Is Rule of Thirds incompatible with decompression diving?

[NothingClever]

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

I’d think that goes without saying.

What I was implying about the note “need to leave 30 bar” is that it’s a simplified recreational rule that doesn’t require much planning. It’s not the appropriate start point for technical diving.

Good luck in your learning journey.

 

[pisauron]

I’d think that goes without saying.

What I was implying about the note “need to leave 30 bar” is that it’s a simplified recreational rule that doesn’t require much planning. It’s not the appropriate start point for technical diving.

Good luck in your learning journey.

Assumptions are dangerous. Good luck avoiding them.

30 bar is not rec rule. The rec rule I think you might be mistaking is "turn the dive at 100". Or sometimes called "100+10".

 

[PfcAJ]

Lots goin on here.

Y’all remember this thread when there’s talk about “agency doesn’t matter”. This kind of stuff just doesn’t fly with some agencies.

For the OP: “rules” are simplifications that may or may not work depending on the particulars. Rule of thirds is a good example. It can be applied well, and as you’re seeing, it can be applied wholly inappropriately.

You need to reserve enough gas to get you up and out with whatever failures you’re willing to plan for. Me? Two major failures is kind of where I land. For some, you might want to be able to tolerate 3 major failures. Let’s remember that some folks out there only plan to tolerate 1 failure.

You can quickly plan yourself out of a dive if you decide to plan for a high number of failures OR you are so saddled with equipment that you can’t even manage the dive itself.

If I were you, I’d figure out how to actually plan for how much gas need without relying on deco software to tell you that value. This will help foster understanding of it all.

 

[Jacob BV]

I do think BSAC teach rule of thirds un-optimally (I'm not saying all BSAC instructors do, but the instructor materials/slides leave issues with dives with deco). BSAC teach rule of thirds from their first qualification all the way up to mixed gas (and do not use minimum gas). Here's the extract from the twinset course (admittedly this is being re-written but fixing the rule of thirds issue was resisted as was introducing minimum gas):

Now a BSAC sports diver can dive to 35m and do deco. A common set up within BSAC for this would be using a 15L. Assuming a 220bar fill 2/3s of this would be 2200L, which on my multideco settings (50/80 and 15L/min consumption) allows a diver on air to do 23mins of bottom time, hitting the surface with their 1100L reserve. However, just the ascent portion of this dive needs ~650L so in the emergency that your buddy is out of gas, even if you both ascend breathing 15L/min (I preffer to increase breathing rate for at least the ascent portion of gas sharing in my calculations for minimum gas/emergencies) there wouldn't be enough gas. This mismatch increases with the more bottom gas you carry or if you use anymore conservative GFs.

Furthermore, I don't see people in BSAC use the rule as above calculating how much gas the bottom time and ascent would use to arrive at the surface with 1/3, and if they do they are already doing a calculation that is as complicated as minimum gas (and the complication of calculation of minimum gas is one of the reasons using it is resisted). I think rule of thirds is a bad rule, that minimum gas is much more intuative, and it leads to better thinking divers in practice.

 

[Wibble]

I think rule of thirds is a bad rule, that minimum gas is much more intuative, and it leads to better thinking divers in practice.

Rule of Thirds is excellent for the intended use: NDL overhead environments. It's exceedingly simple to use and apply: check your gas pressures (sidemount), round down to nearest whole 10 bar divisible by 3 (e.g. if 220 bar, drop to 210, if 200 bar drop to 180), divide by 3, subtract that from your current gas pressure and that's the turnaround pressure. Available gas is adjusted for flow, restrictions, mismatching teammates (if applicable), etc., where thirds becomes quarters, fifths, sixths, as required.

Outside of that, the Rule of Thirds is pretty much irrelevant and confusing for decompression diving where planning considerations must factor in specific bottom gases, decompression gases, contingencies and failure modes -- not to mention max dive times, skipper's requirements, etc.

 

[rongoodman]

Assumptions are dangerous. Good luck avoiding them.

30 bar is not rec rule. The rec rule I think you might be mistaking is "turn the dive at 100". Or sometimes called "100+10".

It has been requested by the crew of every liveaboard I've ever been on. It's very much a "rec rule".

 

[Jacob BV]

Rule of Thirds is excellent for the intended use: NDL overhead environments. It's exceedingly simple to use and apply: check your gas pressures (sidemount), round down to nearest whole 10 bar divisible by 3 (e.g. if 220 bar, drop to 210, if 200 bar drop to 180), divide by 3, subtract that from your current gas pressure and that's the turnaround pressure. Available gas is adjusted for flow, restrictions, mismatching teammates (if applicable), etc., where thirds becomes quarters, fifths, sixths, as required.

Outside of that, the Rule of Thirds is pretty much irrelevant and confusing for decompression diving where planning considerations must factor in specific bottom gases, decompression gases, contingencies and failure modes -- not to mention max dive times, skipper's requirements, etc.

Yes totally agree, I should have clarified I think it's a bad rule in the context BSAC use it. Very effective and simple for NDL everhead.

 

[pisauron]

It has been requested by the crew of every liveaboard I've ever been on. It's very much a "rec rule".

I never been on a rec liveaboard so can't help you with that. You're welcome to our tech one tho.

 

[halocline]

So, the diver plans a square profile with staged decompression using a 3000psi Twinset of aluminum 80's. He immediately descends to 135ft and upon reaching depth, swims into a long horizontal cave for 20 minutes before he hits 2000psi. He then turns his dive around because he was told to always obey rule of thirds to be safe.

So he retraces his steps at exactly the same swim speed until reaching the ascent/descent line and starting decompression. But due to the decompression burden, he needs to use more gas for the total return trip than he used on the initial descent and penetration.

I was surprised to read in a later post that you have finished AN/DP. Because you should have been taught that the dive you propose here is unsafe, and no well trained technical diver would ever plan one as such.

I really hope your class included better instruction on gas planning for deco dives. If not…, let’s say you didn’t get your money’s worth. But, a basic, bumper sticker version of deco dive gas planning is that first you calculate how much gas you need for ascent and deco, double that, and reserve that amount, i.e, don’t use it for anything else. (Assuming both teammates have the same tanks and SAC) This way if one diver loses ascent/deco gas, there is still enough for both divers to safely ascend. This also assumes that an early ascent is possible in the event of emergency during the bottom phase of the dive. All this info is right there in the TDI written materials; I don’t know which agency your class was from.

The other part of your scenario, the cave dive portion, would be planned differently, and apparently you are not a trained cave diver. So the first and most important thing is that diving in a cave without proper overhead training (including gas planning for that environment) is a huge no-no. Like, people die type of no-no. But, a basic, bumper sticker version of gas planning in caves (really simplified, there are lots of other factors) is the rule of thirds. But that assumes no deco obligation; it’s only for the ‘working’ part of the dive.

Since you have proposed a dive that is both overhead and carries a deco obligation, you must follow safe gas planning rules for both. If you are actually planning dives like this, it sounds like you need more/better training. If you’re just bringing this up out of curiosity, then I hope you’ve received the explanation you’re looking for.

 

[J-Vo]

Implied by others, but I'm just going to say it. Rule of thirds for open water deco dives is dumb. I can come up with examples where it is both overly conservative and dangerously underestimates gas needs. Learn to plan min gas to get you and a teammate out safe. If you have an overhead portion of the deco dive rule of thirds applies to just that portion and must exist inside the overall min gas plan.