how to dive with non-DIR divers?

[rjack321]

Rjack, Read Post #93. I believe you wrote it, didn't you? That's where you want to use average ATAs = 2.0 on a dive with max depth = 45fsw. However, rounding to the nearest tenth, avg ATAs = 1.7. Now, 2.0 - 1.7 = .30 ATA, which we all know is 3 meters or about 10 fsw. So far so good or do you want to question that?

Based on avg ATA = 2.0, Min Gas = (2 divers x 4 minutes x 1 min SAC x 2 ATAs) =
16 cf. However, if you use 1.7 avg ATAs instead of 2.0, Min. Gas = 13.6 cf. No? Now, 16 - 13.6 = 2.4 cf. 2.4/13.6 = approx. 17%. See how this works?

Correct, I said to ignore hundredth and thousandth and then used to the closest tenth for my own calculations. Generally, the decision to "round up" should be made at the level of the quarter of an ATA IMO.

E.g.
2.1 goes to 2.25
2.4 goes to 2.5
2.7 goes to 2.75
2.8 goes to 3.0

Any finer than that and you're splitting hairs. For my own calculations I use: "a bit less than 2 = 2" or a "quite a bit less than 4 = 3.5"

And for the case in question 2.0 ATA was "good enough". The entire premise of scuba math is to be able to do it in your head on a bouncy charter boat or underwater. Knowing what "good enough" is part of the system.

1.8xyz vs. 2.0 is irrelevant.

Round a couple numbers up and a couple down and everything works out fine. E.g. even thinking along the lines of 1.685 ATA x PDQ is unnecessary and an impediment to using this regularly and quickly.

 

[Phil K.]

Correct, I said to ignore hundredth and thousandth and then used to the closest tenth for my own calculations.

That's where you're loosing me rjack. If your rounding convention is "to the closest tenth," are you saying that a raw avg ATA of 1.675 rounds to 2.0, rather than 1.7. I can't help thinking that avg ATA = 2.0 requires rounding to either the nearest or, because we never round down, what you probably mean is the "next highest" whole number. Also, if you're going to round up at the level of the next highest quarter, you've just put back those pesky hundredths again and 1.675 should round up to 1.75, not 2.0. No?

Just so we understand your position, if my raw avg ATA = 1.165 and we round "to the closest tenth," do you get 2.0 or 1.2? If you round to 1.2, then I rest my case and the rest of this should be worked out over a couple of beers. What say you?

 

[mdb]

That's where you're loosing me rjack. If your rounding convention is "to the closest tenth," are you saying that a raw avg ATA of 1.675 rounds to 2.0, rather than 1.7. I can't help thinking that avg ATA = 2.0 requires rounding to either the nearest or, because we never round down, what you probably mean is the "next highest" whole number. Also, if you're going to round up at the level of the next highest quarter, you've just put back those pesky hundredths again and 1.675 should round up to 1.75, not 2.0. No?

Just so we understand your position, if my raw avg ATA = 1.165 and we round "to the closest tenth," do you get 2.0 or 1.2? If you round to 1.2, then I rest my case and the rest of this should be worked out over a couple of beers. What say you?

What a total anal waste. Turn around @ 50% in open water, 33% in enclosed. Not that difficult.

 

[Carribeandiver]

MDB, the problem with your logic is that it doesnt account for enough gas to support both you and your teammate should an OOG incident occur. The DIR philosophy is that your gas is your buddy's gas and vice versa. If you make your turn at 1/2 tank, and at approximately that moment, you or your buddy has an OOG incident, you will be short on gas for the return.
And I dont know about all those other calculations but I do know that turnaround pressure should be calculated accounting for the gas you and your buddy will need for a safe return including safety stops and a small margin reserve.

 

[mdb]

MDB, the problem with your logic is that it doesnt account for enough gas to support both you and your teammate should an OOG incident occur. The DIR philosophy is that your gas is your buddy's gas and vice versa. If you make your turn at 1/2 tank, and at approximately that moment, you or your buddy has an OOG incident, you will be short on gas for the return.
And I dont know about all those other calculations but I do know that turnaround pressure should be calculated accounting for the gas you and your buddy will need for a safe return including safety stops and a small margin reserve.

True.

 

[rjack321]

That's where you're loosing me rjack. If your rounding convention is "to the closest tenth," are you saying that a raw avg ATA of 1.675 rounds to 2.0, rather than 1.7. I can't help thinking that avg ATA = 2.0 requires rounding to either the nearest or, because we never round down, what you probably mean is the "next highest" whole number. Also, if you're going to round up at the level of the next highest quarter, you've just put back those pesky hundredths again and 1.675 should round up to 1.75, not 2.0. No?

Just so we understand your position, if my raw avg ATA = 1.165 and we round "to the closest tenth," do you get 2.0 or 1.2? If you round to 1.2, then I rest my case and the rest of this should be worked out over a couple of beers. What say you?

We're saying the same thing beer buddy. Tenthish, Quarterish its all good.

I guess I get a bit concerned when people start saying 1.675 ATA * XYZ = something.

That level of precision is really unwarrented. Just say 1 and 3/4s (same as 1.75 but its understoof you're working fractions now not decimal to 2 decimal places),

or an even 2 if you math skills are bad.

e.g.
1-3/4 ATAs * 2cf/Min = 3-1/2 cf per min at depth average. Close enough.

Of course when your working differences between and average of 33ft and 23 or 40ft it doesn't really matter much cause your rounding up to 500psi in the end anyway.

Cheers!

 

[TSandM]

Just for perspective . . . I sat through gas planning in Rec Triox yesterday, and the instructor very clearly told us that all these calculations are inexact and there is no need to do them to hundredths (or possibly even anything closer than the nearest half, or even whole number). He also said the degree of conservativism you build in depends on a lot of factors -- the experience of the divers, the complexity of the dive, the variability in conditions, and the risk-tolerance of the team.

That's from a recently minted Rec Triox instructor, who did his training for this class under JJ.

 

[Carribeandiver]

for a thorough understanding of gas and how it effects you, consult your local anesthesiologist or TS&M, whichever (whoever?) is easist to locate.

 

[Mike Edmonston]

for a thorough understanding of gas and how it effects you.

You can visit your local mexican restaurant for a first hand demonstration of "Burrito Decompression and offgassing":mooner:

Cheers

Mike

 

[MXGratefulDiver]

You can visit your local mexican restaurant for a first hand demonstration of "Burrito Decompression and offgassing":mooner:

Cheers

Mike

... I was wonderin' how long it was gonna be before someone made that connection ...

... Bob (Grateful Diver)