Gas Planning Discussion

[Kevrumbo]

What factors DIR divers take into account for planning minimum gas reserve (rock bottom) for deco dives?

Here's an MGR (minimum gas reserve) example for a deco dive to 54meters on twin 11 L tanks:

Max SCR: 28.3 L/min*ATA [same as 1 cf/min*ATA for my Imperial Brethren]

"Emergency/Mad-Minute" at 54m: consumes 181 litres [multiply Max SCR of 28.3 by 6.4 ATA equals 181 litres]

Time to ascend from 54m to 21m (Nitrox 50% deco stop) at 10m/min ascent rate: 3.3 minutes [54 minus 21 divided by 10 equals 3.3 minutes].

Average ATA for range 54m to 21m: 4.7 ATA [6.4 plus 3.1 divided by 2 equals 4.7ATA]

Therefore backgas needed for ascent 54m -->21m: 440 litres [that's Max SCR 28.3 multiplied by Time of 3.3 minutes, multiplied by Average ATA 4.7]

Total MGR: 181 litres plus 440 liters multiplied by two divers gas sharing: 1240 litres
Total MGR in bar pressure: 1240 litres divide by 22 litres per bar equals 56 bar (~approx 60 bar absolute is your MGR reading on your SPG)

Total Backgas: 200 bar multiplied by 22 litres per bar equals 4400 litres. [a full twinset of 11 litre tanks]

Usable Backgas less MGR: 4400 litres minus 1240 litres equals 3160 litres.

So you have 3160 litres of gas for the mission requirements of your dive. . .

 

[Kevrumbo]

To reiterate, my calculated example is the absolute minimum backgas to get to the deco bottle switch for Nitrox 50% from 54 meters, hence the term Minimum Gas Reserve (MGR).

(Include your parameters for deep stops, refined ascent rates, gas switches etc. per your agency's requirements & training standards above as needed to pad your MGR and turn it into a Contingency Gas Reserve. . .)

 

[Sloth]

To reiterate, my calculated example is the absolute minimum backgas to get to the deco bottle switch for Nitrox 50% from 54 meters, hence the term Minimum Gas Reserve (MGR).

(Include your parameters for deep stops, refined ascent rates, gas switches etc. per your agency's requirements & training standards above as needed to pad your MGR and turn it into a Contingency Gas Reserve. . .)

Ummm this is the DIR forum.... In GUE and I imagine UTD Min Gas is Min Gas you don't throw out deep stop ascent rates when calculating Min Gas.

Why you would neglect deep stops is beyond me but Im not going to question another agencies training. From a GUE stand point your time is off by about 5 minutes.

 

[Kevrumbo]

Why would you infer that Sloth? The question was asked about a Rock Bottom/Minimum Gas Reserve (MGR), and I gave an example as such . . . a CGR (Contingency Gas Reserve) on the other hand, is a gas plan including proper deep stops etc. --and that's the terminology I learned to differentiate the two-- why quibble rhetorical over semantics?

Go ahead and tabulate a full Contingency Gas Plan for us then and account for those 5 minutes. . . [I would estimate then 80 to 90 bar for a full CGR updating my given example]. . .

 

[Sloth]

Why would you infer that Sloth? The question was asked about a Rock Bottom/Minimum Gas Reserve, and I gave a example as such as I understood the term to be defined . . . why quibble rhetorical over semantics?

Go ahead and tabulate a full Contingency Gas Plan for us then and account for those 5 minutes. . . [I would estimate then 80 to 90 bar for a full CGR updating my given example]. . .

He asked for the DIR approach to calculating min gas (rock bottom) for a deco dive. IMO DIR means GUE or UTD approach but of course that is up for debate. Honestly throwing out the deep stop ascent rate is what scares me in your approach but I'm not going quibble. 30ft per min was the norm in the old days so it obviously can work. Doesn't jive with the DIR approach to deco diving though.

 

[Kevrumbo]

Well . . .there's your "MGR ranges" then Sloth for this example to 54m:
60 bar (Rock Bottom) to 90 bar (including Deep Stops).

I don't throw out deep stops as you rhetorically infer; for pre-planning, I calculate both a Rock Bottom and a full Contingency Gas Reserve for comparison. I always implement the CGR evaluation on the actual dive.

 

[Sloth]

Well . . .there's your "MGR ranges" then Sloth for this example to 54m:
60 bar (Rock Bottom) to 90 bar (including Deep Stops).

I don't throw out deep stops as you rhetorically infer; for pre-planning, I calculate both a Rock Bottom and a full Contingency Gas Reserve for comparison. I always implement the CGR evaluation on the actual dive. . .

What does any of that have to do with DIR and why are you attempting to answer a question about how a DIR diver does something?

It literally seems like every thread in the DIR forum is hijacked by someone that does not apply DIR concepts or worse thinks he does.

 

[Kevrumbo]

Simply a personal approach for a "thinking diver" that I offered to share. . . please pardon me if that doesn't meet your standards for this (your) thread.

Ask questions to clarify Sloth . . .not reactionary rhetoric. We're all here to learn. . .

(That's the DIR way).

 

[Kevrumbo]

http://www.scubaboard.com/forums/4680914-post16.html

(My, my Sloth . . .such an insular, arrogant and presumptuous idea & group you belong to )

 

[Sloth]

it would better describe the current state of this forum unfortunately. Then you can freely tell us all about your methods without hearing a single peep from me