How To Calculate Minimum Gas Requirements

[laikabear]

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.

 

[Kevrumbo]

Hi guys,

<snip>

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 pressure 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 pressure 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 pressure first would call the dive, the question is just WHAT is the turn pressure for Diver 2?

<snip>

First thing needed . . .What are your Tank Factors (cuft/100psi) for the dissimilar cylinders in your example? To get a Turn Pressure (TP) from a limiting Turn Volume (TV) which is determined by the teammate with the smallest total volume of Usable Gas (i.e. minus MGR), we need to be given the Tank Factors. . .

 

[KevinNM]

Which is why life is so much easier if you dive bar and liters.

 

[laikabear]

Actually you don't need the tank factor to answer my question.

Tank factor is used to convert between cubic feet and PSI. However, if we keep the example in cubic feet, there is no need for the tank factor. Just like if we were using metric.

I mean, sure I could make up a tank factor and we could convert to PSI, but that doesn't have any bearing on my question.

 

[laikabear]

Hmm, KevRumbo you edited your post while I was replying.

It looks like you are answering my question by saying that you will use the teammate with the smallest amount of usable gas and apply that amount of usable gas to each diver. So you are using method #1. Am I right in that interpretation of what you wrote?

 

[Kevrumbo]

Hmm, KevRumbo you edited your post while I was replying.

It looks like you are answering my question by saying that you will use the teammate with the smallest amount of usable gas and apply that amount of usable gas to each diver. So you are using method #1. Am I right in that interpretation of what you wrote?

Yes . . .please understand though -->You're asking for Turn Pressure, but only giving us units in terms of Turn Volume.

Hi guys,

<snip>.

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 pressure 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 pressure 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 pressure first would call the dive, the question is just WHAT is the turn pressure for Diver 2?

I figured this should go in Basic Scuba since calculating turn pressure should be pretty basic, right? <snip>.

The Rule of Modified Halves (in your example) --the common normalized Turn Volume for the Team is 70 cubic feet based on the lesser amount of total Usable Gas Supply of Diver #1 (less "Rock Bottom"/MGR). So Diver 1's Turn Pressure is going to be 70 cubic feet divided by the Tank Factor and then subtract that from his starting initial SPG reading; similarly Diver 2's Turn Pressure is going to be 70 cubic feet divided by his Tank Factor and then subtract that from his starting SPG reading. . .

 

[Sloth]

Im doing a scuba reboot and I am going through rec 1 with my wife in May and looking at the Rec 1 material it is also a bit vague hinting at both methods.

Rec 1 is very conservative were you use a scr of .75 for all planning so with that in mind, if I ask this question in rec 1 I would think the answer would be you use method 1 so that diver 2s turn pressure would be after she consumes the same cuft that should put diver 1 at their turn pressure since they both are theoritically breathing at the same rate.

I cant recall this from my tech 1 days at all since we always dove in teams with double 80s.

Fundies class might provide a different answer. As always, best thing to do is talk it through with your instructor.

 

[Kevrumbo]

Method 1 is called Modified Halves (or optionally Third's); Method 2 is Gas Matching on Halves (or Third's).

Not really critical yet, assuming y'all are in Open Water --but if you're going into Overheads (Cave/Wreck), you should know the differences between the two methods & how to specifically apply them.

 

[laikabear]

OK, turn volume, my bad. I edited my first post to say turn volume instead of pressure. Thank you for correcting me. I want to use correct terminology.

I understand how to convert between PSI and cubic feet, was looking more for the philosophy behind it.

If the answer is that we apply the smaller amount of usable gas to the larger tanks, I guess my question is WHY? If the diver with the smaller tanks is still 10 cu ft away from their turn volume but the diver with the larger tanks has hit the turn volume (of the smaller tanks) then it would seem like they still have enough gas to continue diving for 10 more cubic feet of use.

It seems overly conservative.

If you are accounting for dissimilar gas consumption rates with the calculation for minimum gas, then I don't see why you need to apply the smaller amount of usable gas to the diver with the larger tanks.

 

[Kevrumbo]

OK, turn volume, my bad. I edited my first post to say turn volume instead of pressure. Thank you for correcting me. I want to use correct terminology.

I understand how to convert between PSI and cubic feet, was looking more for the philosophy behind it.

If the answer is that we apply the smaller amount of usable gas to the larger tanks, I guess my question is WHY? If the diver with the smaller tanks is still 10 cu ft away from their turn volume but the diver with the larger tanks has hit the turn volume (of the smaller tanks) then it would seem like they still have enough gas to continue diving for 10 more cubic feet of use.

It seems overly conservative.

If you are accounting for dissimilar gas consumption rates with the calculation for minimum gas, then I don't see why you need to apply the smaller amount of usable gas to the diver with the larger tanks.

Let's restate the question to add some additional factors to consider: "How do we know in proper Modified Third's [Halves in your example] that the team's reserve gas volume is enough for an Emergency Gas Sharing Contingency, and will it be conserved in a case of dissimilar tanks and uneven SAC rates?" A very critical question -not just philosophically but obviously vital- especially in the Overhead Environment. . .

Modified Halves/Thirds method is overly conservative for a reason because it automatically accounts for & takes out the complications of dissimilar tanks and unequal SAC/SCR's between teammates. To understand how it works mathematically, read this thread over in DIR Explorers (it's all in Metric though, so good luck! If you don't understand it, then accept & practice the method as is and it will be proven later if you decide to go on to Tech/Cave). . .