Thirds

[Blackwood]

Asking people who have never taken a GUE course why or why not GUE changed a certain aspect of their training...you probably won't get the best answers.

I think you may be the only one who paid attention to what the OP actually asked.

(Even asking people who have at one time taken GUE/F and T1 won't necessarily be in a position to give the correct answer. GUE is the proper party to whom that particular question should be directed.)

 

[DA Aquamaster]

I agree, if a strict GUE answer was desired it would/should have been posted in the practitioners forum.

That said, there have been other issues raised or asked that are technically off topic and also a little more nubulous than the OP's original question and are more or less fair game in the general DIR forum as opposed to the practitioners forum.

 

[rjack321]

Won't your gas requirement for backgas deco almost always be greater than your rock bottom to your first gas switch?

Nope.

Minimum gas (getting up to 70ft, hoovering at 1cf/min x2) from a 150ft dive is about 65cf. This is plenty to do backgas deco with because while you need 2x the time, you reshape the deco into an "exponential" shape.
25min 50% deco = 50mins backgas shaped roughly like:
10-25
20-12
30-6
40-3
50-2
60-1
70-1

Average depth = ~15ft = 1.5 ATA, planning consumption 0.75cf/min SAC = 54cf

All assuming you don't share your buddies 50% which I would, but how/where to share that is a whole nuther discussion.


Regarding the 1/3rds...
Historically some other agencies have taught 1/3rd down(in), 1/3rd back(up), and 1/3rd(reserve) which is not necessarily adequate. The GUE/UTD way is to calculate the amount of gas you need to ascend and then figure out how much gas you have left to actually dive with and 1) whether you can ascend at any time, 2) want but don't 'have to' retrun to an upline, or 3) must return to your start point. I haven't looked at the book in years, but my recollection is that's its merely vague on this topic it doesn't preclude planning like this.

 

[b1gcountry]

RJ-
I think you might be forgetting to add in the time it takes to get to your 70' stop into the backgas calculation.

Tom

 

[Blackwood]

RJ-
I think you might be forgetting to add in the time it takes to get to your 70' stop into the backgas calculation.

Tom

Minimum gas (getting up to 70ft, hoovering at 1cf/min x2) from a 150ft dive is about 65cf

Assuming a bottle failure, if rock bottom (figured as Richard did) is 65CF, you're only really going to need something like 20CF to make it to 70 ([65-2*{1+150/33}*.75/2]).

So if double deco (due to loss of 50%) takes 54CF, and ascending to 70' takes 20CF, and you began your ascent with 65CF, you may be in gas-share city.

 

[AndyNZ]

I'll get around to asking the question at some stage when I next sit down for a beer with our most local Tech 1 instructor - it may be some time away, as it involves a trip to Aus!

I'm actually finding the discussion fascinating, even if it is going away from the scope of the OP. I'm always rationalising and refining my dive practices in accordance to good ideas picked up from all sorts of sources - so roll on the greater discussion!

Having not done Tech 1, but having done TDI deco courses it is interesting to see how the various philosophies compare.

Regarding the 1/3rds...
Historically some other agencies have taught 1/3rd down(in), 1/3rd back(up), and 1/3rd(reserve) which is not necessarily adequate. The GUE/UTD way is to calculate the amount of gas you need to ascend and then figure out how much gas you have left to actually dive with and 1) whether you can ascend at any time, 2) want but don't 'have to' retrun to an upline, or 3) must return to your start point. I haven't looked at the book in years, but my recollection is that's its merely vague on this topic it doesn't preclude planning like this.

Thanks for that - this was pretty much what I figured. The text is a little dated, and quite general. But still an interesting read.

 

[Meng_Tze]

Rule of 1/3's is a guide line. Whether you calculate 1/3 of total volume or 'usable' volume, you should always double check that you actually DO have the volume to complete the dive safely. Just taking a rule of thumb and applying it without safety checks will end up in tears eventually.

 

[coldsmoke]

RJ-
I think you might be forgetting to add in the time it takes to get to your 70' stop into the backgas calculation.

Tom

Isn't that planning for two failures?

Hunter

 

[Blackwood]

Isn't that planning for two failures?

Hunter

If you're meaning "add rock bottom," yes. However, you absolutely need to account for the transit to the bottle when determining that you have enough backgas to cover a loss of deco gas.

 

[rjack321]

RJ-
I think you might be forgetting to add in the time it takes to get to your 70' stop into the backgas calculation.

Tom

65cf was my rock bottom for 150ft from memory. Here it is laid out step by step:

1min for the failure on the bottom
1min up to 120
1min up to 110 (conservative rounding)
1 at 110 (a bit deep but officially part of the 80% of ATAs or 75% of depth rule)
1 at 100
1 at 90
1 at 80
1 for the switch at 70

Total = 8min ascent
Average depth = 105ft = 4 ATA
4ata * 8min * 1cf/min = 32cf
32cf * 2 divers = 64cf rock bottom

Your normal ascent should use less gas
4ata * 8min * 0.5cf/min (typical deco SAC) = 16cf

64cf - 16cf = 48cf remaining which is roughly the amount you need to deco out on backgas if your 50% were completely empty and you had no buddy. You need 54cf to deco out on backgas at 0.75cf/min but only 38cf if your deco SAC is 0.5. Somewhere in there is the true value which you won't know until you're actually in this (remote) situation.

I use 66cf/1100psi as rock bottom in double 100s although I rarely get that low on a T1 dive nowadays. Really all the math is just an educated guess at what kinds of reserves you need. As Joe T would say "its scuba math", don't use a calculator.

For bigger "T2" dives with experienced buddies we don't use a stressed SAC of 1cf/min because the reserves become obscene and we know we don't use those kinds of volumes stressed. If we did get that stressed, we probably shouldn't be doing 200ft dives.

By all means carry larger reserves if you feel you need them, and calculate your turn/ascent pressures accordingly.