or minimum gas. That's the reserve you need to carry to get you and your buddy to the surface from the deepest part of the dive. A reasonable rule of thumb is 20 cu ft for 60 fsw, and 40 from 100fsw. Knowing the volume, you can figure what proportion of your tank it is -- for example, 40 cu ft is half of an Al80 which is normally filled to 3000 psi, so it's 1500 psi. If you know the tank factor for your tank, you can figure it that way.
So -- you take out the rock bottom, and figure out how much gas you have left. That's your "usable" gas.

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Easy so far.

Now you have to decide how you are going to portion out that gas. If you are diving off a charter boat that will come and get you wherever you end up, you can use it all! Your rock bottom is plenty to get you to the surface, even if you have to share with a buddy. So you head up when you hit rock bottom.

If you are shore diving, most of the time, you would prefer to get back to where you started underwater, rather than do a long surface swim. But you CAN do the long surface swim if you have to. So you divide your usable gas in half. Whatever number you come up with, you add that to your rock bottom, and that's your turn pressure (you turned when you had enough gas to get back to the shore, and to ascend if you needed to).

If you are in a position where you MUST get back to your starting point, then you have to consider that you might have to bring your buddy back there on your gas, AND do the ascent on your gas, so you have to divide that usable gas into thirds. There aren't many situations where open water divers are faced with these constraints, but diving wrecks in high current off an anchored boat is one of them.

Don't make it too complicated in your head. It's really simple. Put away a safety reserve that you won't use. Then decide whether the dive is one way, two way, or "absolutely have to get back to the upline". That tells you how to divide the rest.

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That all makes perfect sense - no questions so far.

Quick examples:

Diving off a Bandito charter at Sunrise. Max expected depth is 60 fsw. You're diving an LP95, with a tank factor of 3.5 (rounded off for easy math).

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I take it that tank factors are something that I'll look up somewhere and that larger or higher pressure tanks hold more gas so they have higher tank numbers.

You need a safety reserve of 20 cu ft, which is roughly 600 psi (6 x 3.5 = 21). So you put 600 psi away. The boat will come and get you wherever you surface, so you can use your tank down to 600 psi, so you have 2000 psi usable (assuming you got the 2640 fill the tank is rated to). You have no turn pressure. You ascend at 600 psi, or when you get cold, whichever comes first.

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Makes Sense - got it.

Diving at Edmonds. Max expected depth is probably 33 fsw, and rock bottom is never less than 500 psi, so that's what it will be for this dive. This is a halves dive, because you'd really LIKE to swim back to the shore, although if you have to surface, you can. So you take that 2640 you have, remove the 500 psi reserve, and that gives you 2140 -- rounding down to 2000, you have 1000 to use going out, and 1000 to use coming back. So add the 1000 you need to come back to the 500 you aren't going to touch outside of an emergency, and you have a turn pressure of 1500 psi.

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Makes sense - got it.

Diving off the Dash on the Possession Point ferry. Max depth is 80 fsw, and the boat is anchored, and surface currents can be fierce. So if you have a problem at depth, you would like to share gas to the upline and ascend there if you possibly can. So, using the 100 fsw value of 40 cu ft, you need to set aside 900 psi for rock bottom.

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Where did the 900 psi come from? If I understand the above it should be 40/3.5 =11.4 yielding 1200 psi to be conservative. Or is it that 60 fsw=20 cu ft, 100 fsw=40 cu ft so 80 fsw=>30 cu ft and 9x3.5= 31.5? I'm figuring that this is it but it wasn't specifically explained that way. Please don't get me wrong, I LOVE TS&M's posts, they are always very intelligent and well written, so I'm not flaming her, just trying to make sure I understand correctly.

That leaves you with 1700 psi usable. Now you want to divide that into thirds, and you have to make a decision. 1700 isn't divisible by 3, so you either have to call it 1500 or 1800 to make the math easy. 1800 reduces your safety cushion; 1500 is very conservative. What you do may depend on how much you trust your buddy, how much experience you have, or how worried you are about drifting in the open water until the boat finds you Say we want to be conservative and careful -- we'd call our outbound leg 500 psi, and turn at 2100.

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So if I'm right above then I understand this but I have a question:
Does the 60 fsw=20 cu ft, 100 fsw=40 cu ft take into account the fact that being a new diver I'll be sucking a lot of air, and if both divers are new, and we're sharing air due to a problem we'll be using a TON of air? In other words, do I have to be even more conservative if it's a situation where longer underwater travel is required.

Thanks much.

jtpwils:

I take it that tank factors are something that I'll look up somewhere and that larger or higher pressure tanks hold more gas so they have higher tank numbers.

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A "tank factor" is the ratio of service pressure to rated volume, usually written as CF/100PSI (since most analog gauges aren't precision enough to read in increments of 1 or 10PSI, but 100 is pretty simple).

Examples (from Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan)

Luxfer 80
At 3000PSI (its service pressure), a Luxfer 80 holds 77.4CF.

77.4CF/3000PSI = .0258CF/PSI, or 2.58CF/100PSI. A conservative "tank factor" would be 2.5, but 3 makes the math easier and is probably okay used in conjunction with "rock bottom" since it's rather conservative for most divers anyway.

PST E7-100
At 3442PSI, a PST (high pressure) 100 holds 100CF. Let's say you double them up. At 3442PSI, your set of double PSI 100s hold 200CF.

200CF/3442PSI = .058CF/PSI, which most of us call a tank factor of 6.

Faber 95
95CF/(2400*1.1) <- tank factor of 3.5 seen above

Or is it that 60 fsw=20 cu ft, 100 fsw=40 cu ft so 80 fsw=>30 cu ft and 9x3.5= 31.5?

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Probably. Although rock bottom isn't linear, for these profiles that's probably a fair approximation.

jtpwils:

Does the 60 fsw=20 cu ft, 100 fsw=40 cu ft take into account the fact that being a new diver I'll be sucking a lot of air, and if both divers are new, and we're sharing air due to a problem we'll be using a TON of air? In other words, do I have to be even more conservative if it's a situation where longer underwater travel is required.

Thanks much.

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It more than likely reflects both divers breathing at a SAC (surface air consumption) rate of 1CF/min (the "S" implies /ATM).

You can ratio it up if you want more conservatism. If RB for 100feet is 40 at a combined SAC rate of 2, then it's 60 at a combined SAC rate of 3.

This article may be able to help answer some of your questions ...

NWGratefulDiver.com

... Bob (Grateful Diver)

Tank factor is defined as the capacity in cubic feet divided by the service pressure multiplied by 100. Blackwood did a nice job of explaining it.

Although Lynne's explanation is very clear, Lamont has posted an excellent article with more details regarding how Rock Bottom is calculated. I'm sure you'll enjoy it.

Rock Bottom refers to the minimum reserve required for a specified depth. When most people discuss Rock Bottom during pre-dive planning, they are referring to the reserve for the maximum depth of a given dive. It is possible to dive within Rock Bottom limits for the maximum depth...and then violate Rock Bottom at a shallower depth. For example, on a 100 fsw dive, Rock Bottom at max depth might be 1400 psi for an AL80. With 1400 psi showing on their gauges, the buddy team ascends to 60 fsw (where Rock Bottom is 900 psi) but do not begin their ascent until their gauges display 600 psi. Is the dive team in violation of Rock Bottom?

Breathing rates can be referred to as surface air consumption (SAC) or, perhaps more rigorously, as respiratory minute volume (RMV) in units of cubic feet per minute (cfm). Rock Bottom calculations allow for the elevated breathing rates of two stressed divers breathing from the same tank. The typical estimated stressed breathing rate for one diver is 1.0 cfm. An average diver's RMV (normal conditions) is 0.5-0.75 cfm. A more experienced diver can have an RMV in the 0.35-0.4 cfm range. A novice diver might have an unstressed RMV around 1.0 cfm.

You can pad your Rock Bottom numbers as much as you want to suit your needs...but understand that the trade-off for increasing your safety reserve is decreasing the amount of usable gas for the dive. You'll have to adjust your goals for the dive accordingly. There are no scuba police when it comes to gas management, so you'll have to make your own decision on whether to increase the level of conservatism.

Gas management (beyond having enough to surface with 500psi) is not taught in the standard PADI curriculum. I wouldn't get too bogged down in the details for now. You've been introduced to the concept of Rock Bottom. Let's leave it at that.
For now, I'd advise you to focus your efforts on locating a good instructor, getting OW certified, and finding some good diving buddies. That's far more important than doing simple arithmetic. For diving (and many other things in life), there is no substitute for quality training and frequent practice.

Thanks all for your great replies and the leads on the other articles.

BTW - I completely agree with bubbletrouble's comment that I should concern myself more with the details of finding a good instructor, and getting certified, etc. so fear not - I'm not trying to get too far ahead of the curve, but I am the kind of person that needs to understand why I do what I do. It's a personality thing.

So, tooting my own horn, here is a cheatsheet I've published -- with thanks to Bob and Lamont and Scott and....for their help in teaching all this to me.

Air Supply Management Cheat Sheet

Where did the 900 psi come from? If I understand the above it should be 40/3.5 =11.4 yielding 1200 psi to be conservative.

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I don't know where the original post was, but if I was still using an LP95 in the example, I simply did the math wrong, and you are right: 1100 psi would be a better answer.

All the math I did assumes a stressed gas consumption of 1 cubic foot per minute for each diver. If you are brand new and are breathing at that rate when you AREN'T stressed, then these figures are not conservative enough. You can quickly see why it's a good idea for newer divers to do shallower dives!

(BTW, I'd love the link to the thread where I wrote this stuff, so I can go fix the math and not confuse anybody else.)

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NWGratefulDiver:

This article may be able to help answer some of your questions ...

NWGratefulDiver.com

... Bob (Grateful Diver)

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I just linked your site to mine, great info, thanks.

TS&M - Here is the link
http://www.scubaboard.com/forums/ne...hings-i-havent-learned-yet-3.html#post4370311

You can quickly see why it's a good idea for newer divers to do shallower dives!

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For that reason and also, if my thinking is correct, the faster you breath the more N2 you're putting into your system, and depth (pressure) obviously makes that happen faster.

Thanks for that cheat sheet Peter and yes, Bob that link to your site is fantastic - thanks for putting that together...very educational.