Why Surface with 500 PSI?

[Dan]

I've seen that in our transAtlantic counterparts as well. Perhaps @stepfen or someone would care to comment?

I did a quick comparison between the metric RBP & imperial one by entering metric equivalent of
AL80 = 11 liters,
tank pressure = 207 bar,
Normal SAC = 20 L/min for 0.7 cfm,
Initial SAC Safety Factor = 3
Min tank bar = 10 (instead of 50)
Depth = 30 m (100 ft)
The results show RBP = 68 bar or 985 psi
The Imperial version shows 1025 psi
Those look close enough for me!

 

[Altamira]

I appreciate @rsingler generous sharing of his spreadsheet. I just wish more DMs were aware of, or paid attention to, his data. As a diver that most frequently dives in the Caribbean, usually with an insta-Buddy, I can’t remember the last time a DM said anything other than “…let me know when you are at 1/2 tank” which is often less than the rock bottom time for a diving pair. And good luck getting their attention at 1/2 tank!

 

[rsingler]

I did a quick comparison between the metric RBP & imperial one by entering metric equivalent of
AL80 = 11 liters,
tank pressure = 207 bar,
Normal SAC = 20 L/min for 0.7 cfm,
Initial SAC Safety Factor = 3
Min tank bar = 10 (instead of 50)
The results show RBP = 45 bar or 652 psi
The Imperial version shows 809 psi
Why is that?

I'll look into it. @stepfen ?

 

[Dan]

I'll look into it. @stepfen ?

My bad. I forgot to enter the Depth = 30m (100 ft). The results come out pretty close. RBP ~ 1000 psi (~ 1/3 of an AL80 tank pressure).

I corrected my previous post accordingly.

I like it!

I did a quick comparison between the metric RBP & imperial one by entering metric equivalent of
AL80 = 11 liters,
tank pressure = 207 bar,
Normal SAC = 20 L/min for 0.7 cfm,
Initial SAC Safety Factor = 3
Min tank bar = 10 (instead of 50)
Depth = 30 m (100 ft)
The results show RBP = 68 bar or 985 psi
The Imperial version shows 1025 psi
Those look close enough for me!

 

[jadairiii]

You can estimate where 3/4 mark from 0 to 50 bar on your SPG, can’t you?

the issue was "why 50 bar?", my point was merely for DM's it is easier just to say 50/500 be back on the boat. 50 bar is clearly marked on the spg (like 500). Remember, you're not dealing with Rocket Scientists if you have to tell someone what tank pressure to be back on the boat with.

Read my first post.

 

[rsingler]

My bad. I forgot to enter the Depth = 30m (100 ft). The results come out pretty close. RBP ~ 1000 psi (~ 1/3 of an AL80 tank pressure).

I corrected my previous post accordingly.

I like it!

No. You were right. There's an error in the metric cell H3. I've corrected it and uploaded new spreadsheets (the max ascent rate cells were locked too).

 

[Fastmarc]

Unlock the spreadsheet with "scuba" and change column width. I sized it pretty tight to fit on my phone screen. Perhaps that was a mistake.

@Neokian , thank you for your error checking. I'm working on corrections now.

Thanks, that did the trick.

 

[johndiver999]

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?

 

[rsingler]

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?

YES! That's exactly the point for the beginning divers reading this forum!

When you start diving deeper, if your buddy goes OOA from a malfunction at depth and comes swimming over to you in a panic at the end of your dive at 80 feet,
if you haven't got at least 1374 psi in your tank, you may both go OOA before you reach the surface!
Most divers who start going deeper do NOT start up at 1375. And they get away with it because, like me in Bonaire, they're moseying up the reef with a constantly decreasing Rock Bottom as their depth decreases.

If they take that habit to the Spiegel Grove on a cattle boat, and have a rental reg malfunction on the top of the wreck at 85 feet, they may not make it up without a fast final ascent, risking DCS. My son guides on the Spiegel, and has donated his long hose more than once.

Be a thinking diver! If you are completely calm at 85 feet with 800 psi, you'll be just fine for a normal ascent, and there's even enough for your calm buddy if you skip your safety stop. But there's nothing if you have a real problem. 'Jes sayin.

You're betting. Given the reliability of equipment, you're okay 99.9% of the time. Until you're not. And one in a thousand is about the right frequency. Every so often, we hear a story. Sometimes it ends well; sometimes it doesn't. The issue is real.

You can game the spreadsheet:
use the best SAC you've ever had;
make your stressed RMV only 1.3 x normal instead of 3 or 4;
make the time it takes to "handle" your emergency 0 instead of 2 minutes at depth.
Do that, and you'll have a more palatable Rock Bottom Pressure.
But if you do, and there's a problem right at the end of your bottom time, you have to execute as calmly and quickly as you planned.
Can you do it?

At the very least, when you reach Rock Bottom, make eye contact with your buddy. If everything is nominal, you can make a risk/reward decision to keep having fun, since you're both okay. That's your decision, and dives are extended past RBP all the time. They go okay because nothing comes up. But at least ask the question, and make your risk-taking deliberate, instead of a surprise.

 

[Neokian]

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