Rule of Thirds & Shallow Rec diving

[DaleC]

I just started thinking about Lamott's rule of thumb again, and that it really should be in Metric (which is the natural unit system for a civilized society ).

For me metric is funny stuff. I was in grade school when it became our new standard so I have half a metric brain and half imperial. I easily do distance and weight in both but still cannot do temperature or volume (I convert and visualize as imperial). And I'm 50!

When it first came out I distinctly remember putting little kph stickers over my Dads mph speedometer.

I have learned bars because I have a bar spg but still see it in terms of psi. And I have no idea what a 11L tank looks like. If you say it's an Al 80 all I see is 80 (77). I thus assume an Al40 is 6L, Al19 about 3L etc...

 

[gcarter]

For me metric is funny stuff. I was in grade school when it became our new standard so I have half a metric brain and half imperial. I easily do distance and weight in both but still cannot do temperature or volume (I convert and visualize as imperial). And I'm 50!

When it first came out I distinctly remember putting little kph stickers over my Dads mph speedometer.

I have learned bars because I have a bar spg but still see it in terms of psi. And I have no idea what a 11L tank looks like. If you say it's an Al 80 all I see is 80 (77). I thus assume an Al40 is 6L, Al19 about 3L etc...

54. I am good with distance, temperature and volume, terrible with weight. Put stickers on my own speedometer. My SPG is marked with both bar and psi. Guess which one I use? I keep switching my computer between metric and imperial, can't make up my mind.

 

[Foxfish]

You didn't read the article properly did you and you're not reading what I've said properly. I'm not arguing against the rock bottom method in terms of its validity or correctness. Like I said a long time ago, I think it has merit and expect that in some forms of diving outside OW/AOW limits it would seem essential. What I have been questioning is the suitability of OW and AOW recreational divers relying on this method to set their ascent pressure. If you do the rock bottom calculations, I believe you will find the 50 b plus the pressure required for a normal ascent provides adequate pressure for an emergency ascent from 30 m. In other words, the 50 b rule is just a simplified way of getting what you'd expect from the rock bottom calculation without the hassle.

Here is a calculation exercise to demonstrate what I mean.

Assume the following:

Tank volume: 12 L
Breathing rate per diver during emergency: 30 L/min
Normal breathing rate: 15 L/min
Maximum depth: 30 m
Ascent is directly to the surface from maximum depth with a safety stop.
1 minute on bottom getting organized
Ascent rate is a constant 9 m per minute
Safety stop is 3 minutes at 4m
No deep stops required

Step 1. Break the dive up into the following time increments and for each determine the air consumed at that depth (L/min), average depth (m) and then the free air volume of air consumed at that depth (L).

The intervals will be as follows:

A. 1 minute at 30 m
B. Ascent from 30m to 4m safety stop
C. Safety stop
D. Ascent from safety stop to surface

Step 2. Sum the free air volumes for each time increment to determine the total free air volume required.
Step 3. Determine the pressure required for an emergency ascent from 30 m.
Step 4. Do the same calculation for a normal ascent and calculate the pressure that you will surface from the dive assuming that you started with the emergency ascent pressure calculated in step 4.

Post the answers you get for each step in metric units.

Some of you were asking how I'd do an emergency ascent. I think this is the kind of information you were wanting. While I'm not entirely happy with the approach it seems fairly standard. A couple of responses are almost there.

Some pointers on how to do the calculation:

• Average depth over a given range is the (maximum + minimum depth)/2 assuming a constant ascent rate.
• The air you consume at a given depth gets converted to the volume of air at a given reference pressure which is usually taken as the atmospheric pressure using Boyle's 'Ideal Gas' Law P1V1=P2V2. You need to work in absolute pressures when using this equation.
• The absolute pressure at a given depth in bars is approximately the depth/10+1. This is P1. The absolute atmospheric pressure at the surface is approximately 1 b. This is P2.
• The 'free air' volume or the volume of the air you breathe at depth converted to a volume at atmospheric pressure is then calculated as V2=V1*P1/P2. In other words you multiply the volume you breathe at the average depth V1 by the ratio of absolute pressures P1/P2.
• Once you have the quantity of air you breathe at the average depth for each interval you sum them to get the total air you need for the emergency. What pressure do you need in your tank to get that volume of free air? Again by using Boyle's Law, P2=P1.V1/V2. Now P1 is 1 b and V1 is the calculated volume of free air. V2 is the actual volume of your air cylinder.
• This may look complicated but it reduces down to a few simple formula which you can use without understanding the underlying principles, a bit like using the 50 b rule.

Can we stick with the numbers I gave so we can all get on the same page.

The tank volume is 12 L which is a standard steel tank size.

Ascent rate is 9m/minute.

Allow for safety stop at 4 m.

AOW maximum depth is 30 m unless you do the deep diver specialty.

Over to you.

 

[GCullen94]

Let me post this YET AGAIN foxfish
1 minute at 30M
3 minutes to get to 5M
3 minutes at 5M
30 seconds to go to surface...

30Lx2x4ata= 240L
30Lx6x2.8ata= 504L
30Lx6x1.5ata= 270L
30Lx1x1.25ata= 38L



If you were talking about a 12L 232B cylinder then you need 88B (round up to 90 so you can actually read the gauge) again 10B will make a difference.​

 

[D_B]

Metric ? ... that's for tire measurements , oops, I mean tyre measurements

I use the thumb rule because it works in my brain ( that's saying a lot) it drops as I ascend and it's easy to recalculate it during a non-square profile dive as I find myself at whatever depth that something catches my eye
I've added another 200 psi to my calculations to allow safety stops (depthx10 + 500psi) .. I do understand that at the deep recreational limits it's not conservative enough and would make an allowance for it

in OW we were told to surface with 500 psi, and were told/shown how much faster depth increased our consumption rate/pressure drop .. other than allowing us to see (and drilling into us) that increased monitoring of SPG would be necessary the deeper we went, no info on exactly how to make sure that we were surfacing with 500 psi

 

[victorzamora]

So, Foxfish, let me clarify....I did NOT say what would you do to calculate two people needing to ascend from 30m. We all know how to do this. I asked specifically how you would ascend if you were AT your normal ascent pressure and your buddy went OOA unexpectedly. That would mean you're at 80b (according to you) at 30m and your buddy signals OOA.

Secondly, AOW does NOT limit you to 30m. You can dive to 40m with OW cert. There is no limit placed on your cert other than within NDL and no overhead.

 

[Foxfish]

So, Foxfish, let me clarify....I did NOT say what would you do to calculate two people needing to ascend from 30m. We all know how to do this. I asked specifically how you would ascend if you were AT your normal ascent pressure and your buddy went OOA unexpectedly. That would mean you're at 80b (according to you) at 30m and your buddy signals OOA.

Secondly, AOW does NOT limit you to 30m. You can dive to 40m with OW cert. There is no limit placed on your cert other than within NDL and no overhead.

Again, you didn't read my post properly did you. Got back and read post #236. Working through the calculation may help you understand what I've been trying to say for about the last 100 posts.

 

[GCullen94]

Fox fish in post 202 YOU outline YOUR procedure as "50B + 10B for every 10M" therefore in a 30 meter dive you'd be starting your ascent with 80B. Why don't you work through the calculations on here, as I have, so we can see how you get your numbers.


Sent from my iPhone using Tapatalk

 

[Storker]

Metric ? ... that's for tire measurements , oops, I mean tyre measurements

Since I'm not British and my English is more US English than UK English, my cars and my bikes have tires, not tyres. But their pressures are in bar. So metric works perfectly well for tires, too

I've added another 200 psi to my calculations to allow safety stops (depthx10 + 500psi) ..

This, and my discovery of the rule of thumb you quoted, are two good things that have come out of this trainwreck of a thread. How was that saying again? That there's an ill wind that blows nobody any good?

 

[victorzamora]

Okay, Foxfish, since you've been dodging my question and have been very condescending, let me describe to you what would happen in my scenario. I'll even do it in metric, which I'm very uncomfortable with, just to prove my point.

YOU said you would start ascent at 80b from 30m to end the dive with 50b. You and your buddy are both at 80bx11L when you're about to start your ascent, he signals OOA. That's 4ata with 880L total of gas supply for both of you. Standard breathing rate is 15L/min per diver. Stressed is double. You're using 60L/min total.

Time to sort out problem: 1min@4ata*60L/min=240L
Ascent at 9m/min means 2.8min ascent to 5m safety stop. Average depth is 17.5m. So 2.8min@2.75ata*60L/min=462L
Now you're talking about 3min@1.5ata*60L/min=270L
Total needed air with ZERO safety margin is 972L. That means you've got -92L of reserve. Well, what if your SPG is also reading 10b low (not uncommon). Wow, now you're in need of 200L. That doesn't bode well for one of you.

So that means you're either bent or dead. Congratulations. Twenty seconds worth of effort is keeping you and your buddy from going home safely tonight. THAT is why your methodology is so woefully insufficient. I love my wife, and I agree to care for my buddy when we buddy up. I don't want anything bad to happen to him, and I want to get to see my wife at the end of the day. When my wife is my buddy, it's even more serious. THAT is what's wrong with your strategy....if something goes wrong, it will ALL go very wrong.