OP
Sharing the 40 and then going to backgas...we'll see who comes in and corrects us..
Gas Management Question
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O-ring once bubbled...
Sharing the 40 and then going to backgas...we'll see who comes in and corrects us..
I don't carry that much extra gas. however, on a 200' dive I would normally carry a 70 and a 20 bottle, not just a 70.
WW
OP
That leaves more of a safety margin too since you aren't using much of the 70' bottle...
ericfine50
Contributor
On the 200 ft. dive - an 80 would be the bottle of choice for a single deco gas. 2 gases, still use a 40. This is all just planning. Need the mix cert to do the dives with the right gas. We are still talking about shorter dives, therfore shorter deco??? :wacko:
Eric :mean:
Eric :mean:
OP
We never got into bottom times, but you are right...any significant stay at 200' is going to put you over 40 cf3 of deco gas pretty quickly...
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Assuming a 20 minute BT at 200' my gas planning says I need this:
165.1 cu ft Trimix 18.0/45.0
21.6 cu ft Nitrox 50.0
16.2 cu ft Oxygen
so I'm actualling using more of the 70 bottle than the 20. I'm also getting a shorter (82 min RT) and cleaner (due to high PO2 at 20') deco by using both. RT is 98 min using only a 70 (plus gas req jumps to 45.2 cf, a problem with a 40 cf bottle). Shortening the BT is an option but not a good one. I want at least a 20 min BT.
WW
OP
I meant that you would be using less of the 40cf EAN50 bottle since you are switching to o2 at 20' instead of staying on one gas the whole time. I agree with you on the BT issue too...Shortening below 20 mins sucks if you have paid all that money and taken a massive boat ride to get out somewhere..
My numbers:
I calculated the same dive the old fashon way.... pen and paper.
Here are my numbers and tell me if i am too far off.
Rock Bottom Calulations:
1 minute for emergency
1 min for each 30 FSW... (4.33 min or 4 mn 20 sex)
1 Cuft per min sac rate. This is for 2 divers. I cannot remember if GUE taught 1Cuft/min for planning, or 1 Cuft/min per diver. (This is worse case senario)
Average ATA = (7.1+3.1)/2......5.1ATA
Calculations:
1Cuft/min*5.1 AVG ATA*(4.3min Ascent + 1min emergency)=
27.03CuFt of Back Gas to reach the first deco stop.
If each diver used a 1Cuft/min SAC, then it would be 54.06CuFt of back gas.
27CuFt Back Gas*2700PSI/208CuFt [This is your tank ratio]=
=350PSI Rock Bottom Pressure.
If you planned for 2 divers with a 1Cuft/min SAC, then your rock bottom would be 700PSI.
*****sorry I am a little fuzzy on which one it is, so if someone would tell me, that way I don't tell anyone the wrong SAC for rock bottom*********
so now you have 350PSI Rock Bottom. Take your working pressure of 2700-350 = 2350PSI. You then plan for either thirds or halves, depending on what type of dive you are doing.
Or once again using the 1CuFt/min per person and not just the 1CuFt/min you would plan starting at 2000 PSI.
Turn Pressure =2350/3*2= 1567PSI for thirds or Halves would be 1175PSI.
Using the higher SAC rate= Turn pressure =2000/3*2 =1333PSI for thirds or 2000/2=1000 PSI for halves
Now this is what I got from my numbers and what I remember. Soemone please tell me if I am wrong b/c I would hate to be responsible for the wrong data.
I calculated the same dive the old fashon way.... pen and paper.
Here are my numbers and tell me if i am too far off.
Rock Bottom Calulations:
1 minute for emergency
1 min for each 30 FSW... (4.33 min or 4 mn 20 sex)
1 Cuft per min sac rate. This is for 2 divers. I cannot remember if GUE taught 1Cuft/min for planning, or 1 Cuft/min per diver. (This is worse case senario)
Average ATA = (7.1+3.1)/2......5.1ATA
Calculations:
1Cuft/min*5.1 AVG ATA*(4.3min Ascent + 1min emergency)=
27.03CuFt of Back Gas to reach the first deco stop.
If each diver used a 1Cuft/min SAC, then it would be 54.06CuFt of back gas.
27CuFt Back Gas*2700PSI/208CuFt [This is your tank ratio]=
=350PSI Rock Bottom Pressure.
If you planned for 2 divers with a 1Cuft/min SAC, then your rock bottom would be 700PSI.
*****sorry I am a little fuzzy on which one it is, so if someone would tell me, that way I don't tell anyone the wrong SAC for rock bottom*********
so now you have 350PSI Rock Bottom. Take your working pressure of 2700-350 = 2350PSI. You then plan for either thirds or halves, depending on what type of dive you are doing.
Or once again using the 1CuFt/min per person and not just the 1CuFt/min you would plan starting at 2000 PSI.
Turn Pressure =2350/3*2= 1567PSI for thirds or Halves would be 1175PSI.
Using the higher SAC rate= Turn pressure =2000/3*2 =1333PSI for thirds or 2000/2=1000 PSI for halves
Now this is what I got from my numbers and what I remember. Soemone please tell me if I am wrong b/c I would hate to be responsible for the wrong data.
OP
You aren't wrong...you just did it the "right" way using the 1 min emergency, etc. etc. whereas I did it the lazy way and added up the cf3 required in decoplanner to reach the 70' stop. We were after different things, I guess...me not so much a discussion of what rock bottom is, but more a discussion of how people are using these things and what levels of conservatism people are using in their own diving in the ocean and in varying levels of penetration scenarios.
Good show on the math though...as I remember it, it was 1 min for emergency, 1 min for every 30 fsw up to deco stop or surface, and a 1 cf3/min SAC for emergency.
I'm going to make a couple of general statements. We could find situations that they might not fit but that's not the point.
Rock bottom = The portion of back gas that will be used for ascent and decompression. It may need to be enough for two and/or lost gas cintingincies.
The purpose of the rule of thirds it to calculate a turn presure. This is done after rock bottom is backed out. However, if your swimming in circles around the down line the concept of "turn presure has no meaning so niether does the rule of thirds.
The rule that says we carry 1.5 times the deco gas needed is there to take provide gas for things like increased sac rates or the need to do more decompression than planned.
Rock bottom = The portion of back gas that will be used for ascent and decompression. It may need to be enough for two and/or lost gas cintingincies.
The purpose of the rule of thirds it to calculate a turn presure. This is done after rock bottom is backed out. However, if your swimming in circles around the down line the concept of "turn presure has no meaning so niether does the rule of thirds.
The rule that says we carry 1.5 times the deco gas needed is there to take provide gas for things like increased sac rates or the need to do more decompression than planned.
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