Is Rule of Thirds incompatible with decompression diving?

[rongoodman]

From "I am not returning with 0 bar tanks"

And if you return with that last 1/3 untouched, as you should barring an emergency, you won’t be. Your reserve is either the 30 bar you mention or 1/3 of your usable gas, not both.

 

[pisauron]

@pisauron​

​

That phrase “need to leave 30bar” sounds like a recreational planning principle.​

​

The rule of thirds totally applies to OW technical dives but loss of a deco gas and the necessary volume to decompress myself with backgas are what really drive my depth and duration.​

​

I encourage you to take a basic technical course to develop the gas planning skills you’re hungry to understand.​

​

Despite some great answers above from reputable and reliable forum​

members, trying to piece things together from ScubaBoard forum responses won’t be efficient or complete.​

I sounds like good practice not to empty your tanks in the water.

 

[Rilelen]

I sounds like good practice not to empty your tanks in the water.

What? I don’t think anyone is disagreeing with that - we leave a reserve in both recreational and technical diving. What’s different is how MUCH of a reserve we leave.

In rec diving, the reserve is small (500 psi, or about 10-15cf on a typical AL80). In technical diving the reserve is large (typically ~ 1200psi if you’re diving thirds on cave-filled steel, or about 70cf on double HP100s).

 

[inquis]

The principle behind the Rule of Thirds is to always have at least 2x the gas need to exit. In the simplistic case of no deco and the exit essentially at the surface, that means penetrate on 1/3. Rarely is it that simple, though.

For example, if there is a substantial ascent, 2x that required amount is taken off the top, with the remainder split into thirds for the bottom portion. Even an OW might employee this strategy while looking at a wreck to ensure they can return to the anchor line.

I like to think of the Rule of Thirds as Out, Back, and Reserve -- 3 parts. Mathematically, it may sometimes work out that Out=Total/3, but that's very short-sighted in many other cases.

 

[jstotz]

Your interpretation of the rule of thirds in this scenario is very flawed. Additionally, the rule of thirds is the minimum safety factor you can utilize - many divers might use 1/4 or 1/6 depending on the dive profile.

You need to account for decompression in your gas planning. Also, if you’re executing a decompression dive, you will probably have a decompression mix (I.e. 50%, pure O2) which is included in your overall gas plan for the dive.

My advice would be to actually take a technical diving course from a reputable instructor where you will learn this.

I'll give more specifics so maybe you can spot my specific confusion:

I just finished TDI AN+DP. As I recall, the basic way we went about deco dive planning was to only work with 2/3 of our back gas. We kinda did a guess-and-check with the Multi Deco app. We'd enter in a depth and bottom time, then ask: What total back gas usage does the app calculate? Is it less than 2/3 our back gas? Good. Now run the algorithm again assuming lost deco gas. Is the new calculated back gas (now also used at the extended 20ft and 10ft stops) still equal to or less than 2/3 our back gas? Good. We also ran a third schedule for a +5 minute bottom time. Still good. i.e. still just under 2/3 predicted back gas usage.

So, if we follow the original plan, I should still surface with at least 1/3 (1,000psi) remaining in my twinset even if one or the other mishap happen.

My confusion starts when the instructor also insisted we turn the dive when I hit thirds(2,000psi). I havn't yet decided if that made the dive more or less conservative than what was originally planned in multi-deco, but in my mind it seems like adding in this additional rule will definitly change the established dive plan, BECAUSE the app only told us that it predicted we'd use up to 2/3 of our back gas, but it didn't make any promises about the proportion of that 2/3 that would be used on penetration vs exit.

 

[Wibble]

Your main premise is fundamentally flawed and way too simplistic.

It is true that for a typical non-decompression cave/mine/overhead/"must return to shotline" dive is based on the rule of thirds: one third in, one third out, one third in reserve.

However, you've neither considered decompression nor your team mate (assuming you have them). The rule of thirds is modified if you've a heavy breather as a team-mate; similarly if you've dissimilar gas volumes where the "third in reserve" may not be sufficient to get both you and your team-mate out alive.

As for decompression, you'd be taking additional gas with you for to decompress on, OR you would be severely curtailing your "third" amount to ensure there's sufficient gas for the decompression and "third in reserve".


Most things can be grossly oversimplified reductio ad absurdum. Cave training covers these calculations which can be quite complex with different diver gas consumption and gas volumes.

 

[inquis]

I should still surface with at least 1/3 (1,000psi) remaining in my twinset even if one or the other mishap happen.

Seems odd that you still want 1/3 remaining untouched after dealing with an emergency -- i.e. the missing deco gas. However, if you want to be that conservative, that's your call. You're effectively planning for more than one failure.

 

[PEDiver]

My confusion starts when the instructor also insisted we turn the dive when I hit thirds(2,000psi). I havn't yet decided if that made the dive more or less conservative than what was originally planned in multi-deco, but in my mind it seems like adding in this additional rule will definitly change the established dive plan, BECAUSE the app only told us that it predicted we'd use up to 2/3 of our back gas, but it didn't make any promises about the proportion of that 2/3 that would be used on penetration vs exit.

I feel like covering these questions with your instructor should be your first step. Especially now that you have completed the class and are looking to plan and execute decompression dives on your own (without an instructor's help).

Decompression planning programs are great to help plan a dive and also account for emergencies (like a lost deco gas or +5 minutes at depth). This helps determine how much minimum gas you need to bring to complete the dive, barring any safety/conservatism factors (which should have been covered in the class).

The rule of thirds (or any similar interpretation of such a rule) is to help you with the underwater portion of the dive. The last thing I want to be doing is math underwater. If you are planning a dive using the rule of thirds, you would have already done the surface work to determine how much gas you need for that dive (including deco gas). Now, when executing the dive, you can use the rule of thirds to determine your turn pressure and your dive plan to get from Point A to Point B, and then back to Point A, with 1/3 in reserve for emergencies (like a lost deco gas, or a buddy OOA, or +5 minutes at depth, etc.).

 

[rjack321]

I'll give more specifics so maybe you can spot my specific confusion:

I just finished TDI AN+DP. As I recall, the basic way we went about deco dive planning was to only work with 2/3 of our back gas. We kinda did a guess-and-check with the Multi Deco app. We'd enter in a depth and bottom time, then ask: What total back gas usage does the app calculate? Is it less than 2/3 our back gas? Good. Now run the algorithm again assuming lost deco gas. Is the new calculated back gas (now also used at the extended 20ft and 10ft stops) still equal to or less than 2/3 our back gas? Good. We also ran a third schedule for a +5 minute bottom time. Still good. i.e. still just under 2/3 predicted back gas usage.

So, if we follow the original plan, I should still surface with at least 1/3 (1,000psi) remaining in my twinset even if one or the other mishap happen.

My confusion starts when the instructor also insisted we turn the dive when I hit thirds(2,000psi). I havn't yet decided if that made the dive more or less conservative than what was originally planned in multi-deco, but in my mind it seems like adding in this additional rule will definitly change the established dive plan, BECAUSE the app only told us that it predicted we'd use up to 2/3 of our back gas, but it didn't make any promises about the proportion of that 2/3 that would be used on penetration vs exit.

Why are you relying on an app for this? You should be calculating this with a pencil in your wetnotes - especially in class.

You (and your instructor) both don't seem to understand gas planning, planning for 1 failure or contingency time.

 

[34109411]

You're starting out with 4800 liters, not 4080. Not sure where "I need to leave 30 bar" is coming from.