Rule of Thirds & Shallow Rec diving

[supergaijin]

Then that's not diving the 'rule of thirds'.... it's a rule.

 

[Storker]

What I am trying to say is it doesn't always work like that. a "third for emergencies" isn't the best idea. It's way too conservative for shallow dives and not enough for deeper dives.

Some time ago, when I was teaching myself rock bottom gas planning, I made myself a spreadsheet. I wanted to know the minimum pressure I needed at different depths, making sure I started the ascent - or at least ascended to shallower waters - before I would've run out of gas in an emergency involving gas donation.

I based the SAC rate on my own logged rates and used some pretty standard parameters for the calculation:

• One minute at bottom to assess situation
• Need to share air on ascent
• Ascent rate of 10 m/min (30 ft/min)
• Doubled SAC rate for myself due to stress
• Doubled SAC rate for buddy due to stress
• Safety stop (3 minutes at 5m/15ft) optional
• Both divers are within the NDL, so no time has been allocated for deco stops

Here are two graphs from that spreadsheet, for different tank sizes and pressures (with some non-Metric unit translations for the benefit of you 'murricans ). The first graph allows for a 3 minute safety stop, the second assumes that we're just going straight to the surface:





I guess those graphs pretty much illustrate GCullen94's point...

 

[victorzamora]

I was in the process of plotting those out myself! Thanks for saving me the hassle. Those things look great! A few things I'm unclear about, though....how many 300bar tanks do you use? I've never come across a tank rated for higher than 3442psi (232bar). 300bar is ~4400psi.

As for Rule of Thirds, I know this has been mentioned before, but it's not optimal for rec/OW diving. Why? It wasn't DESIGNED for OW diving. It was specifically designed for caves. Even then, it's not considered conservative enough.....it's thought of as the bare minimum.

 

[rongoodman]

As for Rule of Thirds, I know this has been mentioned before, but it's not optimal for rec/OW diving. Why? It wasn't DESIGNED for OW diving. It was specifically designed for caves. Even then, it's not considered conservative enough.....it's thought of as the bare minimum.

It works(barely) in caves where there is substantial flow speeding your exit. I would never use it in Mexico, where the caves usually have no or low flow.

 

[Foxfish]

Rock bottom gas will have you on the surface with nothing in your tanks if you are sharing gas and have an increased SCR.

I'd be interested to know:

1. Where the various requirements for 'rock bottom' gas planning originated;
2. Which of the dive instruction agencies are now teaching this method; and
3. On which courses they are teaching the method.

 

[GCullen94]

Victor- 300 bar tanks are in the EU and Australia.


Foxfish- Rock bottom gas is derived from what it will take to get you and a buddy to the surface when things go wrong. It is planned using the following criteria

1. You and your buddy are sharing gas
2. One minute at depth to assess the situation
3. An SCR of 1 for each diver because of stressed situation

Any DIR agency teaches this style gas planning. It has many names but the most common are "min gas" and "rock bottom" it is taught from the very beginning. I know some NAUI instructors teaching this style gas planning, especially in their more advanced classes.



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[Storker]

A few things I'm unclear about, though....how many 300bar tanks do you use?

My son has a 10L 300bar. I have one 10L 300bar (which my son borrows when we go on two-tank trips), one 15L 200bar and one 15L 300bar. I guess that that adds up to about three I threw in the 232bar rating since that's also a pretty common pressure rating. And as to why we have such a diverse assortment of tanks, the answer is simple: We bought used gear, and except for my 10x300 the tanks were those that came with the sets we bought.

I've never come across a tank rated for higher than 3442psi (232bar). 300bar is ~4400psi.

Here in Norway the most common rec tank was 15x200 steel just a handful of years ago. The current default is 10x300 steel. The 10x300 is pretty neat to handle, less buoyant than a 15x200 - which is an advantage when you're diving dry, because you need about 4kg less weight - and holds nominally the same amount of gas as the 15x200. In reality, though, the 10x300 holds about 10% less than the 15x200 due to the compressibility factor of air in the range from 200 to 300 bar.

I just checked my LDS' web store, and they stock 10 and 12L 300 bar, and 12, 15, 18 and 20L 232 bar. Plus Al ponys, Al stage bottles and steel doubles. And the standard tank valve is DIN, of course :cool2:



Incidentally, the diversity of pressure ratings is why I prefer the Euro way of giving tank capacity: Volume x pressure rating. Since it's all SI units, you can calculate the capacity easily (10x300 and 15x200 are both nominally 10x300 = 15x200 = 3000 liters, 10x232 is nominally 10x232 = 2320 liters and 12x300 is nominally 12X300 = 3600 liters). And when you calculate turnaround - or ascent - pressure, you don't worry about pressure rating, only tank volume. One turnaround pressure for a 10L, one for a 12L and one for a 15L, irrespective of pressure rating. So my turnaround pressure is the same whether I dive my 15x200 or my 15x300.

 

[rongoodman]

Foxfish- Rock bottom gas is derived from what it will take to get you and a buddy to the surface when things go wrong. It is planned using the following criteria

1. You and your buddy are sharing gas
2. One minute at depth to assess the situation
3. An SCR of 1 for each diver because of stressed situation



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4. You need to have enough to get you both to the next breathable gas(surface or deco) while allowing enough gas to do any required stops.

 

[victorzamora]

I just checked my LDS' web store, and they stock 10 and 12L 300 bar, and 12, 15, 18 and 20L 232 bar. Plus Al ponys, Al stage bottles and steel doubles. And the standard tank valve is DIN, of course :cool2:

Incidentally, the diversity of pressure ratings is why I prefer the Euro way of giving tank capacity: Volume x pressure rating. Since it's all SI units, you can calculate the capacity easily (10x300 and 15x200 are both nominally 10x300 = 15x200 = 3000 liters, 10x232 is nominally 10x232 = 2320 liters and 12x300 is nominally 12X300 = 3600 liters). And when you calculate turnaround - or ascent - pressure, you don't worry about pressure rating, only tank volume. One turnaround pressure for a 10L, one for a 12L and one for a 15L, irrespective of pressure rating. So my turnaround pressure is the same whether I dive my 15x200 or my 15x300.

Well that's fantastic! I'm a little upset that the standard over here is 163-232bar and mostly Yoke. As an engineer, I hate the Imperial system (you know, the one the Brits made up) and much prefer the metric system. My only complaint is the metric system doesn't mean as much to me innately as the imperial system does....but that's a matter of having gotten used to the imperial system.

My biggest complaint with metric system for tanks is people call tanks "12L" tanks but don't specify the rated pressure. If everybody called their tanks 12Lx300 I'd be MUCH more supportive of metric for diving and would probably buy BAR SPGs and dive my computer in metric!! In the imperial system, you have the AL tanks which are 3000psi (besides a few oddball 3300psi tanks or super old tanks) and your steels are LP or HP (besides a few oddball tanks or super old tanks) so it specifies exactly which tank you're diving.

 

[Storker]

My biggest complaint with metric system for tanks is people call tanks "12L" tanks but don't specify the rated pressure.

OTOH, people around here have ten-threehundreds, or twelve-two-three-twos, or fifteen-twohundreds. I've hardly met anyone who just has a ten, twelve or fifteen. At least when we talk about single tanks.