Why a 120 tank?

[tracydr]

Post dive trip thoughts, and I can't noodle this out. 4 dives off Hatteras this past weekend with a single 100 (and 19cu pony). On 28% I got about 14mins bottom time (~120FSW), which seemed the same as the 120 guys. My computer was flipping between gas time and NDL time as my limiting factor at depth.

So what am I missing? What's gained by a 120? Is it just that their 120= my100+pony? (and no, I wasn't burning into my pony)....any insight is appreciated. Thanks

Not everyone has the same SAC. It's also nice to have extra gas, because when you plan your gas you should be planning for you and your buddy and an emergency reserve.
If you have an "H" valve with two regs and a side-slung deco tank you could also do some decompression dives with the 120. Lots of reasons the 120 can come in handy.
Mainly, though, gas is like money. You can never have too much.

 

[rhwestfall]

taking on to James' post,

300-500 held in reserve or gauge accuracy (keep in mind, 135 psi means no air delivery), that is 2300 psi

3442 (assuming HP) - 2300 = 1142

That is your down, out, and back gas......

Starting rule of thumb in rock bottom is tank volume = depth.....

 

[maniago]

What jcaplins said. Yeah I've read rockbottom planning several times. I understand it and I understand that it assumes you've not done any risk management in your specific dive. Its a tool. But its not a religion. YMMV

 

[rhwestfall]

....yet it is the reason one may select the 120 you asked about.

 

[tracydr]

There's the mandatory safety stop for a dive greater than 100 feet to consider and/or dives within 3 pressure groups of NDL so a dive this deep is not a true "no-ceiling dive" at least not in the purest sense of the word.
I think a 120 ( with redundancy of some sort would be nice), or double smaller tanks would be a nice choice on a dive like this for somebody with advance nitrox or other intro-tech training or even for somebody who simply wants extra conservatism in their dive plan. Just because you're carrying extra gas doesn't mean that you have to use it to stay at depth long enough to get bent.
Gas is meant to stay safe, not to get hurt. The number one reason for scuba fatality is running out of air.

Whenever I encounter a problem under water that causes me stress, I just remind myself that I have plenty of air and that I can still breathe. ( hopefully that's the case). If I'm still breathing and have air, I can still solve my problem. ( hopefully) It's worked to calm me down lots of times, especially in cold water when I've had mask floods or kelp entanglements.

 

[beanojones]

[Rock bottom is] a tool. But its not a religion.

Let me just say that the last sentence there is absolutely wrong. It is a religion, absolutely.

Ritual, incantation, belief, condemnation, zeal, evangelism.

Yep, religion.

 

[fdog]

Post dive trip thoughts, and I can't noodle this out. 4 dives off Hatteras this past weekend with a single 100 (and 19cu pony). On 28% I got about 14mins bottom time (~120FSW), which seemed the same as the 120 guys. My computer was flipping between gas time and NDL time as my limiting factor at depth.

So what am I missing? What's gained by a 120? Is it just that their 120= my100+pony? (and no, I wasn't burning into my pony)....any insight is appreciated. Thanks

To get back to your question, given your dive profile you should be arriving back at the line with ~33 cubic feet in your cylinder. 33+19 = 52 cubic feet, close enough to rock bottom.

So you are following roughly the same plan as the guys with 120's who are running Rock Bottom, albiet unintentionally.

I understand that most folks get "cubic feet sticker shock" when they first get exposed to Rock Bottom. Still, I find myself somewhat religious about NOT breathing water, so I calculate out an appropriate gas management plan before every dive, whether that be thirds, RB, Team +1, or whatever will get us back to the surface.

You mentioned something that piqued my interest:

<snip> it assumes you've not done any risk management in your specific dive. <snip>

I'm not trying to be adversarial, I'm genuinely interested in what risk management you're refering to that would take the place of Rock Bottom in this dive profile?

All the best, James

 

[jcaplins]

There's the mandatory safety stop for a dive greater than 100 feet to consider and/or dives within 3 pressure groups of NDL...

I have never heard this before (at least I can't remember... my initial OW training was through NAUI almost 20 years ago.) so I googled it, and could only see it being referred to from PADI training using the eRDPML electronic dive planner.

NAUI seems to recommend a 3 min safety stop at 15 feet for dives greater than 100'. But it's not required/mandatory as part of the dive plan to keep from getting bent, just a good idea.

So, If I am not using the eRDPML nor PADI certified, and I have my own computer to beep at me, technically there is no mandatory safety stop for the purposes of safely getting to the surface in an emergency for a dive greater than 100'.

The reality is, I always do at least a 3 min stop at 15' regardless of depth or time. I try to use a HP120 whenever I can for the max bottom time I can get, and whatever is left while sitting at 15' I'll gulp down watching things swim by till I hit 500psi.

So, after all that.... It's still a "no-ceiling dive."

 

[KD8NPB]

I go by the safety idea of 1 cf per ft of depth.

120 ft dive = 120 cf of backgas, minimum.

Hence why I dive double 72s. They're perfect for a beginning nitrox / deep / technical diver.

 

[eelnoraa]

Let's do a quick calculation for gas planning. Say at 120ft, you buddy's gas supply has issue. He signals you. You comes to help, spend 2 minutes sorting out the issue, share air and start to ascent from 120ft. You do 30ft/min ascent to 15ft, this will take you 3.5 min. 3 min safety stop at 15ft. Then ascent to surface at 30ft/min. Since you and your buddy have issue, your SAC will likely increase. Let's say 1cf/min for both you and your body. If you do this math, it will take 50cf. This is your rock bottom air. This leave your 70cf to use. SAC=0.75 mean = 4.6cf/min at 120ft. 70cf gives you 20 min at 120ft. Yes, you gas supply will still last longer than NDL, but not by much.

Keep in mind, if your normal SAC=0.75, under stress, you are more likely go double or even more. So the margine further reduce.