How To Calculate Minimum Gas Requirements

[laikabear]

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.

AHA! So it is not a mistake but 2 different methods.

Yes, I am in open water and would like to someday move up to some wreck penetration (maybe). I don't know about caves. They seem scary. I've read some cave diving books and I just don't think it's for me.

Thank you Kevrumbo for answering my question! And for providing further reading! I will check out your link.

 

[Sloth]

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.

In the real world If a dive team has scrs that are that far apart then yes, you would probably talk about using method 2 knowing that it will complicate dive planning.

Having said that I wouldnt be surprised if the fundies answer will probably be that you use the conservative method 1.

It seems overly conservative.

Welcolme to GUE dive planning.

 

[Kevrumbo]

In the real world If a dive team has scrs that are that far apart then yes, you would probably talk about using method 2 knowing that it will complicate dive planning.

Having said that I wouldnt be surprised if the fundies answer will probably be that you use the conservative method 1.

Welcome to GUE dive planning.

Method 1 is what GUE teaches explicitly in their Cave/Tech Courses. Even in a dive team with large disparate SCR's along with dissimilar tanks, just understand & accept without rigor for now --it all still works out. . .

Method 2 in more detail is taught optionally as well by IANTD Instructors (also called SAC Ratio Factor "SRF"/Gas Matching Method):
Dive Table, Gas Managment | Dive Gear Express®
http://wetlands.simplyaquatics.com/d/14074-1/sac_ratio.pdf

 

[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.

AHA! So it is not a mistake but 2 different methods.

Yes, I am in open water and would like to someday move up to some wreck penetration (maybe). I don't know about caves. They seem scary. I've read some cave diving books and I just don't think it's for me.

Thank you Kevrumbo for answering my question! And for providing further reading! I will check out your link.

The quick answer is this: With dissimilar tanks AND a buddy team in which one member has a SCR that is twice as much or greater than the other, the other teammate with the lower SCR must ensure he has enough reserve gas capacity especially if he has the smaller volume capacity cylinders. He might have to conserve more in reserve, and this may very well be critical in an Overhead. . .

GUE's method of Modified Thirds -as an initial introduction to Overhead Turn Pressure Gas Planning- automatically by rote takes this into account as a starting point. There may be additional factors to consider in the overhead such as tight passages & restrictions to negotiate while emergency gas sharing, opposing current flow on egress etc. -that you may have to use even more conservative parameters in planning like Modified Fourth's or Sixth's. . .

 

[Sloth]

Kevin,

Remember the context here is recreational diving. Rules of thirds is mentioned in rec and fundies classes but isnt expanded on or allowed for dive planning.

recreationally, for rules of halves you can use method two safely as exit isnt mandatory. As she mentioned the material is kind of vague as it does mentioned planning should be based on lowest volume but has examples of rules of halves planning were each diver calculates thier own turn pressures based on their own useable gas.

Its odd its documented this way since rules of thirds requires calculating all turn pressures on the lowest volume of useable in the team for the reason mentioned so I am surprised they dont force rules of halves to be calculated the same way even though it isnt needed for safety.

Its possible that the material is that way to stress dissimilar tank calculation with actual dive planning to be taught based on lowest volume but the rec 1 examples at least do seem to use divers calculating turn on their own usable gas for rules of halves.

 

[Kevrumbo]

That's fine then - either method of Modified Halves is good for Recreational/Open Water Turn Pressure Planning. The important point to remember is of course don't go beyond MGR/Rock Bottom of your operational depth as well as subsequent depths on nominal ascent to your Minimum Deco Stops and the surface.

 

[eelnoraa]

Well, the best thing to do is to take the class. You are paying for the instructor to go through the material in the proper way. If you learn it all on the Internet, why do the class. Especially this gets asked in the "basic" forum, you are likely the get a wrong answer. My suggestion is to go through the the material, note down any questions, then ask them in the class.

 

[Kevrumbo]

Well, the best thing to do is to take the class. You are paying for the instructor to go through the material in the proper way. If you learn it all on the Internet, why do the class. Especially this gets asked in the "basic" forum, you are likely the get a wrong answer. My suggestion is to go through the the material, note down any questions, then ask them in the class.

Actually, it's more likely that the OP will get useless cynical and non-contributory rhetoric like yours above eelnoraa . . .

Laikabear's GUE Instructor will probably be Steven Millington, and I know for a fact that he will be an excellent teacher & mentor --I was his teammate when he took Fundies back in 2007. . .

 

[Lorenzoid]

Well, the best thing to do is to take the class. You are paying for the instructor to go through the material in the proper way. If you learn it all on the Internet, why do the class. Especially this gets asked in the "basic" forum, you are likely the get a wrong answer. My suggestion is to go through the the material, note down any questions, then ask them in the class.

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.

Also, the PDFs that are downloadable as class materials when you register may not be up to date. GUE sometimes makes small changes, and it takes a while for the changes to make their way into the materials. The instructor will explain what way to do the calculations is currently preferred.

 

[rivers]

And this is why it pays off to have the same size cylinders as the rest of your team...