Why Surface with 500 PSI?

[rsingler]

Cells C10 and A10 are still incorrect in the new metric version...

Back to work!

EDIT: cells corrected. I'm waiting for corroboration from @Neokian before I post them, to see if I can avoid yet another version...

DONE! @Neokian came through (and found two more errors).
I've just posted v6 of the metric thread in Post #1. Thank you, @Neokian !!

 

[BRT]

NO-DECO ROCK BOTTOM GAS CALCULATION

Tank: 3000 psi 77.4 cu ft Tank factor = 38.8 psi/cuft
Normal RMV 0.7 cfm = 27.1 psi/min SAC
Initial RMV Safety Factor 3 = 2.1 cu ft/min initial RMV
Depth 80 ft = 3.4 atm max
Asct (fpm) Time (min) Mean Atm RMV Cu Ft Used PSI Used
Problem Solving: 0 1 3.4 2.1 7.2 279
Ascent Calc #1 (max depth to 60'): 30 0.67 2.6 2.1 3.7 142
Ascent Calc #2 (60' to 15'): 30 1.5 2.1 1.4 4.5 174
Safety Stop 0 3 1.5 0.7 3.1 118
Surfacing: 30 0.5 1.3 0.7 0.4 17
Total ascent cu ft = 18.8 730 Total ascent psi
150 Min tank psi
ROCK BOTTOM PRESSURE (1 Diver) 880 psi
ROCK BOTTOM PRESSURE (2 Divers) 1610 psi
ROCK BOTTOM PRESSURE (1 Diver) w/o Safety Stop 762 psi
ROCK BOTTOM PRESSURE (2 Divers) w/o Safety Stop 1374 psi

So I captured a screen shot of the version 5 of the sheet.

What this is telling us is that for a dive to just 80 feet and without a safety stop, a diver (with a buddy) will need to leave the bottom with around 1,400 psi in aluminum 80 tank.

Who does that? Seriously who?

I dive with a pony bottle, so I can use a different approach. For the people who don't use a pony bottle, what pressure do you leave the bottom with?

Depends on conditions. If things were really knarley and I wasn't sure about the boat, bad viz, high on nitrogen, high current, I might leave the bottom at half a tank. But normally? If there was no way to gradually come up the slope I'd start up at 5-600 lbs.

 

[Neokian]

If there was no way to gradually come up the slope I'd start up at 5-600 lbs.

Quite an ascent at 5-600 pounds!

 

[tursiops]

Quite an ascent at 5-600 pounds!

Agreed. I do hope he means psi, not pounds.

 

[rsingler]

So, for an experienced diver, how could 600 psi work?
Well, assume a good SAC of 0.5cfm. Assume an equally calm buddy.
Assume that even though you've got good air consumption, he's breathing double at the beginning of the OOA event, while you're still calm. That averages out to a stressed RMV of 1.5x.
Take 1 minute to get joined up on your octo and start up quickly (60fpm) until you reach 60 feet:

You could both ascend and make it to the surface with 150 psi left in the tank if you skipped your safety stop.
Most beginning divers couldn't do this, but at least it explains why someone might say they can dive that long at 85 feet. And with no OOA event and a standard ascent and safety stop, this experienced diver would finish with 450 psi in the tank.

 

[BRT]

Quite an ascent at 5-600 pounds!

Do you really think so? Diving with the fishermen I've left the bottom, 110 feet, at 600 or so when I had no more bottom time and then stairstepped up with my computer while following the guy I was with until I was just following his bubbles, then came to 15' and did an extended safety stop. Got on the boat with 100-200 psi. Always (barely) within my computers NDL's, but wasn't interested in surfacing with my Oceanic maxed out. Yes, I consider this more extreme than I really want to do. But if I had not stairstepped up and kept following him I would have been able to stay on a safety stop for as long as I wanted to. As soon as I quit trying to fight the current and keep up with the fisherman my air consumption drops way down. A couple hundred pounds goes a long way at 15 feet. And if it doesn't.....go up to the big air tank. So long as there is a slope to follow like we usually have in Cozumel, I'd rather get shallower and extend the dive time.

 

[rsingler]

But remember, this is the Basic Forum.
Do we really want to advocate diving 85 feet to a tank pressure of 600psi?
No matter what we do personally.

 

[BRT]

But remember, this is the Basic Forum.
Do we really want to advocate diving 85 feet to a tank pressure of 600psi?
No matter what we do personally.

You are correct. On the other hand do you think the new divers you are on the boat with will believe you are leaving the bottom with 1400 psi? My buddy, who is not very good on air and always trying to figure out how to do better is often hanging above everybody else to extend his dive time. He often tells me "All those people who don't use much air are skip breathing". You see, he watches their bubbles.

 

[johndiver999]

But remember, this is the Basic Forum.
Do we really want to advocate diving 85 feet to a tank pressure of 600psi?
No matter what we do personally.

I don't necessarily jump to the conclusion that when someone states what practices that they follow it is intended to be construed as advocacy. To do so,may have a stifling impact on discussion and honesty.

This sheet should show a smart, basic diver that if they are following typical gas reserve requirements, that there ain't no way they can afford to get excited by the air share, they need to not screw around for even one second on the bottom and the ascent had better have the computer beeping a little here and there.

I had an incident which really surprised me. I was diving with a good buddy in 90 or so feet on a drift dive catching lobsters or something in Florida. It was getting toward the end of our dive. I look up and he is flying toward me, waving and has his regulator out of his mouth. I was so surprised and I guess, reasonably confident that he could handle things that I did nothing for the few seconds it took to approach me.

Then I proceeded to do nothing. I knew I had a good bit of air and just paused for a few moments and let him decide if he was going to snatch the reg from my mouth or take the octopus. In retrospect, that was probably pretty irresponsible. He later indicated that his reg. suddenly and for no reason, completely stopped delivering any air - but on the surface - on the boat afterwards - it worked.. anyway.. back to the story.

He took my octopus and I pressed my up button and grabbed onto him. He was breathing like a freight train and I was still totally calm and somewhat amused that he had screwed up so bad. So my air use was completely normal. I think I had like 1000 psi or something on the bottom.

So when we (quickly) got to the safety stop (with ZERO time screwing around on the bottom) I was SHOCKED to see I only had like 200 or 300 psi or something. He had sucked down my air, much, much faster than I would have imagined.

So even if YOU are the best diver in the world and can remain completely calm, there is no way to predict or control how hard the idiot next to you is going to be sucking on YOUR air. Keep that in mind when you select input parameters.

 

[BRT]

I don't necessarily jump to the conclusion that when someone states what practices that they follow it is intended to be construed as advocacy. To do so,may have a stifling impact on discussion and honesty.

This sheet should show a smart, basic diver that if they are following typical gas reserve requirements, that there ain't no way they can afford to get excited by the air share, they need to not screw around for even one second on the bottom and the ascent had better have the computer beeping a little here and there.

I had an incident which really surprised me. I was diving with a good buddy in 90 or so feet on a drift dive catching lobsters or something in Florida. It was getting toward the end of our dive. I look up and he is flying toward me, waving and has his regulator out of his mouth. I was so surprised and I guess, reasonably confident that he could handle things that I did nothing for the few seconds it took to approach me.

Then I proceeded to do nothing. I knew I had a good bit of air and just paused for a few moments and let him decide if he was going to snatch the reg from my mouth or take the octopus. In retrospect, that was probably pretty irresponsible. He later indicated that his reg. suddenly and for no reason, completely stopped delivering any air - but on the surface - on the boat - it worked.. anyway.. back to the story.

He took my octopus and I pressed my up button and grabbed onto him. He was breathing like a freight train and I was still totally calm and somewhat amused that he had screwed up so bad. So my air use was completely normal. I think I had like 1000 psi or something on the bottom.

So when we (quickly) got to the safety stop (with ZERO time screwing around on the bottom) I was SHOCKED to see I only had like 200 or 300 psi or something. He had sucked down my air, much, much faster than I would have imagined.

So even if YOU are the best diver in the world and can remain completely calm, there is no way to predict or control how hard the idiot next to you is going to be sucking on YOUR air. Keep that in mind when you select input parameters.

I agree, but even in that situation your 1000 psi got you both out, with a safety stop, from deeper than 80'.