Question re: minimum air volume when decompression diving

[90to90]

I'm hoping I don't embarrass myself with this question, but I was having a discussion with a fellow diver who is interested in taking the first steps into tech diving, and the topic of air volume requirements came up. I am not (yet) a tech diver, but am familiar with some of the basic concepts of deco diving. I told my buddy that when I take ANDP, if I don't do it in my sidemount kit, I will probably get two steel 50's for a backmount rig, since I am a smaller woman and honestly probably don't have the strength to put two steel 80's on my back in addition to carrying a bail out. This buddy told me that 100 cuft of air wouldn't be enough for a deco dive. By my understanding, air volume requirements are dictated mostly by depth, bottom time and one's SAC rate, no? So couldn't someone with a pretty low SAC rate who wasn't trying to hang out at 130' for half an hour plan a safe, shallower, shorter deco dive with 100 cuft of air, while still respecting the rule of thirds? Again I haven't taken ANDP myself so haven't planned out a deco dive yet, but requiring you carry a minimum of 160 cuft of air, not including bail out, for a conservative deco dive seems excessive to me.

 

[tursiops]

What is your SAC rate? How long might you spend at 130 ft? How much gas would that alone take?

 

[Marie13]

You may find your future AN/DP instructor won’t allow double 50s.

 

[ColinT3]

I'm hoping I don't embarrass myself with this question, but I was having a discussion with a fellow diver who is interested in taking the first steps into tech diving, and the topic of air volume requirements came up. I am not (yet) a tech diver, but am familiar with some of the basic concepts of deco diving. I told my buddy that when I take ANDP, if I don't do it in my sidemount kit, I will probably get two steel 50's for a backmount rig, since I am a smaller woman and honestly probably don't have the strength to put two steel 80's on my back in addition to carrying a bail out. This buddy told me that 100 cuft of air wouldn't be enough for a deco dive. By my understanding, air volume requirements are dictated mostly by depth, bottom time and one's SAC rate, no? So couldn't someone with a pretty low SAC rate who wasn't trying to hang out at 130' for half an hour plan a safe, shallower, shorter deco dive with 100 cuft of air, while still respecting the rule of thirds? Again I haven't taken ANDP myself so haven't planned out a deco dive yet, but requiring you carry a minimum of 160 cuft of air, not including bail out, for a conservative deco dive seems excessive to me.

Something to consider, especially in technical diving, it's not just about you, but the team. You'll cover gas planning in your training, but remember you need to be able to service the needs of others in the team in the event of them losing gas for any reason. And no reason not to do it in sidemount (as someone who Tec dives sidemount ).

 

[Rilelen]

I dive (privately) on double 50s on deep deco dives; my SAC rate is low enough (< .30) that they give plenty of bottom time plus reserve, even on thirds.

That said, I’ve used LP85s for all my classes. If nothing else, doing so gives us more “class time” underwater, and allows for proper gas matching with the instructor and classmates.

(I am also a petite woman)

 

[scubadada]

Hi @Rilelen

Have you voted in the Average Gas Consumption poll? Only a dozen divers with an average RMV <0.30 cu ft/min.

Average Gas Consumption

There has been extensive discussion lately regarding average gas consumption. A poll may help give us a better idea of what the normal distribution of RMVs is. 1 cu ft = 28.32 liters 1 liter = 0.035 cu ft 28.3 l/min

scubaboard.com

 

[Angelo Farina]

My wife has a very low SAC, around 10 liters/minute or less (I am around 13).
Years ago, she conducted routinely recreational deco dives in air, down to 50m max and with deco stops (never deeper than 6m) using a single 15 liters steel tank (3000 liters when full).
At the time, I was using a twin tank of 9+9 liters (3600 liters total).
Of course deco procedures mandate for an additional air tank at the deco bar at 6m, for emergencies.
So yes, within recreational limits, a deep (and so, short) deco dive does not require a very huge amount of back gas.
OK, a dual 50 means just 2830 liters, a bit short of the 3000 liters my wife was using. But if your SAC rate is truly small, it could be enough.
Still, an additional smaller tank (say, 8-10 liters) is always recommended for bailout - extended emergency deco.

 

[Rilelen]

Hi @Rilelen

Have you voted in the Average Gas Consumption poll? Only a dozen divers with an average RMV <0.30 cu ft/min.

Average Gas Consumption

There has been extensive discussion lately regarding average gas consumption. A poll may help give us a better idea of what the normal distribution of RMVs is. 1 cu ft = 28.32 liters 1 liter = 0.035 cu ft 28.3 l/min

scubaboard.com

I have…I’m one of those dozen lol!

 

[boulderjohn]

As ColinT3 pointed out, it is not just you on the dive; your team will plan dives based on the amount of gas carried by the most compromised diver, and that would be you. Your dives even in those shallow depth will be very short.

I have done a number of technical dives with petite females. One was well known on ScubaBoard years ago (TSandM--Lynne Flaherty) and the other is Natalie Gibb, owner of Under the Jungle in Mexico. Both carried the same gear the rest of us did on our dives. I would guess Natalie is about 4-11 tall. When I was bothered by a knee injury once, Natalie offered to carry my tanks to the site. (I declined.)

 

[rjack321]

I'm hoping I don't embarrass myself with this question, but I was having a discussion with a fellow diver who is interested in taking the first steps into tech diving, and the topic of air volume requirements came up. I am not (yet) a tech diver, but am familiar with some of the basic concepts of deco diving. I told my buddy that when I take ANDP, if I don't do it in my sidemount kit, I will probably get two steel 50's for a backmount rig, since I am a smaller woman and honestly probably don't have the strength to put two steel 80's on my back in addition to carrying a bail out. This buddy told me that 100 cuft of air wouldn't be enough for a deco dive. By my understanding, air volume requirements are dictated mostly by depth, bottom time and one's SAC rate, no? So couldn't someone with a pretty low SAC rate who wasn't trying to hang out at 130' for half an hour plan a safe, shallower, shorter deco dive with 100 cuft of air, while still respecting the rule of thirds? Again I haven't taken ANDP myself so haven't planned out a deco dive yet, but requiring you carry a minimum of 160 cuft of air, not including bail out, for a conservative deco dive seems excessive to me.

Step one: How much gas does it take to get you and your stressed OOA buddy from the bottom to your next breathable gas source? (deco gas or the surface depending on what you you're carrying)
Step two: How much gas do you (unstressed) need for the planned bottom time and depth?

Add those volumes together. Your unstressed SAC rate is only one small part of the overall planned gas needs.