CptTightPants21
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It's not that SAC doesn't matter, but that it is accounted for IN the volumetric reserve.
Rule of 1/3s and different tank sizes
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It's not that SAC doesn't matter, but that it is accounted for IN the volumetric reserve.
victorzamora
Contributor
When you're planning thirds, SAC is completely irrelevant. You and your buddy(buddies) are each allowed to use one third of the smallest gas supply.
If you're planning MinGas, RockBottom, or some other OW-style volumetric reserves scheme, then it matters. For scooter gas planning or CCR bailout planning, SAC also matters.
CptTightPants21
Contributor
We are saying the same thing. I was attempting to clarify for others who might be confused by the "SAC doesn't matter comment".
Kevrumbo
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Vic, SAC is not completely irrelevant per se ("completely irrelevant" is an incorrect qualitative term) -but it is actually "transparent", conserved and automatically accounted for without explicitly evaluating or implementing it in proper practical application of limiting Third's Turn Volume. The higher nominal SAC rate usually determines who reaches the team's Turn Volume first based on the diver with the smallest usable gas supply.
Makes sense?
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@stuartv I'll disregard your lack of helium in that mix, so hopefully you have a good narcosis tolerance if you plan on staying that deep with 24% He in the mix. Personally I would have 35-40% in there, but that's just me.
Now, IIRC they tie into the top of the stack. The swim from the bow is going to be close to 500ft, that swim is 10 minutes alone assuming no current. That gives you 0 minutes to resolve conflict with your current plant
Why are you using 70% deco instead of 50% deco? Running 50/80 with a 25 minute bottom time at 170ft, you actually save a minute of deco by using 50% instead of 70% as well as lowering the gas requirements for your OOA scenario.
Why would you give a 700psi reserve if the dive goes well? Based on the rough numbers, that 2700/3500 psi should give you roughly 13 minutes before you turn around, so hopefully you actually plan on using all of the expensive gas that you bought...
Now, all of that said, you need 2 pressures for your "turn" pressures. One is the bow turn pressure, that is the 2000psi that you already calculated. The other needs to be your "ascend" pressure which is basically rock bottom. This gives you a pressure that says "i have to get closer to the mooring line" and then one that says "i have to start going up the mooring line". That gives you the time where you can poke around at the bottom.
From there, you have to figure out how many minutes it would be to suck down the tanks to that pressure. I typically round that up to the next 5 minute interval to account for any exciting things that happen. You need to do this with your expected SAC rate, not an elevated one to round your bottom time to "safety" which is longer. THEN you can run that bottom time through your deco software to make sure you actually have enough deco gas with you. Planning the actual dive is easy assuming things go mostly to plan with time, planning the rest of it for your contingency plans is the complicated bit
Now, IIRC they tie into the top of the stack. The swim from the bow is going to be close to 500ft, that swim is 10 minutes alone assuming no current. That gives you 0 minutes to resolve conflict with your current plant
Why are you using 70% deco instead of 50% deco? Running 50/80 with a 25 minute bottom time at 170ft, you actually save a minute of deco by using 50% instead of 70% as well as lowering the gas requirements for your OOA scenario.
Why would you give a 700psi reserve if the dive goes well? Based on the rough numbers, that 2700/3500 psi should give you roughly 13 minutes before you turn around, so hopefully you actually plan on using all of the expensive gas that you bought...
Now, all of that said, you need 2 pressures for your "turn" pressures. One is the bow turn pressure, that is the 2000psi that you already calculated. The other needs to be your "ascend" pressure which is basically rock bottom. This gives you a pressure that says "i have to get closer to the mooring line" and then one that says "i have to start going up the mooring line". That gives you the time where you can poke around at the bottom.
From there, you have to figure out how many minutes it would be to suck down the tanks to that pressure. I typically round that up to the next 5 minute interval to account for any exciting things that happen. You need to do this with your expected SAC rate, not an elevated one to round your bottom time to "safety" which is longer. THEN you can run that bottom time through your deco software to make sure you actually have enough deco gas with you. Planning the actual dive is easy assuming things go mostly to plan with time, planning the rest of it for your contingency plans is the complicated bit
Agree with Tom on 50%.....just my preference to go to deco gas earlier. Also curious, what's your lost deco plan?
I didn't check run time with 50%. I checked with 60% and it was longer than with 70%, so I assumed 50% would be even longer. I'll check it, though. If it's faster, I'll use it.
My helium mix is because I am only certified to get a max helium fraction of 20%. So, I don't think I can get a shop to give me more than that - other than just being off a bit. I dived the JB King at 160' in the Saint Lawrence on EAN23 with no discernible effects of narcosis. I felt clear as a bell during and after the dive. I reviewed the dive with my buddies after and confirmed I could clearly remember all the details of the dive. And that dive was darkish and coldish. Thus why I feel okay with only 20% helium on the O.
Lost deco gas plan would be for my buddy to also carry an AL80 of the same deco gas as me. With the plan being for each of us to need less than half the cylinder.
My helium mix is because I am only certified to get a max helium fraction of 20%. So, I don't think I can get a shop to give me more than that - other than just being off a bit. I dived the JB King at 160' in the Saint Lawrence on EAN23 with no discernible effects of narcosis. I felt clear as a bell during and after the dive. I reviewed the dive with my buddies after and confirmed I could clearly remember all the details of the dive. And that dive was darkish and coldish. Thus why I feel okay with only 20% helium on the O.
Lost deco gas plan would be for my buddy to also carry an AL80 of the same deco gas as me. With the plan being for each of us to need less than half the cylinder.
Kevrumbo
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Some initial MGR parameters on @stuartv Oriskany Plan:
1.5 cf/min x (108/33 + 1)ATA x 6min = 38 cf
Therefore MGR for two Divers: 38 cf x 2 = 76 cf (round to 80 cubic feet for an actual Rock Bottom SPG reading of 1100psi). So 80 cf to get to Eanx70 switch depth of 45' from 170' in an emergency gas sharing contingency.
Total initial fill at 3500psi in double hp120's is 240 cf.
Hence Usable Gas Amount for Keep It Simple Scuba dive profile mission on Oriskany is:
240cf minus 80cf = 160 cubic feet usable.
No penetration, and keeping AL80 of Eanx70 clipped-on: use Rule of Halves -with 80 cf consumed outbound towards bow at 170' alongside Hangar Deck, and 80 cf back to tower mooring/ anchorline at the top of the Island/Flight/Bridge Superstructure. Delta Turn Pressure is 1100psi consumed and down from 3300psi for an actual Turn Pressure SPG reading of 2200psi. Option of using all 160 cf, traversing all the way at 170' to the bow and ascend up at bow's location mooring/ anchorline if Sac Rate & currents are in your favor -but you better be sure to close within the distance of the bow, and can see & reach the mooring/anchorline at the bow with 1100psi actual reading on the SPG, or else you've got a blue water ascent to deco under a deployed SMB.
If all is nominal & benign with no current and easy & low Sac Rate range of 0.6 to 0.7, you should have some time and gas to explore the Island/Bridge Superstructure at 140' to 80'. For a normal no current, no physical exertion ascent to your Eanx70 deco stop at 40fsw, expect to use only 25% of your MGR volume (20 cf); that will leave you with 60 cf of backgas to use after you complete your deco profile should any big pelagics or marine mammals come cruising by shallow to play with you.
Do you have a Nautilus Lifeline VHF broadcast beacon and/or a direct to SAR satellite PLB? And for exploring something this big, how come you aren't utilizing a scooter ? (For the WWII USS Saratoga Aircraft Carrier in Bikini Atoll 2013, we all brought DPV/Scooters to cover the grand tour over 888 feet of it's entire length, and depth ranges from 50' to 190' deep for eight consecutive days). . .
Also, your 20/24 Trimix has a gas density of 5.57 g/L at 170fsw: per Dr. Simon Mitchell's recommendations, a value of less than 6 g/L with plenty of margin is best for preventing CO2 retention, especially if you're expecting a high physical activity heavy breathing Sac Rate at 170fsw approaching 1 cf/min (again, you and your buddy should be using scooters for this reason as well, especially if working against a current at depth!). For future reference when you get full Trimix certified, a standard mix of 21/35 Trimix has a gas density of 4.94 g/L, and 18/45 of 4.33 g/L respectively at 170'.
What made you decide only on one big AL80 of Eanx70 with switch at 40fsw, instead of an AL40 of Eanx50 and switch at 70fsw? The MGR backgas volume going from 170' to 70' is 60.3 cf (round it to 65 cubic feet for a SPG actual reading of 930psi), instead of 80 cf from 170' to 45'. Also you start your decompression & off-gassing sooner as @tbone1004 and @decompression above stated at 70fsw with Eanx50, and together with an AL40 Oxygen deco stop at 20fsw, you get a cleaner more efficient inert gas elimination from those stubborn supersaturated slow tissues, as well as two deco gases backing each other up if you lose one or have an unusable deco bottle scenario. . .
What's your deco schedule & profile look like for 170' with 23min BT and Eanx70 deco gas?
For 21/35 with Eanx50 and O2:
Buhlmann ZHL-16B 50/80 (from Deco Planner 3.1.4):
170', 25min;
(ascent rate 30'/min);
70', 1min (Eanx50);
60', 1min;
50', 2min;
40', 2min;
30', 5min;
20', 10min (Oxygen);
10', 6min -->same as slow 3 foot per min ascent to surface on O2.
And how many deco dives do you plan on doing over how many days?
Finally and most important, where is the nearest recompression chamber that will treat emergency diving accidents 24/7? (There's NEDU in Panama City, but the Navy might only treat civilian casualties as a last resort). . .
- Eanx70 gas switch depth at 40fsw (45fsw @ 1.6ATA)
- 5 to 6min ascent time including 1 min to problem solve and donate reg for emergency gas share at 170', and then ascend 30 fpm from 170fsw to 45fsw
- Emergency stressed SAC rate given above as 1.5 cf/min
- Average depth 108fsw on ascent from 170' to 45'
1.5 cf/min x (108/33 + 1)ATA x 6min = 38 cf
Therefore MGR for two Divers: 38 cf x 2 = 76 cf (round to 80 cubic feet for an actual Rock Bottom SPG reading of 1100psi). So 80 cf to get to Eanx70 switch depth of 45' from 170' in an emergency gas sharing contingency.
Total initial fill at 3500psi in double hp120's is 240 cf.
Hence Usable Gas Amount for Keep It Simple Scuba dive profile mission on Oriskany is:
240cf minus 80cf = 160 cubic feet usable.
No penetration, and keeping AL80 of Eanx70 clipped-on: use Rule of Halves -with 80 cf consumed outbound towards bow at 170' alongside Hangar Deck, and 80 cf back to tower mooring/ anchorline at the top of the Island/Flight/Bridge Superstructure. Delta Turn Pressure is 1100psi consumed and down from 3300psi for an actual Turn Pressure SPG reading of 2200psi. Option of using all 160 cf, traversing all the way at 170' to the bow and ascend up at bow's location mooring/ anchorline if Sac Rate & currents are in your favor -but you better be sure to close within the distance of the bow, and can see & reach the mooring/anchorline at the bow with 1100psi actual reading on the SPG, or else you've got a blue water ascent to deco under a deployed SMB.
If all is nominal & benign with no current and easy & low Sac Rate range of 0.6 to 0.7, you should have some time and gas to explore the Island/Bridge Superstructure at 140' to 80'. For a normal no current, no physical exertion ascent to your Eanx70 deco stop at 40fsw, expect to use only 25% of your MGR volume (20 cf); that will leave you with 60 cf of backgas to use after you complete your deco profile should any big pelagics or marine mammals come cruising by shallow to play with you.
Do you have a Nautilus Lifeline VHF broadcast beacon and/or a direct to SAR satellite PLB? And for exploring something this big, how come you aren't utilizing a scooter ? (For the WWII USS Saratoga Aircraft Carrier in Bikini Atoll 2013, we all brought DPV/Scooters to cover the grand tour over 888 feet of it's entire length, and depth ranges from 50' to 190' deep for eight consecutive days). . .
Also, your 20/24 Trimix has a gas density of 5.57 g/L at 170fsw: per Dr. Simon Mitchell's recommendations, a value of less than 6 g/L with plenty of margin is best for preventing CO2 retention, especially if you're expecting a high physical activity heavy breathing Sac Rate at 170fsw approaching 1 cf/min (again, you and your buddy should be using scooters for this reason as well, especially if working against a current at depth!). For future reference when you get full Trimix certified, a standard mix of 21/35 Trimix has a gas density of 4.94 g/L, and 18/45 of 4.33 g/L respectively at 170'.
What made you decide only on one big AL80 of Eanx70 with switch at 40fsw, instead of an AL40 of Eanx50 and switch at 70fsw? The MGR backgas volume going from 170' to 70' is 60.3 cf (round it to 65 cubic feet for a SPG actual reading of 930psi), instead of 80 cf from 170' to 45'. Also you start your decompression & off-gassing sooner as @tbone1004 and @decompression above stated at 70fsw with Eanx50, and together with an AL40 Oxygen deco stop at 20fsw, you get a cleaner more efficient inert gas elimination from those stubborn supersaturated slow tissues, as well as two deco gases backing each other up if you lose one or have an unusable deco bottle scenario. . .
What's your deco schedule & profile look like for 170' with 23min BT and Eanx70 deco gas?
For 21/35 with Eanx50 and O2:
Buhlmann ZHL-16B 50/80 (from Deco Planner 3.1.4):
170', 25min;
(ascent rate 30'/min);
70', 1min (Eanx50);
60', 1min;
50', 2min;
40', 2min;
30', 5min;
20', 10min (Oxygen);
10', 6min -->same as slow 3 foot per min ascent to surface on O2.
And how many deco dives do you plan on doing over how many days?
Finally and most important, where is the nearest recompression chamber that will treat emergency diving accidents 24/7? (There's NEDU in Panama City, but the Navy might only treat civilian casualties as a last resort). . .
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@Kevrumbo for that dive I'd take an 80 of 50% and not bother switching between a pair of 40's. Bit too much to go wrong, and it's also possible that his certification doesn't allow for 2 decompression gases. The deco efficiency may be better with O2 at the top, but the total deco time won't be reduced significantly. Total deco goes from 31 mins to 25 mins by adding O2. Not enough for me to deal with a second bottle personally.
Depending on surface conditions as well, I may not want to have to finish my deco at 20ft. I'd rather spend an extra 5 minutes down at 30ft to stay out of any surge and chop instead of being bounced around depending on how exciting surface conditions are.
For deco planning
6.2ata*.6cfm SAC rate=3.7cfm at depth.
160cf available is a 45 minute total bottom time.
That is important because it is your sh!t really hit the fan decompression obligation.
On 50/80, if you lose deco gases, you're in for a 177 minute deco, requiring about 160cf of gas assuming .5cfm. You really don't want to have that happen.
On 50/80 with 70%, that yields 68 minutes of deco, and 55cf
On 50/80 with only 50%, that gives 72 minutes of total deco, with a gas requirement of 69cf.
50% and 100% gives you 56 minutes, with 36cf out of the 50% bottle, and 29 out of the 100%
100% only gives you 89 minutes and 34cf.
The 70% does get you out of the water 4 minutes earlier with the longer bottom times, but the 50% gets you off of the deep mix about the same 4 minutes earlier which allows for a bit more oh sh!t moments at the bottom for your gas planning and is a worthwhile trade off in my opinion. It's only 10cf of backgas saved, but it doesn't hurt.
My gas plan for that dive? I'd carry an 80 of 50%, and clip off a 40 of 100% at 20ft. You already blew your time limit out of the water, and the second failure would be a loss of one of the 50% bottles. Third strike and you're out is not getting on the anchor line to get to the O2 bottles and in that case you get up as fast as you can to the shallow stops to save gas in the 50% bottles and deco as long as you can on that, switch to back gas for as long as you can, then pray to a God that you may or may not believe in to keep you from getting bent.
If all goes according to plan with the 25 minute bottom time, you'll only take 20cf each out of the 50% and 100% bottles, and have a deco of 24 minutes, or 34cf/31mins out of only a 50% bottle if you are behaving to your certification with a single deco gas. In either scenario though, I'd have 50% in an al80 on me, and at least a 40 of O2 each, or an AL80 with a pair of second stages clipped off to the mooring line just in case.
Depending on surface conditions as well, I may not want to have to finish my deco at 20ft. I'd rather spend an extra 5 minutes down at 30ft to stay out of any surge and chop instead of being bounced around depending on how exciting surface conditions are.
For deco planning
6.2ata*.6cfm SAC rate=3.7cfm at depth.
160cf available is a 45 minute total bottom time.
That is important because it is your sh!t really hit the fan decompression obligation.
On 50/80, if you lose deco gases, you're in for a 177 minute deco, requiring about 160cf of gas assuming .5cfm. You really don't want to have that happen.
On 50/80 with 70%, that yields 68 minutes of deco, and 55cf
On 50/80 with only 50%, that gives 72 minutes of total deco, with a gas requirement of 69cf.
50% and 100% gives you 56 minutes, with 36cf out of the 50% bottle, and 29 out of the 100%
100% only gives you 89 minutes and 34cf.
The 70% does get you out of the water 4 minutes earlier with the longer bottom times, but the 50% gets you off of the deep mix about the same 4 minutes earlier which allows for a bit more oh sh!t moments at the bottom for your gas planning and is a worthwhile trade off in my opinion. It's only 10cf of backgas saved, but it doesn't hurt.
My gas plan for that dive? I'd carry an 80 of 50%, and clip off a 40 of 100% at 20ft. You already blew your time limit out of the water, and the second failure would be a loss of one of the 50% bottles. Third strike and you're out is not getting on the anchor line to get to the O2 bottles and in that case you get up as fast as you can to the shallow stops to save gas in the 50% bottles and deco as long as you can on that, switch to back gas for as long as you can, then pray to a God that you may or may not believe in to keep you from getting bent.
If all goes according to plan with the 25 minute bottom time, you'll only take 20cf each out of the 50% and 100% bottles, and have a deco of 24 minutes, or 34cf/31mins out of only a 50% bottle if you are behaving to your certification with a single deco gas. In either scenario though, I'd have 50% in an al80 on me, and at least a 40 of O2 each, or an AL80 with a pair of second stages clipped off to the mooring line just in case.
victorzamora
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