Rule of Thirds & Shallow Rec diving

[GCullen94]

Foxfish here's a little quiz... Where do the following items come from?

The Octopus regulator?
Nitrox gas?
The BP/W system?
DIN regulators?


Just wondering where you think those items come from?

 

[Foxfish]

NO WE DO NOT AGREE!!!!!!!!!!!!!! using the METRIC VERSION WE GET 86B and you will RUN OUT OF GAS AS YOU BREAK THE SURFACE. we ROUND UP TO 90B. STOP CONVERTING FROM METRIC TO IMPERIAL USING CONVERSION FACTORS.

You previously said in post 315:

Now we will calculate gas in different tanks: For metric divided L required by tank capacity in L to get B required. For imperial we use tank factors, take CF required divided by tank factor and multiply by 100 to get PSI needed.
...
HP100/12L= 1200PSI, 90B(1166, rounded up to more conservative 100 PSI)
...

Before you said 1200 psi (82.74 b) for 30 m with a 12 L tank was safe. Now you're saying 80 b is unsafe. That is a variation of about 3 b or 40 psi and frankly not something I'd readily pick up off my pressure gauge. Do you seriously believe that this kind of discrepancy is significant?

This silly quibbling over numbers highlights the characteristic lack of judgement and perspective often present in novice divers which is another negative for the 'minimum gas' method.

 

[GCullen94]

Foxfish this is what I am trying to tell you... over 1200 psi, the conversion factor you are using is NOT THAT ACCURATE. There is also a fundamental difference in the ascent pattern from metric and imperial

 

[Foxfish]

Am I correct in saying that you previously calculated for a 12 L tank you need 82.7 b (1160 psi) from 30 m and you rounded that up to 1200 psi.

 

[GCullen94]

I will outline the differences in the ascent profiles AGAIN and hope that you can see they are different

Imperial (1200PSI for HP100)

Each diver 1CFM
Ascent rate 30FPM
3 minutes at 15 feet

Metric (86B, rounded up to 90 so you can actually read the gauge)
Each diver 30L/M
Ascent rate 10M/m
3 minutes at 5M.

While they are close, they are NOT THE EXACT SAME SO USING A CONVERSION IS NOT ACCURATE.

10M/M= 33FPM
30L/M=1.06CF/M
15M=16.5ft.

All of these differences make a fairly large difference in the required minimum pressure. And your conversion factor is probably rounded as well and over a large number IS NOT ACCURATE.
There is no silly "quibbling" when metric or imperial is used, only when trying to use a stupid conversion factor instead of doing the equation properly.

The fact is, you still maintain that you need to leave the bottom to hit with 50B, Min gas is all about cubic footage, not hitting the surface with a center pressure if nothing goes wrong.

 

[Drin]

I will outline the differences in the ascent profiles AGAIN and hope that you can see they are different

I think we're long past the point where Foxfish might be convinced that his "50b standard" is inadequate for safety and inaccurate for calculation purposes. He has chosen instead to dig his heels in and be stubborn, and no amount of logic or common sense will bring him out of his foxhole (no pun intended). He simply does not have an open mind and will unfortunately either end up bent or bend his dive buddy when a diving emergency occurs that his limited imagination insists simply can't happen.

In short you're probably better off not wasting your time posting any more.

On the positive side I'm very happy to see that this thread has educated some other divers as to the benefits of gas management planning. That has made the pain of reading Foxfish's comments worthwhile, so thank you to everyone who has positively contributed to the conversation.

-Adrian

 

[DaleC]

When all caught up, the best advice is not to run with the line but to be still. Often, the hook will work free that way and you will not be so attached.

 

[shoredivr]

Scott, somewhere I still have my OW and AOW PADI books from 1998. Can't remember anything in there remotely resembling even simple gas calculations....I remember feeling like something was missing, like how DO I figure out when to leave the bottom, so I can arrive at the surface with 500psi. True, it could also have been the instructor. Maybe the great decline in gas planning happened between early and late 1990s.
IIRC, in the AOW it was mentioned that over time you would recognize when to head up. Now I'm going to have to dig those books out of a box in the basement.

There fixed it for you.
While the recent changes in the course lays it out for the instructor, many will be learning for the first time, it was taught in my IDC in the early 90's. Not RB and the calculations that are being discussed here but a 'simpler' version but still more complex than be back with 500 psi.

 

[Foxfish]

I will outline the differences in the ascent profiles AGAIN and hope that you can see they are different

Let me also refer you back to post #321

Storker when I ran the calculations my way, I had that you actually need 90B in a 12 to surface with 0 reserve. Just a heads up, check my math though as I am not as good in metric however I cross referenced with the imperial calculations and I got 90B/1200 PSI

Clearly you intended that the 90 b and 1200 psi were both valid pressures based on calculations you had done in metric and imperial. The numbers you gave for the 12 L tank referenced in my previous post were among others you'd calculated for other tank sizes. When I pointed out the variation you responded:

Foxfish if I was to take 1043/12 I get 86.9B needed. I don't know a gauge that reads in single bar so the calculation is rounded up for conservancy. Something you must have overlooked, yet again. I am not using conversion factors but rather solving each gas solution for metric and imperial. There will be slight differences in the numbers as the ascent profiles are slightly different... IE 30L/M= 1.06CF, 10M/M=33FPM so there will be slight differences because of the ascent profile.

Back then you called them slight differences. And now:

All of these differences make a fairly large difference in the required minimum pressure. And your conversion factor is probably rounded as well and over a large number IS NOT ACCURATE. There is no silly "quibbling" when metric or imperial is used, only when trying to use a stupid conversion factor instead of doing the equation properly.

So back when you did the calculation they were slight differences. Now any error in the metric calculation is somehow my fault, is not accurate and is a fairly large difference.

Even Storker got the same results that I got. You're the only one that seems to have a problem with them.

Min gas including safety stop: 240L + 450L + 270L = 960L. That's:

• 960/15=64b for a 15L, round to 65 or 70
• 960/12=80b for a 12L, round to 80
• 960/10=96b for a 10L, round to 100

You are confused and have contradicted yourself.

I estimated that using the 50 b method, you'd need to start an ascent at around 80 b for a 12 L tank from 30 m. For the minimum gas method you said you'd need 82.7 b or 1200 psi. The difference here is 3.3 % and less than the amount you'd be able read off a typical pressure gauge. To quibble over these sorts of petty differences just highlights a lack of perspective and judgement.

 

[DaleC]

Afraid the prey has slipped the hook, the wily fisherman gives the line a quick set of jerks.