Now that you say this, I think we had this discussion before on another thread and ended up agreeing to disagree.
That said, I'd like to get clarification on the quote above.
I fully understand the part about gas logistics because RD is tuned to suit DIR standard gasses. That said, if you enter the mix in a computer then the computer is also "tuned" to the standard gasses, so to me that point is kind of like saying that since a sundial is tuned to the sun that it is somehow more practical than a watch. However, we previously agreed to disagree so I'm not going to debate the nuances of this. I will concede the point that RD is, indeed, tuned to standard gasses.
It's the second part I don't get. RD is an ascent strategy, as opposed to a model. When you say it takes diver training level into consideration it triggers me to think that this whole time we've been comparing apples and oranges.
Since RD is not a deco model it is, indeed, unfair to compare it to something like Buhlmann. I think this is where your comment about practicality comes from and your comment above. Am I right about this?
I hadn't thought on how to phrase and elaborate this.
I'll use an example.
Using Nx50 at 21m
instead of O2 at 6m is not sensible from a purely physiological decompression perspective, unless you subscribe to a notion that the ppO2=1.6 has significant weight over the tissue supersaturation level. And even so, even if one thought that tissue supersaturation level means absolutely
nothing, if the ppO2 is equal, one might as well have it closer to the surface.
Right?
From a physiological decompression perspective, there is little to suggest that the 21m-option/NX50 (assuming you're limited to 1 deco tank here - for instance due to diver training limits) would ever be preferable over 6m/O2. And, as far as it's feasible, we prefer O2.
However, there comes a point (1:1, as it happens) where a double-set of 12Ls or 80s won't cover your bottom gas and Rock Bottom all the way to 6m. So we install a "next available gas" much deeper, at 21m, so we can reach that sooner and thereby reserve less gas.
This is a gas logistics-driven decision, not a physiological decompression-driven one.
In this example, we're taking a step away from what's physiologically most sensible (optimal) because we don't want two divers to drown if I blow an O-ring (practical).
I think it would then be blurring the picture to compare the ascend using O2, to the ascend using Nx50 and discard Nx50 for being suboptimal in terms of physiological decompression.
I'm not sure if that answers your question..?
If I zoom out to the big picture then I actually think you're right in a way. An ascent strategy IS a necessary part of the diver's repertoire. "Blindly" following a computer is probably not wise. In fact blindly following ANY ascent strategy is not wise. What you're saying here seems to be along those lines. Where the disconnect happens is when you assume that since a computer suggests a certain ascent approach that you are *obligated* to follow that approach. You are not.
Lately, a great deal of energy has been put into the discussion about *efficiency* of different ascent strategies and the corresponding risks. That said, you *could* make a deep stop using Buhlmann. You could, in fact, make an entire RD type ascent using Buhlmann. In some cases you may (probably would) have to add extra time to the last stop in order to compensate for the inefficiency of the ascent, but it could be done and I think it could be done safely as long as you considered your last stop a "mop up" stop to fix whatever you did before that.
Absolutely. I mean, I don't kid myself that all the heavy-end divers out there using computers are just around because they're extremely lucky.
Of course they're taking their computers and algorithms into a greater context, which includes an armada of practical considerations.
The thing I like about RD is, a lot of that has already been done "out of the box". Not the optimal deep stop emphasis, but the ppO2s, rock bottoms and diver training concerns are already catered to off-the-bat.
Compared to my experience in my initial tech diving, with more traditional dive planning methods (computer, backup, wetnotes, laptop, V-planner), I find RD much easier to use. But, in fairness, also
far more taxing to learn!
I personally went back to rec diving while learning it, and then re-expanded my comfort zone back to tech diving when I was ready.
By that time, I was comfortable doing so and I found it about as easy to plan dives and calculate adaptations as looking at a computer read-out. But that wasn't the case in the beginning.
My view is RD
should stress the diver's learning curve - but
should not stress the diver's capacity on the dive.
For me, that meant I needed to put any Billy-Big-Bananas diving on hold while I developed my RD/RockBottom-savvy on cozy flower-dives.
Fine by me.
Then I got to thinking about why....
At first, I thought what you did..... That the computer and the table dive could be compared on equal terms.
But then I remembered the one most important thing about computers.... they give you longer bottom times because they eliminate the rounding errors built into the tables! We all preach that to our students now. Buy a computer for longer bottom times... nobody makes a bucket profile.....
Even though this was a wreck dive I clearly didn't spend 100% of my time at the maximum depth and the computer knew that. The table was based on an erroneous assumption that this would be the case. The deco times were different because (aside from differences in deco model) the computer knew what I had ACTUALLY done... I, however, did not.
Even calculating average depth on the fly you're going to be off. You have a data point every now and then but the computer samples your depth every few seconds and recalculates the entire dive based upon a very accurate average depth. A human could never come close to that. My conclusion, after thinking it all through, is that, like tables, the average depth calculation has enough "slop" in it to have a significant effect on your deco times.
Maybe not enough to get you bent (on every dive) but enough that trusting in-your-head-average-depth-calculations isn't sounding wise to me when you can strap something to your wrist that does it perfectly.
Yeah, for sure! I mean, computers will always be more accurate.
Let me use another example to illustrate my thinking on it:
When I check my gas, I calculate what it
ought to be, based on my time and depth.
I then look at the SPG and confirm.
If my calculated number and the one on the SPG don't align, that's an alarm - either my SPG is busted, or my gas consumption is different to what I'd expected it to be ("respiration system compromised")
I like that approach because it gives me 1) early warning if my gas consumption is off plan, and/or 2) confirmation that my SPG is/isn't working as intended.
When I check my depth, I calculate what it
ought to be, based on my time and depth.
I then look at the gauge and confirm.
If my calculated number and the one on the gauge don't align - either my gauge is busted, or my thinking is different from I'd expected it to be ("thinking system compromised").
I like that approach because it gives me 1) early warning if I'm approaching the fringes of my comfort zone, and/or 2) confirmation that my gauge is/isn't working as intended.
I agree that I'm sacrificing a bit of accuracy if I'm going solely by depth averaging instead of gauge.
But in practice, I'm actually not relying solely on either.
What I'm doing instead, is mine my mind for as much data as possible because I believe it may give me an advantage (i.e. early warning of stress and control of gauge function).
However, translating it to a standard deco will probably always be suboptimal, just like using a table.
I personally think it's worth it, and try to cut the loss - most often by erring on the shallow/"aggressive" side.
I land on the conclusion that the above makes sense, because my belief is that the single greatest threat to my personal safety, is my own brain.
