stage diving

[Dive-aholic]

Maybe some aren't seeing how the half+ 2 thing works.
Dan didn't do a great job of presenting the math. Don't get mad at me Dan.

If we call an al80...80 cu ft (it's close), we use half (save a little because guages don't accurately read zero) and half on the way back. Now the stage is empty but did we violate the rule of thirds? No because we reserved an extra 40 cu ft of our back gas. Essentally we subtract that 40 cu ft before calculating what a third is.

If the reserve is on your back, in theory, you have some redundancy there and you are not going to ditch it or fail to pick it up. the idea is that on your back is a more reliable place to keep your reserve gas.

My comment about Dans math is just related to the fact that it might not have been obvious what he was doing. 40 cu ft is about 500 PSI in lp 104's (about 8 cu ft/100 PSI). subtract 500 psi, calculate 1/3 on that useable supply and subtract that 3rd from the starting pressure to get turn pressure rounding to the more conservative side to the nearest 100 psi. If using more than one stage, you just reserve 40 cu ft (5-- PSI in lp 104's) for each stage. Obviously if the doubles aren't lp 104's the math changes accordingly. My wife dives hp 100's so she is dealing with about 6 cu ft/100 PSI so she reserves 700 psi for each al80 stage.

I too have used a stage bottle to get two wreck dives out of one set of doubles. Here I do it different. I drain the stage on the first dive so I don't have to carry it on the second dive because we usually don't drop tanks in OW like we do in a cave and why carry a stage and two decompression gasses on both dives if you don't have to? This is a bit different than a cave dive where the stage is to add distance or time. With 20 - 30 minute bottom times just about the whole first dive can be done on the stage reserving almost the total volume in the back gas...maybe you use a little on the deep part of the ascent before the first gas switch. We still never come close to pushing "the rule of thirds".

I think you're referring to my math not Dan's. I'm not mad , but I guess I should have included that I usually dive 85s so for simplicity I did the math based on the pressures roughly being equal in volume.

 

[PvilleStang]

Wow, going from "Should I leave my valve on or off" to the battle between thirds and half + 2

 

[Dan Gibson]

Mike,

I think we are on the same page, but your math is slightly different than what I was getting at (and obviously didn't explain). In the example I don't reserve that much extra in my backgas. The reserve is what is the difference between the 1/3 of the stage (1000 psi used) and 1/2+200 or 1300 psi used. That is equivalent to 300 psi or approximately 2.5 cft x 3 or 7.5 cft (use 8 cft). Since the tank factor for the doubled up 104s is 8, I need 100 extra psi subtracted from the turn pressure which now makes it 2500 psi instead of 2400.

Now why did I add back 200 instead of 100 as I just showed. It has to do with when the dive really goes south. After running through the math (which I doubt I could do anymore without fully thinking it through again) you could end up short by 8 cft in the bakgas with that gas being present in the stage. Buddy breathing a stage would not be a good idea. I will have to go back and look at some notes I wrote down once showing how this all works. I do know some people have been taught only adding 100 psi back into there backgas calcs. I'll continue to use adding 200 psi too my backgas.

Besides, I really like diving the stages and barely toughing the backgas so I usually don't cut my supply too close anyway. We also like the three manteam best as it gives an OOA a much better chance and keeps the donors with additonal gas.

Maybe some aren't seeing how the half+ 2 thing works.
Dan didn't do a great job of presenting the math. Don't get mad at me Dan.

If we call an al80...80 cu ft (it's close), we use half (save a little because guages don't accurately read zero) and half on the way back. Now the stage is empty but did we violate the rule of thirds? No because we reserved an extra 40 cu ft of our back gas. Essentally we subtract that 40 cu ft before calculating what a third is.

If the reserve is on your back, in theory, you have some redundancy there and you are not going to ditch it or fail to pick it up. the idea is that on your back is a more reliable place to keep your reserve gas.

My comment about Dans math is just related to the fact that it might not have been obvious what he was doing. 40 cu ft is about 500 PSI in lp 104's (about 8 cu ft/100 PSI). subtract 500 psi, calculate 1/3 on that useable supply and subtract that 3rd from the starting pressure to get turn pressure rounding to the more conservative side to the nearest 100 psi. If using more than one stage, you just reserve 40 cu ft (5-- PSI in lp 104's) for each stage. Obviously if the doubles aren't lp 104's the math changes accordingly. My wife dives hp 100's so she is dealing with about 6 cu ft/100 PSI so she reserves 700 psi for each al80 stage.

I too have used a stage bottle to get two wreck dives out of one set of doubles. Here I do it different. I drain the stage on the first dive so I don't have to carry it on the second dive because we usually don't drop tanks in OW like we do in a cave and why carry a stage and two decompression gasses on both dives if you don't have to? This is a bit different than a cave dive where the stage is to add distance or time. With 20 - 30 minute bottom times just about the whole first dive can be done on the stage reserving almost the total volume in the back gas...maybe you use a little on the deep part of the ascent before the first gas switch. We still never come close to pushing "the rule of thirds".

 

[Dan Gibson]

I don't think it's a battle, but a good discussion on gas management that I'm sure most never learn.

Now, if I said only stupid farm animals touch their backgas...

Wow, going from "Should I leave my valve on or off" to the battle between thirds and half + 2

 

[in_cavediver]

Mike,

I think we are on the same page, but your math is slightly different than what I was getting at (and obviously didn't explain). In the example I don't reserve that much extra in my backgas. The reserve is what is the difference between the 1/3 of the stage (1000 psi used) and 1/2+200 or 1300 psi used. That is equivalent to 300 psi or approximately 2.5 cft x 3 or 7.5 cft (use 8 cft). Since the tank factor for the doubled up 104s is 8, I need 100 extra psi subtracted from the turn pressure which now makes it 2500 psi instead of 2400.

Now why did I add back 200 instead of 100 as I just showed. It has to do with when the dive really goes south. After running through the math (which I doubt I could do anymore without fully thinking it through again) you could end up short by 8 cft in the bakgas with that gas being present in the stage. Buddy breathing a stage would not be a good idea. I will have to go back and look at some notes I wrote down once showing how this all works. I do know some people have been taught only adding 100 psi back into there backgas calcs. I'll continue to use adding 200 psi too my backgas.

Besides, I really like diving the stages and barely toughing the backgas so I usually don't cut my supply too close anyway. We also like the three manteam best as it gives an OOA a much better chance and keeps the donors with additonal gas.

Dan,

Just to clarify your process and where it comes from:

80cf Stage: 1000 is third, 1300 is 1/2+200.

When you use it, you use 1300psi, leaving 1700psi to exit. IE you 'overused' you third by 300psi one way and 300 psi the other for a total of 600psi over? 600psi works out to 16cft in an 80 or 200psi in 104's. Reserving another 100psi from the 104's would account for the 'dead' air (last 300psi) in the stage?

Thus for each stage carried and used for the 1/2+200 rule, you drop 200psi from your backgas (in 104's) for backgas thirds. and whatever dead air reserve you are comfortable with.

Just wanting to make sure I understand it correctly?

 

[MikeFerrara]

Dan, notice that if you calculate it my way, you get the same turn pressure.

3600 - 500 = 3100
1/3 of 3100 is about 1000...so I'm going to use 1000 psi in the doubles before turning.
3600 - 1000 = 2600...is my turn pressure.
2600 was your turn pressure too right? I'll dive with you. LOL

I'm not reserving any more than you are...at least not in this example. Whatever I'm not reserving in my stage I am reserving in my doubles...it's as simple as that.

Algebraically I can show where your 200 psi comes from. Want to see?

I'll write an equation for how I calculate turn pressure with one stage...for al80 stage and double lp 104's filled to 3600

3600 - ((3600 - 500)/3) = turn pressure
3600- (3600/3 - 500/3)
3600 - 3600/3 + 500/3
3600 - 1200 + 166.667
round the 166.667 to 200 and you're there. so...
3600 -1200 + 200 = 2400 + 200 = 2600
Yes? It's the same thing.

 

[Dan Gibson]

Just a different way of saying the same thing I guess.

Dan, notice that if you calculate it my way, you get the same turn pressure.

3600 - 500 = 3100
1/3 of 3100 is about 1000...so I'm going to use 1000 psi in the doubles before turning.
3600 - 1000 = 2600...is my turn pressure.
2600 was your turn pressure too right? I'll dive with you. LOL

I'm not reserving any more than you are...at least not in this example. Whatever I'm not reserving in my stage I am reserving in my doubles...it's as simple as that.

Algebraically I can show where your 200 psi comes from. Want to see?

I'll write an equation for how I calculate turn pressure with one stage...for al80 stage and double lp 104's filled to 3600

3600 - ((3600 - 500)/3) = turn pressure
3600- (3600/3 - 500/3)
3600 - 3600/3 + 500/3
3600 - 1200 + 166.667
round the 166.667 to 200 and you're there. so...
3600 -1200 + 200 = 2400 + 200 = 2600
Yes? It's the same thing.

 

[Dan Gibson]

From what I remember (which isn't too much today as I was sedated this morning for a procedure) you add back 200 psi into the backgas for each stage you used.

Dan,

Just to clarify your process and where it comes from:

80cf Stage: 1000 is third, 1300 is 1/2+200.

When you use it, you use 1300psi, leaving 1700psi to exit. IE you 'overused' you third by 300psi one way and 300 psi the other for a total of 600psi over? 600psi works out to 16cft in an 80 or 200psi in 104's. Reserving another 100psi from the 104's would account for the 'dead' air (last 300psi) in the stage?

Thus for each stage carried and used for the 1/2+200 rule, you drop 200psi from your backgas (in 104's) for backgas thirds. and whatever dead air reserve you are comfortable with.

Just wanting to make sure I understand it correctly?