Modified ratio deco

[crofrog]

Ratio Deco is at best extremely risky and goes against everything we understand about decompression theory.

It does? What part of decompression theory does it go against?

All of the BS about you know your body is just that, BS, you can't do decompression by feel. You need to learn how you feel after dives to learn how to adjust your personal gradient factors, sure, but you aren't just going to say "ya know what, this part of my body doesn't feel great so I should probably spend some more time at this depth", that's nonsense.

It happens, people start experiencing DCS symptoms in the water. There is an entire section devoted to it USN Diving Manual Rev 7 specifically 9-12.11.

 

[tbone1004]

It does? What part of decompression theory does it go against?



It happens, people start experiencing DCS symptoms in the water. There is an entire section devoted to it USN Diving Manual Rev 7 specifically 9-12.11.

It relies on a human paying attention which is inherently risky when compared to a computer. As I stated, it's fine to think of them for sanity checks, but with proper dive planning setting the boundaries for contingencies, it is not beneficial to actually dive them.

All of the current ratios being taught to not match an ascent profile that correlates to the current understanding of minimizing decompression stress on the body as shown in the Spisni study. Moreover the ratios do not actually correspond to any particular gradient factors so do not actually line up to any dive planning software as the GF's move with each ratio. They are great for sanity checks, I use them regularly, particularly in nitrox depths, but for anything serious, proper dive planning with software and execution with a computer is vastly superior.

I'm not denying that people don't experience DCS in the water, but that's not what is generally being discussed about "listening to your body" when people are talking about ratio ascent profiles. The way current proponents of Ratio Deco discuss it they make it sound like you can actually make an ascent profile just by feel, which is completely untrue. On a CCR you can actually make an ascent without anything other than a depth gauge and ppO2 monitor, but that's a bit of a party trick more than a primary means of executing an ascent. The other way they discuss the "feel" is randomly staying for a few extra minutes at random depths. This is one thing if you were comparing it against tables which are rigid, but the dive computers are fully dynamic and are tracking your live exposure profile and adapting accordingly.

Ratio Deco had a place 20 years ago before the computers became capable, and even 10 years ago when they were capable but could be cost prohibitive, that has since been resolved and the cheapest of computers out there are able to run custom GF profiles with multi-gas and that has effectively eliminated any need to really know how to do RD. If you disagree, I would love to hear your argument on why it is better than properly planning dives with software and then executing that dive with a computer that is running the same algorithm as the software.

 

[crofrog]

It relies on a human paying attention which is inherently risky when compared to a computer. As I stated, it's fine to think of them for sanity checks, but with proper dive planning setting the boundaries for contingencies, it is not beneficial to actually dive them.

All of the current ratios being taught to not match an ascent profile that correlates to the current understanding of minimizing decompression stress on the body as shown in the Spisni study.

I'm not entirely disagreeing with you, but I'm not entirely in agreement either. The concept of ratio deco doesn't inherently violate "everything" we know about decompression theory.

Doing math and keeping track of stuff underwater is harder than it is above land, and it does rely on people "paying attention" which I would argue you should be doing anyhow if engaging in technical diving

As for what's safer... That's a whole other ball of wax right?

Shallow water ratio deco would say for a 60 minute dive at 85ft (common Ginnie springs depth) would have you do 30ft/min to 40, 10ft/min to 20 and on dive 1, 8 minutes of O2 at 20ft and a 6min ascent to surface. You'd surface at gf of around 60%. If you did the same dive 90 minutes later, ratio deco would have the same ascent profile to 20ft, but 20 minutes at 20ft and a 6 min ascent to the surface. Surfacing at around 44% GF.

If you just dove something with a GF high of 85% you'd have something like 8 minutes of O2 at 20ft, and then on dive 2 90 minutes later you'd have 13 minutes at 20ft.

Strategy	dive 1 TDT	dive 2 TDT
Ratio deco	14	26
ZHL gf-high 85%	8	12

I think it's pretty safe to say that longer TDT is generally speaking safer, I think that's especially safe to say when we say that more time on O2 is better than less time on O2 when O2 is your only stop.

 

[tbone1004]

@crofrog at nitrox depths the ratios are generally fine because GF-Lo doesn't matter, the issue is at trimix depths when the GF-lo's are much lower than the current recommendation of 70-80% of the GF-hi.

 

[rjack321]

This gets a bit weird on CCR at trimix depths, but since you have to plan your exit and ascent assuming you're on OC, I just plan the dive as if I was on OC the whole time and know that with the CCR I should be trending a bit shorter.

This is fine and is reasonably conservative above about 210ft. For mix dives on CCR below ~210ft your CCR deco is going to be longer than the OC bailed deco - sometimes pretty radically longer.

 

[tbone1004]

This is fine and is reasonably conservative above about 210ft. For mix dives on CCR below ~210ft your CCR deco is going to be longer than the OC bailed deco - sometimes pretty radically longer.

depends on if gas-switch with the breather or not. If you stay on bottom mix then certainly is a lot longer, but if you plug in to knock the helium down it ends up about the same. I like to at least plugin the first nitrox bottle and flush it which not only gets the helium out of the loop and brings the deco obligation way down but it also lets you perform a dil-flush to check for cell function at 1.5-1.6. I will typically bring the ppO2 up to 1.6 then let it fall down to about 1.0 then vent and bring it back up again as I'm going up which helps to keep my lungs from getting too crispy. No way would I go up to 1.6 on anything deeper than 20ft without performing that flush to check for current limiting.

 

[flymolo]

I like to at least plugin the first nitrox bottle and flush it which not only gets the helium out of the loop

how do you know it gets the helium out of the loop?

what about the helium that is flushed out of your tissues and ends up in the loop?

do you have an actual way to measure helium in the loop or are you going off what your computer thinks is in the loop because you switched the gas?

 

[inquis]

what about the helium that is flushed out of your tissues and ends up in the loop?

Open question as to how long it's negligible after the Nx dil flush. I'd love to get a download from a Liberty diver with the He sensor history. My suspicion is by the time you get to 70 ft on a ~100m dive, the off-gassing is predominantly N2.

 

[boulderjohn]

Doing math and keeping track of stuff underwater is harder than it is above land, and it does rely on people "paying attention" which I would argue you should be doing anyhow if engaging in technical diving.

Paying attention to what? Your physical environment? Your teammate(s)? Animal life? Your route? You have lots of things to pay attention to during a dive, particularly a technical dive. Constantly doing math throughout the dive only distracts you from those things. If anything important arises during the dive that seriously occupies your time and attention, what happens to your constant math computations?

Around 15 years ago a very similar debate featured John Chatterton, and he was specifically referring to wreck exploration. One person in the Ratio Deco group (and he was arguing people should use ONLY Ratio Deco) said it was good because it gave you something to think about during the dive. Chatterton fairly exploded, saying he had plenty to think about during a dive, and if you needed to do math to make a dive interesting, you needed to be doing more interesting dives.

 

[flymolo]

Paying attention to what? Your physical environment? Your teammate(s)? Animal life? Your route? You have lots of things to pay attention to during a dive, particularly a technical dive. Constantly doing math throughout the dive only distracts you from those things. If anything important arises during the dive that seriously occupies your time and attention, what happens to your constant math computations?

Around 15 years ago a very similar debate featured John Chatterton, and he was specifically referring to wreck exploration. One person in the Ratio Deco group (and he was arguing people should use ONLY Ratio Deco) said it was good because it gave you something to think about during the dive. Chatterton fairly exploded, saying he had plenty to think about during a dive, and if you needed to do math to make a dive interesting, you needed to be doing more interesting dives.

what about recalculating gas on a stage cave dive should we not attempt to do that? too distracting?