Rule of Thirds & Shallow Rec diving

[Foxfish]

If you use rock bottom calculations to figure out how much air you use to get to the surface with 750 PSI, you are actually combining two different systems. you can do that, but it sin't necessary.

What's the best way to ensure 40 people on a recreational dive charter get back to the surface with enough air for an emergency? Tell everyone to do a rock bottom calculation or get back to the surface with a predefined pressure? So far I've seen rock bottom estimates for when to ascend from a 30 m 100 ft dive with a 11-12 L tank ranging from 70 b 1000 psi to 110 b 1600 psi. Specifying a surface pressure also means that you can also quickly check peoples pressure gauges.

You clearly see the need to simplify the calculation process and I agree. In post #191 I gave some numbers I'd calculated using a rock bottom approach for the varying depths and tank sizes I encounter. They match yours pretty closely and just happen to be similar to the numbers I'd need to use to end up on the surface with 50 b. In other words, by ensuring that I end up at the surface with 50 b I've always got enough air if an emergency situation occurs. I think that is a simple and clever approach.

 

[gcarter]

What's the best way to ensure 40 people on a recreational dive charter get back to the surface with enough air for an emergency? Tell everyone to do a rock bottom calculation or get back to the surface with a predefined pressure? So far I've seen rock bottom estimates for when to ascend from a 30 m 100 ft dive with a 11-12 L tank ranging from 70 b 1000 psi to 110 b 1600 psi. Specifying a surface pressure also means that you can also quickly check peoples pressure gauges.

You clearly see the need to simplify the calculation process and I agree. In post #191 I gave some numbers I'd calculated using a rock bottom approach for the varying depths and tank sizes I encounter. They match yours pretty closely and just happen to be similar to the numbers I'd need to use to end up on the surface with 50 b. In other words, by ensuring that I end up at the surface with 50 b I've always got enough air if an emergency situation occurs. I think that is a simple and clever approach.

Do you mean this bit?

8 minutes to ascend from 30 m seems reasonable. Remember though that the average depth is not simply the total depth divided by two, it is the time weighted average. I calculate a pressure of 80 b or 1150 psi with a 12 L tank. If I do a normal ascent from 30 m starting with 80 b I reach the surface with 50 b.

Let's go back to victorzamora's question. At the point this happens, you have 80 bar. What do you do?

You are diving your current, standard configuration. You are diving to 30m with a buddy (obviously) using your current, standard, gas-planning methodology. Everything on this dive has gone right so far. RIGHT before you start to ascend, your buddy signals OOA due to an unforseeable error, not caused by your buddy. You had a good SAC rate, so you really pushed your NDL. What do you do?

 

[GCullen94]

Fox fish we get the different numbers because we use different ascent profiles. It depends on the persons ASCENT PROFILE that determines the amount of gas they need


Sent from my iPhone using Tapatalk

 

[MXGratefulDiver]

Here are some depth profile numbers for an emergency ascent someone gave in a previous post and which I referred to in the past. The first example was for an emergency ascent from a thirty metre dive. It is based on DIR methodology

Minimum Gas Explained | Theory Articles | Articles | DiveDIR

What do you think?

... I think it looks like a metric version of what I teach. But you DO realize (I hope) that the example makes assumptions and provides numbers based on both divers wearing doubles? You can't apply those same numbers to a single cylinder ... the volume of gas you're dealing with isn't nearly adequate.

Furthermore, the entire explanation isn't intended to give you a quick and dirty number that would apply under all circumstances at that depth. It's intended to help you understand how to arrive at numbers that work for you, based on your consumption rate and depth profile. That's the irony ... the article you just linked is providing you the exact same methodology you've been arguing against for the last 200+ posts ...

... Bob (Grateful Diver)

 

[DaleC]

What's the best way to ensure 40 people on a recreational dive charter get back to the surface with enough air for an emergency?... Specifying a surface pressure also means that you can also quickly check peoples pressure gauges.

This is an example of confirming pressure after the fact. It is not a plan for deriving needed volume.

...In post #191 I gave some numbers... similar to the numbers I'd need to use to end up on the surface with 50 b.

This section of your post suggests an intuitive approach to gas management based on experience. Is this what you suggest for beginner divers in place of a tool to actually calculate needs? Otherwise please describe your methodology so I can replicate the results.
If you can't explain it, and I can't replicate it, it's not really much of a plan for anyone else is it.

 

[Foxfish]

... I think it looks like a metric version of what I teach. But you DO realize (I hope) that the example makes assumptions and provides numbers based on both divers wearing doubles? You can't apply those same numbers to a single cylinder ... the volume of gas you're dealing with isn't nearly adequate.

Furthermore, the entire explanation isn't intended to give you a quick and dirty number that would apply under all circumstances at that depth. It's intended to help you understand how to arrive at numbers that work for you, based on your consumption rate and depth profile. That's the irony ... the article you just linked is providing you the exact same methodology you've been arguing against for the last 200+ posts ...

... Bob (Grateful Diver)

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.

 

[supergaijin]

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
No deep stops required

A. 1 minute at 30 m
30L x2 x4ata = 240L
B. Ascent from 30m to SURFACE at 10m/min
30m-20m (ave 3.5ata) = 210L
20m-10m (ave 2.5ata) = 150L
10m- 0m (ave 1.5ata)= 90L

Total Gas Required = 690L
690/ tank volume. Assuming the divers are using the common AL80 which has an internal volume of 11.11L
690/11.11 = 62.1bar

This seems to be a 'best-case' scenario, an accurate SPG and the tank will be empty upon surfacing. Normally one would pad this minimum and if you want to make safety stops, then you'll need:
3min Safety Stop = 30L x2 x 3 x1.5ata = 270L (additional 24bar)
TOTAL: 86+ bar

Of course the recreational limit for diving is actually 40m.
A. 1 minute at 30 m
30L x2 x5ata = 300L
B. Ascent from 40m to SURFACE at 10m/min
40m-30m (ave 4.5ata) = 270L
30m-20m (ave 3.5ata) = 210L
20m-10m (ave 2.5ata) = 150L
10m- 0m (ave 1.5ata)= 90L

1020L/ 11.11 = 91.8bar
+ 3min Safety Stop = (additional 24bar)
TOTAL 116+ bar

 

[DaleC]

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.

Tsk Tsk. What you are saying is that someone who does the math of calculating gas requirements to surface with 50b achieves the same results as someone who just "decides" to surface with 50b.

I get the conversation:

I will surface with 50b.
How?
Don't ask, I just will!
How?
Look, you do the math. Do you end up with 50b
Yes.
So do I. And I didn't do the math.
How?
Magic...........

 

[GCullen94]

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
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/m).

The intervals will be as follows:

A. 1 minute at 30 m
B. Ascent from 30m to 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.

GOOD GREIF... I have made 3 post with this EXACT profile and you have ARGUED WITH ME EVERY TIME... Ok I'll do it in metric now and maybe.. just MAYBE you'll get it though your head that your way of 50B+10B for every 10M just DOESN'T WORK. You'd be:
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

1052L of gas needed in this ascent using the parameters that YOU gave me. in an 11L (AL80) this is 95B (Lot larger than your 80B requirement) ALSO THE SAME AS 1400 PSI that I was posting earlier for this EXACT SAME PROFILE.

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.

Jeez foxfish. I have no clue why you are having such trouble understanding that the current way of teaching gas management to people is NOT CORRECT. This is why almost all of us use this gas planning strategy.

 

[Storker]

the example makes assumptions and provides numbers based on both divers wearing doubles?

Uh, no. In the 30m example, the divers are using single 12L tanks:

I’m going to run through two examples: a thirty metre dive with a single 12 litre cylinder, where we can ascend directly to the surface; and a 45 metre dive in twin 12's

---------- Post added December 7th, 2013 at 10:42 AM ----------

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 ). So, I looked at my minimum pressure spreadsheet again and did some simple linear regression on my own numbers. Lo and behold, with my personal numbers as the basis, I got some fairly neat and round results.

Again, the basis for my calculations is:
Normal air consumption: 16.0 liters/bar (0.57 cu.ft) per minute
Air consumption doubled due to emergency, for both divers
Time before ascent: 1 minute
Ascent speed: 10 m/min (33 ft/min)
Safety stop: 3 minutes at 5 meters (16 ft)
Remaining pressure after surfacing: 20 bar

The linear regression ended up as:
10L: y = 2.9203x + 36.925, round to 3x depth + 40
12L: y = 2.4336x + 34.104, round to 2.5x depth + 35
15L: y = 1.9469x + 31.283, round to 2x depth + 30

This kind of math is something I can trust myself to do while submerged (at least if I'm shallower than 20-25m...)


I think I'll start calling this "Storker's rule of thumb"