My Stage Diving Strategy

[rjack321]

I was allowed the gap at Ho Tul because its basically traversing the cavern zone. Pretty sure we made it there on 500 psi too. I guess it doesnt really matter because Kalimba is closed now and the jump from Grand to Paso has been that way for 15+ years and I dont see it changing

 

[JonG1]

Apologies for resurrecting this thread but I am interested in understanding the 1/2+200 rule but have struggled to follow it due to being used to the metric system we use in UK.

So i understand that you would use half of a stage plus 15 bar, initially so in a 7 litre at 210 bar this would be 100-10 (to account for interstage pressure issues)+15 so drop/switch off is at 85 bar, but what I can't/don/t get is how you change your back gas calculation in relation to this?

Do you take the 85 bar away from your back gas starting pressure to cover the distance covered by the stage in case it fails?

 

[JohnnyC]

You would drop at half (105) (+10ip, if you go for that) + 15, so breathe to 130, not 85. You're breathing LESS gas out of your stage than 1/2, not MORE. However, it is less than simple 1/3rds, so you make up for that difference in your backgas calculations by turning early on your backgas. If you breathed a stage to 85 bar on the way in, you would have breathed almost 2/3rds.

Bobby's method is 1/3rds and time, plus reserve added to your backgas, and instead of dropping the stage, you carry it another "1/3" worth of time so that you get to the stage faster. You also breathe the stage down more on the out so that you have more backgas to go to in case of emergency.

Did that answer the question?

 

[CptTightPants21]

Apologies for resurrecting this thread but I am interested in understanding the 1/2+200 rule but have struggled to follow it due to being used to the metric system we use in UK.

So i understand that you would use half of a stage plus 15 bar, initially so in a 7 litre at 210 bar this would be 100-10 (to account for interstage pressure issues)+15 so drop/switch off is at 85 bar, but what I can't/don/t get is how you change your back gas calculation in relation to this?

Do you take the 85 bar away from your back gas starting pressure to cover the distance covered by the stage in case it fails?

Half of 210 is 105 Bar + 15 Bar is drop the stage at 120 Bar (90 Bar consumed). If you have 200 Bar in stage, it would be 100+15 Bar, but we like round numbers so it gets rounded up to 120 Bar drop pressure as well (80 Bar consumed).

1.Take the volume of gas that was consumed in the stage, subtract it from your back gas total volume, and then calculate thirds off that reduced back gas volume

 

[JonG1]

Thanks Johnny C I think I messed up the maths so should have been as follows?

7 litre at 210 bar:

210-10ip=200
200/2=100
100+15=115

So drop/off pressure is 115 bar on SPG

210-115=95-10ip=85

So 85 bar is used from the stage on entry

Do I then subtract that 85 from my backgas starting pressure before calculating back gas turn pressure or is it the 15 bar that was added to the stage drop calculation?

 

[JohnnyC]

Thanks Johnny C I think I messed up the maths so should have been as follows?

7 litre at 210 bar:

210-10ip=200
200/2=100
100+15=115

So drop/off pressure is 115 bar on SPG

210-115=95-10ip=85

So 85 bar is used from the stage on entry

Do I then subtract that 85 from my backgas starting pressure before calculating back gas turn pressure or is it the 15 bar that was added to the stage drop calculation?

You need to take it off your backgas based on VOLUME, not pressure. 85 bar out of a 7L is not the same thing as 85 bar out of a set of twin 15L's. Make sense? Then you recalculate backgas.

I'm still not sure why you're taking 10 bar off the top for IP issues, then running your calculations based off that. You'd be breathing MORE gas by basing it off of 200 bar if you switched at 115 because you're not taking that 10 bar into account. You'd be breathing more before your switch because you're actually starting from 210, not 200. 210 math gives you a switch at 120, preserving more gas for the way out, which is where you want it. You're tricking yourself into breathing more gas by taking gas off the top from the get go.

Remember when you first learned 1/3rds you'd round down to take your 1/3rd pressure for ease of mental math, then you're subtract that 1/3rd from your actual SPG pressure.

 

[JonG1]

Right the volume issue makes sense so 85 bar is 595 litres or 25 bar in twin 12s thanks.

The ip deduction was as I was taught when calculating gas on Intro recently, so SP-10, next nearest divisible to work out thirds/quarters/sixths and then that figure off SP.

The theory being that you don't drop below the 10 bar to keep the reg working.

Ergo:

SP 210-10=200
200/4=50 (Assuming quarters)
210-50=160 TP
160-50=110 FP (finishing pressure)
100=Reserve gas 2x50
10=IP

 

[JohnnyC]

Right the volume issue makes sense so 85 bar is 595 litres or 25 bar in twin 12s thanks.

The ip deduction was as I was taught when calculating gas on Intro recently, so SP-10, next nearest divisible to work out thirds/quarters/sixths and then that figure off SP.

The theory being that you don't drop below the 10 bar to keep the reg working.

Ergo:

SP 210-10=200
200/4=50 (Assuming quarters)
210-50=160 TP
160-50=110 FP (finishing pressure)
100=Reserve gas 2x50
10=IP

Ok, you were subtracting your turn pressure from your "IP less" pressure in your stage examples above twice, both to get your turn pressure and gas used so you were double dipping and got me confused.

If how you do it is the way your instructor taught you do it, I just don't know anybody that actually plans that way. It's a new one to me.

 

[JonG1]

So is the plus 15 the IP allowance

 

[CptTightPants21]

There is no 10 Bar for inter stage pressure issues

So is the plus 15 the IP allowance

I'm going to be honest, I have never heard of the 10 Bar allowance being explicitly used in any gas calculations (at least in US/Mexico). I guess you could say the 10 Bar is in the 15 Bar allowance---I would say all gas reserve calculations take into account the 10 Bar in one way or another, especially with the added conservatism of rounding up, etc.

The official answer is that the 15 bar is there to provide a margin of safety and provide the gas needed during the actual gas switch procedure. It also means that if all goes well, you would end the dive with 400 PSI (30 Bar) in each stage bottle.