How To Calculate Minimum Gas Requirements

[C Dub]

To the original poster:

I've taken GUE Fundies through Tech 1 over the past year here in the LA area. Kevrumbo mentioned Steve Millington, he was my instructor and was *excellent*.

Anyway, we all were VERY frustrated with the written materials. They really need to be redone, especially the worksheets. You will be taught a much simpler way to do min gas and turn pressures in class, without using paper, calculators, etc. My sincere recommendation is to read the materials a couple of times then call it good. It will save you a lot of grief and confusion.

 

[KevinNM]

I took a couple of primer classes from Steve Milington, he's very good. There is at least one other GUE instructor in LA, who seems to be held in high regard by other instructors. But then again, all the GUE instructors I've met seem to be very good.

The worksheet was terrible. I did the class a year ago, hopefully they fixed it.

 

[eelnoraa]

Maybe a better way to phrase this is that students aren't expected to show up to class knowing how to do the calculations. If you have figured it all out ahead of time, that's great, but not necessary. I agree with your last sentence.
.....

Thank you!. I should have worded it better.

 

[Lorenzoid]

To the original poster:

I've taken GUE Fundies through Tech 1 over the past year here in the LA area. Kevrumbo mentioned Steve Millington, he was my instructor and was *excellent*.

Anyway, we all were VERY frustrated with the written materials. They really need to be redone, especially the worksheets. You will be taught a much simpler way to do min gas and turn pressures in class, without using paper, calculators, etc. My sincere recommendation is to read the materials a couple of times then call it good. It will save you a lot of grief and confusion.

I recently took a drysuit and doubles primer, and the sheer awesomeness of GUE was in evidence by the instructor's use of this course to review some of what is taught in Fundies. In other words, I got a Fundies refresher along with the new stuff! Anyway, the instructor explained that the minimum gas calculation had been simplified. The people here on SB who are willing to help are a great resource, but there's nothing like having a live instructor spoon feeding you whatever the current GUE thinking is (and probably in a way that's easier to understand than what's in the written materials).

 

[Ayisha]

Some of the download materials had changed last year when I took Fundies, but perhaps they have changed again, because I don't have any file called "Gas Planning Concerns" from last year. I do have the others though.

My instructor had advised us to read the materials and do the worksheets before the course, and we did. I found the worksheets really helpful in breaking down and understanding all the calculations.

Good luck but don't stress over it yet!

 

[KevinNM]

The (metric) copy I have is dated 7/28/15, from a class I did in October.

 

[Kevrumbo]

What was frustrating, difficult to understand and apply from GUE's Fundies text & workbook materials?

Here's an easy very conservative gas planning example & exercise for a novice OW Buddy Team to the Basic Open Water NDL Limit of 18m/60':

Pre-Dive Plan: Given an Emergency "Stressed" 30 liters/min per ATA Surface Consumption Rate (SCR), with 18 meters depth NDL, and with an arbitrary conservative controlled and slow 0.5 minute stops (30sec) every 3 meters ascent rate to a Safety Stop --let's calculate the "Rock Bottom"/Minimum Gas Reserve requirement value to get to the surface starting from 18meters (same as 2.8 ATA):

Depth(ATA) x SCR x Minutes = Liters
2.8 x 30 x 1 = 84
2.5 x 30 x 0.5 = 37.4
2.2 x 30 x 0.5 = 33
1.9 x 30 x 0.5 = 28.5
1.6 x 30 x 2 = 96 [2 min Safety Stop]
1.3 x 30 x 0.5 = 19.5
1.0 x 30 x 0.5 = 15

Sum Total: 313.4 liters gas needed to ascend to surface for an emergency contingency.

Divide the above total by the metric tank rating of the Scuba cylinder in use; for this example let's use the AL80 tank again which has a metric rating of 11 liters/bar. So 313 divided-by 11 = round up to approx 30 bar.

That's 30 bar to get you to the surface --to get yourself and sharing gas with an Out-of-Air Buddy you will need at least twice this amount: 30 x 2 = 60 bar. Therefore your Rock Bottom Reserve pressure is 60 bar showing on your SPG --if there is no emergency air sharing contingency at that instant with your SPG reading 60 bar actual, just continue the dive but start a easy nominal ascent to the shallower depths between 5m and 9m. Finally, be at your 5 to 6m safety stop with your buddy with no less than 50 bar showing on the SPG.

Your usable gas for the dive is your starting pressure subtracted by the Rock Bottom Reserve --so 200 bar minus 60 bar equals 140 bar usable. Let's use 20 bar of this 140 to get squared away in good trim & buoyancy on the descent to 18m which leaves 120 bar usable. Now if your dive plan calls for returning to near the vicinity of your original point of entry (like a beach dive for instance), then turn the dive back around when you use half of 120 bar or 60 bar consumed [Rule of Halves].

So for a dive with a nominal volume SCR of 22 liters/min per ATA on a 11L per bar (AL80) tank, your resulting pressure SCR will be 2 bar/min per ATA [22 divided-by 11 equals 2bar/min per ATA]. Your depth in meters, which converts easily to ATA (simply divide-by-10 and add 1) becomes your multiplier depth factor for your 2bar/min per ATA pressure SCR.

18 meters depth is 2.8 ATA (divide 18 by 10 and add 1 equals 2.8 ATA); your 2bar/min per ATA pressure SCR at depth -or Depth Consumption Rate (DCR)- now becomes 5.6 bar/min. [2.8 times 2bar/min equals 5.6 bar/min]. So 10 minutes at depth 18m on an AL80 (11L/bar) tank in nominal conditions, you would expect to consume 56bar of gas (10min times 5.6 bar/min equals 56bar), and your SPG reading to show a delta of 56bar less for that 10 minute elapsed time interval. . .

Okay, you splash in to start the dive with 200bar and a 60 bar rock bottom. You use up 20bar on descent --initial exertion, inflating your BCD/wing etc. -so you now have 180bar with 60 bar rock bottom at present depth 18m, a net usable of 120 bar (180 minus 60 is 120 bar).

Finning out for 10 minutes, an easy swim looking at all the cool marine life around -- you already know by the end of this 10 minute interval and confirmed with a SPG reading that you've used up 56bar, close to half of a net usable of 120 bar, so you decide to turn the dive coming back around on a reciprocal course. You should now know you have roughly 60 bar of usable gas left before encroaching on your Rock Bottom Reserve, and realize that you must start a nominal ascent from 18m at the end of the next 10 minute interval. (Note: 20 minutes elapsed bottom time at 18 meters depth is well within NDL).

Alright, so while turning back, you see a turtle and decide to chase it for a few minutes, huffing & puffing on your reg, until it dives down below your operational depth of 18m; because of this unplanned turtle excursion & physical exertion, you immediately check your SPG and it reads 60 bar --bingo! Rock bottom has arrived so start ascending to the shallower depths & eventual Safety Stop . . .or if your Buddy just happens to blow his tank neck O-ring at that instant with a catastrophic loss of his remaining gas supply --you know you've got enough breathing gas margin for a controlled air-sharing Emergency ascent profile as described above. . .

Be aware that depending on environmental conditions and physical exertion (cold water, stiff current, long surface swim, thick surface kelp forest, heavy workload at depth etc), you may have to reserve a greater Rock Bottom Reserve --perhaps as high or even over 100 bar on the 11L/AL80 tank to be conservative. (Even better just abort the dive, wait another day or find another easier site to dive!)

 

[Kevrumbo]

Hi guys,

I am going to be taking GUE Fundamentals in 2 months (so excited!) and am currently reading the materials they provide once you register for the class. The materials include several different PDFs and this topic is covered in a few different places. For those interested, the materials are about 1000 pages long in total.

Here's the question:

In one area, it is stated that for divers with dissimilar tanks, usable gas must be calculated for EACH diver individually, and then use the diver with the smallest amount of usable gas to calculate the turn pressure for each diver. (Say Diver 2 has the least usable gas, you then use that number for Diver 1 also). There are a couple of examples done in this style (I will call it Method #1).

In another place (2 places actually), each diver gets their own usable gas calculation, and then their individual usable gas is used to calculate a turn pressure (Method #2).

Here is an example I just made up. Minimum gas is 20 cu ft. Diver 1 has 120 cu ft of gas total, Diver 2 has 220 cu ft of gas total. It's an out and back, rule of halves dive. Diver 1 turn volume would be at 120 - (120-20)/2. Do we say Diver 2 would turn at 220 - (120-20)/2 also, or would it be 220 - (220-20)/2? I think it's the 2nd one.

Now that's all in cu ft and I did not include converting to PSI but that should not matter. It's the basic assumption that is confusing me since it's stated both ways, not the math. I can do math (usually).

If we do it Way #1, Diver 1 turn volume is at 70 cu ft and Diver 2 has to turn at 170 cu ft. Way #2 Diver 1 still turns at 70 cu ft, Diver 2 turns at 120 cu ft. Obviously whichever diver reaches their turn volume first would call the dive, the question is just WHAT is the turn volume for Diver 2?

I figured this should go in Basic Scuba since calculating turn pressure should be pretty basic, right? I wish we had gone into more depth about it in my PADI OW and AOW classes. We did not. We did not cover converting between PSI and cu ft. or really do anything other than say that starting with 3000 psi, you want to ascend at 800 (reserve of 300 for ascent and 500 for emergencies). That is also what is printed in the OW Diver manual (pgs 92-93).

*If you have the GUE Fundies materials and want to look at what I am talking about, there is a PDF called Gas Management which calculates turn pressures on Page 4 via method #2. There is another PDF called Gas Planning Concerns which calculates turn pressures on Pages 1 and 2 via method #1. This type of calculation is also mentioned in the Powerpoint notes, on a slide called Gas Strategies, where method #2 is used. Another mention is in the PDF Beginning With The End in Mind, but I did not see where it explicitly stated either way.

I have mentioned this question to my instructor but haven't heard back from him yet. I am just trying to understand all the classroom material PRIOR to the class, because I know that just trying to learn the physical skills is going to have me well-taxed during the class. Hopefully some of you (GUE or not) will be able to give some advice.

Laikabear --so how did your instructor (Steve Millington?) explain the two methods of calculating Turn Pressure above for Rule of Halves in Open Water?

 

[The Chairman]

If the answer is that we apply the smaller amount of usable gas to the larger tanks, I guess my question is WHY?

The whole point of going to the minimum is to make sure that you have enough gas for both divers to survive. While not addressing individual SACs directly, it prevents the person with the bigger SAC from accidentally killing you both. In OW, this kind of planning is overkill, unless there is a deco involved. In an overhead, the rule of thirds is appropriate... a third in, a third out and a third for return. Your "usable gas" in this case would be 2/3s your tank volume. For the diver with 120 cf, that would be 80 and their turn would also be at 80, leaving 40 to exit and 40 for their buddy. For the 240 diver, that would be a turn at 160, with 80 for the diver and 80 for the buddy. Now say diver 2 consumes 50 cf gas for every 40 ft of gas of diver 1. If both turn their dive when either first consumes 40 cf then all is fine. But say diver two has consumed 50 cf and then loses all of his air? Diver one has only 80 cf left to cover the distance that both consumed 90 cf to traverse. Both are now dead.