Question re: minimum air volume when decompression diving

[tursiops]

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

Something to consider, especially in technical diving, it's not just about you, but the team.

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.

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

We are all on the same page. My post #2 was trying to get there by letting the OP self-discover...first, what are HER needs (I doubt she knows yet) and only then, what are the needs of her buddy? It is the combined needs that count, as we are all saying or (or would have said if there had been a chance to get there....).
And this is why one takes formal training....

 

[inquis]

while still respecting the rule of thirds?

A 1/3 reserve is woefully short in many cases and unnecessarily conservative in others. You might look into the topic of "Rock Bottom" (also known as "Minimum Gas"), as it will walk you through the considerations in Step 1 of @rjack321's list. Moreover, this approach is applicable to recreational diving as well, so you can start to use it even before ANDP.

 

[rx7diver]

Even if you're diving solo (so, no buddy concerns) and very petite, then if you're deco diving either side-mount or BM independent doubles, I think you'll find that Faber LP50's won't provide enough gas for you to safely incur a lengthy deco obligation. (Open circuit diving.)

rx7diver

 

[scubadada]

Even if you're diving solo (so, no buddy concerns) and very petite, then if you're deco diving either side-mount or BM independent doubles, I think you'll find that Faber LP50's won't provide enough gas for you to safely incur a lengthy deco obligation. (Open circuit diving.)

rx7diver

Good point, I do most of my diving solo and do back gas light deco about 5% of the time, deco generally <10 min. My average RMV is not as low as that of @Rilelen at 0.36 cu ft/min. For me, a 17 min dive to 150 feet on air only requires about 43 cu ft of gas and about 10 min of deco. I carry a pony for back up gas, useful at any depth.

Of course AN/DP is more advanced than this with accelerated decompression.

 

[90to90]

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 ).

Ah if only it was so easy. There are no tec sidemount instructors within a 200 mile radius of where I currently reside, unfortunately (PNW). There was one, but he stopped instructing tec courses a year ago So, if I want to learn ANDP up here, I need to go backmount.

 

[90to90]

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.

I certainly wouldn't say I have a "jesus take the wheel" approach to diving- truly curious since, again, I have not yet taken any tech courses.

 

[PfcAJ]

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.

So it’s very important to build strength to carry stuff. Tech gear is heavy. No way around it. Taking tanks that are borderline (or worse) because of fitness isn’t a good idea.

Also, you mention a “bailout”. You probably mean a deco bottle. Walking around with that clipped to your chest isn’t a thing.

 

[90to90]

Thanks for the replies everyone. Looks like any future tech diving I do will be in my sidemount kit, somewhere warm. Hah. Appreciate everyone sharing their knowledge.

 

[rx7diver]

Good point, I do most of my diving solo and do back gas light deco ...

@scubadada,

FWIW, when I plan a deco dive, I will plan also for a delay at depth and for a next deeper depth (if there is no hard bottom at my planned Target Operating Depth). So, for example, using your example with tables, I will plan also for a 30 min Bottom Time (cf. your 20 min BT) and for a 160 ffw TOD (cf. your 150 ffw TOD).

So, those LP50's seem a bit anemic (to me) for your intended air deco dive to 150 ffw for 20 min.

rx7diver

 

[Marie13]

Thanks for the replies everyone. Looks like any future tech diving I do will be in my sidemount kit, somewhere warm. Hah. Appreciate everyone sharing their knowledge.

There’s always CCR…