Rule of Thirds & Shallow Rec diving

[GCullen94]

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

 

[Foxfish]

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

Check your conversion factors. 1 psi = 0.06895 b approximately so 90 b is 1305 psi.

Refer to post 316.

 

[Storker]

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

I'm no good at all in Imperial, so I won't even try to check on those calculations But since you showed me yours, I'll show you mine. If you find any errors, I'd be glad to know.

Assume SHTF at 30m (4 ATA)
Assume constant ascent rate of 10 m/min
Assume normal air consumption = 15 SLM
Assume doubled air consumption due to stress => 30 SLM per diver
Two divers => 60 SLM
Ascent 30m -> 0m => Ascent time: 30/10 = 3 minutes
Constant ascent speed to surface => Average depth is 0.5* max depth, i.e. 15m (2.5 ATA)
Safety stop: 3 minutes at 5m (1.5 ATA)

1 min @ 30m: 1*60*(30/10 + 1) = 240L
3 min @ 15m ave: 3*60*(15/10) + 1) = 450L
Safety stop 3 min at 5m (1.5 ATA): 3*60*(5/10 + 1) = 270L


Min gas without safety stop: 240L + 450L = 690L. That's:

• 690/15=46b for a 15L, round to 50
• 690/12=57.5b for a 12L, round to 60
• 690/10=69b for a 10L, round to 70

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

Min gas including safety stop and 20b reserve at surface (or 20b margin for an inaccurate SPG) gives me:

• (64+20)=84b for a 15L, round to 85 or 90
• (80+20)=100b for a 12L, round to 100
• (86+20)=116b for a 10L, round to 120

In my calculations, I ascend from the safety stop at 10m/min, which clearly is faster than I should. OTOH, it makes the calculations very simple, and my 20b after surfacing safety margin should cover some of that extra time used for ascending slowly from the safety stop.

Can my fast ascent rate after the safety stop be the reason for our discrepancy?

 

[GCullen94]

Check your conversion factors. 1 psi = 0.06895 b approximately so 90 b is 1305 psi.

Refer to post 316.

.

AL80 (TF 2.5)/11L= 1400 PSI, 100B (ROUND UP to more conservative 10B)
HP100/12L= 1200PSI, 90B(1166, rounded up to more conservative 100 PSI)
HP120/15L= 1000PSI, 70B
HP130/18L= 900PSI, 60B

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.




Storker I kept the same ascent speed as you after the safety stop however, I calculated my average depth when moving to be 2.75 ATA, and still taking 3 minutes to get there. I do this because it makes the calculation cleaner, faster and a little (couple bar) more conservative. I was taught this by my fundies instructor. It really makes things cleaner in the if the max depth isn't a perfect ata. I used max-5m/2 +5M to find average depth.

 

[Storker]

Storker I kept the same ascent speed as you after the safety stop however, I calculated my average depth when moving to be 2.75 ATA, and still taking 3 minutes to get there.

OK, that means we've found what causes the discrepancy. Good. I hate it when calculations don't match!

I do this because it makes the calculation cleaner, faster and a little (couple bar) more conservative. I was taught this by my fundies instructor.

After re-checking my math, I've realized that my basic calculations really don't have any conservatism at all. So thanks for that! Which means that I'll continue to use a basic consumption rate which is slightly higher (16 SLM) than my current rate under benign conditions (~14SLM) and keep my extra margin of 20b after surfacing to introduce another little bit of conservatism.

It really makes things cleaner in the if the max depth isn't a perfect ata. I used max-5m/2 +5M to find average depth.

As I've said before, I don't trust myself to do proper rock bottom calculations during the dive. So I'll keep using the table I have on my slate, or the metric version of Lamott's rule of thumb (2x depth + 30b for a 15L, or 3x depth + 40b for a 10L) that I quoted in post #240

 

[Foxfish]

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.

So when you calculated the numbers in imperial you got around 1166 psi (80.3 b you rounded that up to 1200 psi (82.7 b).

Then you did the numbers in metric and got 86.9 b (1260 psi) and rounded up to 90 b (1305 psi).

That means the variation in your numbers due to conversions and rounding off was 90-82.7=7.3b.

Incidentally in previous posts your calculated range varied from 83 b to 88 b.

Personally I don't have a big problem with that though if I really did do the calculation and came up with 90 b I'd stick with that rather than say 80 b.

However you were adamant that variations of this magnitude could kill a diver. So if the correct number was 88 bar (the one you calculated) and I said I'd stick with 80 b you said I'd die.

I said:

To put the matter in perspective, we're quibbling over a few bar to cover a scenario that to the best of my knowledge has never been reported. In contrast BSAC records there have been dozens of incidents caused by problems with drysuits including fatalities. People still dive in drysuits and tell me they don't consider advanced training is a requirement.

and you replied:

Fox fish... The numbers we posted are to the point where YOU WILL RUN OUT OF GAS. It's not "just a few bar" and OOAs get reported all the time, so does running out in the ascent. We just try to manage the risk, you obviously are going to dive your way and safe diving to you. Hope you're still here when SHTF. I will not buddy up with someone who doesn't follow basic gas theory because if I have a problem, that's mine and their gas.

If this really was critical you'd need more signifiant figures in your calcuation and on your computers and air gauges. The reason they don't is because it isn't necessary *to work out calculations to a greater level of accuracy.*

 

[D_B]

it (what? gas planning?) isn't necessary

I would say if it's gas planning, that the percent of injuries and deaths in scuba caused by running out of air leans one in the direction that that is not the case and maybe giving divers the tools needed to determine just how much gas they might need may change that statistic

 

[Foxfish]

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

The numbers I gave for a 12L and 15L tank back in post #134. If you ascend at these pressures on a normal dive from 30m you will get to the surface with about 50 b.

So it's taken hundreds of posts to get to this stage, BUT

how do you know your assumed ascent profile was correct. What if you need 5 minutes at the bottom before an ascent? What if you need 100 L/min for two anxious divers not 60 L/min? What if your ascent rate should be 4 m/minute rather than 9 m/minute in an emergency. Do we need to consider deep stops and extended safety stops during an emergency ascent. If you believe one person's account our gas calculations are way off the mark in an actual OOA emergency. If you believe someone else then a calculation error of 5-10b can be a matter of life and death.

As a novice diver, how do you determine reasonable numbers for a rock bottom calculation?

 

[D_B]

what if this .. what if that .. what if my tank explodes ?
... as a manager here is fond of saying, you cant run a business on exceptions, and one offs

you cant calculate for everything , you can calculate a reasonable scenario ... this thread is full of answers to that very question
I have a calculation that I can use, and it's a reasonable one .. will it work in every situation, of course not , but it's useful to me and much better than guessing on how to "be back on the boat with 500psi"

 

[Storker]

What if [...] ? What if [...] ? What if [...] ?

Ah, the well known discussion technique of "Just asking questions" - or "JAQing off", as it's sometimes affectionately called :cool2: And, of course, with a side order of strawmen.

I've given up discussing with Foxfish, but for the benefit of other, more open-minded divers, I'd like to show that each and every one of those strawmen is easy to deal with as long as you use a sensible method for determining min gas / rock bottom instead of "surfacing with 50 bar, but only God knows how I'm gonna do that":

how do you know your assumed ascent profile was correct.

Assuming an ascent profile of 5 m/min:

Ascent 30m -> 0m => Ascent time: 30/5 = 6 minutes
1 min @ 30m: 1*60*(30/10 + 1) = 240L
6 min @ 15m ave: 6*60*(15/10 + 1) = 900L
Safety stop 3 min at 5m (1.5 ATA): 3*60*(5/10 + 1) = 270L
Min gas including safety stop: 240L + 900L + 270L = 1410L, 117.5 bar in a 12L, round to 120 bar

What if you need 5 minutes at the bottom before an ascent?

5 min @ 30m: 5*60*(30/10 + 1) = 1200L
3 min @ 15m ave: 3*60*(15/10 + 1) = 450L
Safety stop 3 min at 5m (1.5 ATA): 3*60*(5/10 + 1) = 270L
Min gas including safety stop: 1200L + 450L + 270L = 1920L, 1920/12 = 160 bar in a 12L

What if you need 100 L/min for two anxious divers not 60 L/min?

1 min @ 30m: 1*100*(30/10 + 1) = 400L
3 min @ 15m ave: 3*100*(15/10 + 1) = 750L
Safety stop 3 min at 5m (1.5 ATA): 3*100*(5/10 + 1) = 450L
Min gas including safety stop: 400L + 750L + 450L = 1600L, 1600/12 =133 bar in a 12L, round to 140 bar

Incidentally, this is more or less what I did in my earliest min gas calculations, when my normal consumption rate was around 20SLM...

What if your ascent rate should be 4 m/minute rather than 9 m/minute in an emergency.

Ascent 30m -> 0m => Ascent time: 30/4 = 7.5 minutes
1 min @ 30m: 1*60*(30/10 + 1) = 240L
7.5 min @ 15m ave: 7.5*60*(15/10 + 1) = 1125L
Safety stop 3 min at 5m (1.5 ATA): 3*60*(5/10 + 1) = 270L
Min gas including safety stop: 240L + 1125L + 270L = 1635L, 1635/12 =137 bar in a 12L, round to 140 bar

Do we need to consider deep stops and extended safety stops during an emergency ascent.

Deep stop: 1 minute at 15m (2.5 ATA)
Extended safety stop: 5 minutes at 5m (1.5 ATA)

1 min @ 30m: 1*60*(30/10 + 1) = 240L
3 min @ 15m ave: 3*60*(15/10 + 1) = 450L
Deep stop 1 min @ 15m: 1*60*(15/10 + 1) = 150L
Extended safety stop 5 min at 5m (1.5 ATA): 5*60*(5/10 + 1) = 450L
Min gas including deep stop and extended safety stop: 240L + 450L + 150L + 450L = 1290L, 1290/12 =108 bar in a 12L, round to 110 bar

As a novice diver, how do you determine reasonable numbers for a rock bottom calculation?

As a novice diver you should, IMO, use the same approach that I used: Determine your actual gas consumption rate, use stock numbers for ascent rate and safety stop depth/duration and do the math. Just as the min gas/rock bottom procedure tells you to do. It's a lot better than "surface with 50 bar, but only God knows how I'm gonna do that", and the numbers can be adjusted as you gain experience and find out if the stock numbers are appropriate for your diving and you can make an educated decision on your personal need for conservatism. Easy peasy, right?

And, unlike "surface with 50 bar, but only God knows how I'm gonna do that" or the woefully misleading "50 bar plus 10 bar / 10m" so-called rule which Foxfish has been plugging, you can tailor your min gas calculations to fit your own diving. Not bad, eh?