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's 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.