This thought is often posted with a negative connotation. People post about divers "riding their NDL" where the context strongly implies (or even specifies) that the poster regards this as a very poor or unsafe behavior. I'm not saying you've done that here, by the way.
I want to note my own observation that I think this attitude is counterproductive.
What if I set my computer to GF 30/70 (or some equivalently conservative setting for calculating NDL)? And what if I set my AI computer's gas reserve to be 1000 psi? In that case, what is wrong with riding the NDL and the GTR? Those settings seem pretty conservative to me. I want to arrive at the surface with a GF99 no higher than 70% and be at the surface with 1000 psi left in my tank. If I use those settings and ride the NDL and the GTR, what's the problem? If I push the limits and then have an issue, I still have a very large safety margin.
Like I said, I think talking about "riding the NDL" or even "riding the GTR" in a way the implies that that is AUTOMATICALLY bad does a disservice to the community and especially to the divers with less experience. I mean, what tech diver who uses GF30/70 does NOT ride his computer? Riding your computer is NOT inherently bad. Instead of making people feel like it is wrong to ride their computer, shouldn't we instead be talking about making sure people understand what their settings mean and making sure they choose appropriately conservative settings?
It is good that NDL has a pretty standard meaning. We all know that it means time to go to the surface and safety stops are optional. If some computers calculated NDL with an assumption of a 3 minute safety stop and others didn't, that would muddy the waters and make it confusing for everyone.
Sadly, that is where we currently are with GTR. And now Shearwater has really screwed the pooch (in my opinion) by not only calculating GTR differently than any other OEM, as far as I know, but calculating it in a way that is worse and less intuitive than anybody else does it. To the best of my knowledge, some computers calculate GTR as "the time at current depth until you reach the reserve pressure." That is the most simple and easiest to understand. On those computers, if you're at 100' and the GTR reads "1", and your reserve setting is 500 psi, then you know that if you stay at your current depth and keep breathing the same, in 1 minute, you will literally be at 500 psi. Simple. Easy to understand. Not what we really want - but at least it's very simple and easy to understand. Set your reserve setting to whatever you want your pressure to be when you start your ascent (based on the planned depth, etc.) and go.
The Oceanic (et al) GTR is different. Not quite as simple to explain, but gives us (or, most of us, anyway) what we really want. It factors in depth so that it tells when we have to start our ascent to get to the surface with the desired reserve, including all stops that the computer will require or suggest. In the case of the Oceanic algorithm, it has extra safety built in because it includes optional stop times. So, if I follow the computer exactly, I'll hit the surface with my desired reserve. But, if I choose to skip an optional stop, then I'll get to the surface with MORE gas left than my reserve setting calls for. The boat captain says "be back on the boat with 500 psi" and the Oceanic algorithm allows people to do exactly that and without having to do mental math during their dive.
In contrast, the new Shearwater method works in a way that, if I follow the computer, doing all the stops that are required or suggested, I will actually get to the surface with LESS air than my configured reserve setting. To be sure, the difference is expected to be small as we're generally talking about doing a shallow safety stop that is not accounted for in the GTR calculation. You don't expect to use much gas at a 3-minute, 15 foot stop. Nevertheless, a calculation that leaves you LESS gas when following the computer is clearly not AS safe as a calculation that leaves you MORE gas.
Imagine: A rec diver gets a new Perdix AI. They are about to do a dive on a reef with a 60' hard bottom. Their NDL is going to be so long that they know they'll have to end the dive because of low gas. Because it's fairly shallow, they decide they are okay with getting back to the boat with 200 psi left. I'm sure I'm not the only one that has seen numerous divers get back on the boat with that much gas left. So, this diver sets the PAI reserve to 200 and jumps in. When his computer hits 0 on the GTR meter, he begins his ascent, following the computer exactly, doing an exact 30 ft/min ascent. He gets to 20' and the computer tells him to do a 3 minute safety stop. Now, because of the way the PAI works, during his safety stop he actually breathes his tank down past 200 psi and suddenly realizes he's still at 20' depth and has just breathed his tank down so low that he can no longer draw a breath. There is clearly a bad choice being made right off the bat, in this scenario. But, the Shearwater implementation for GTR also clearly contributes significantly to the diver going OOA.
It would be helpful to the whole community if GTR were standardized, like NDL, so that everyone knows what it means and it means the same thing on every AI computer - and it is as accurate as possible when you follow the computer exactly (including doing any safety stops the computer calls for).
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Let's face it, AI is a gimmick. A usefull gimmick, but not something you need or even something you should need. If AI solves a problem for you you better solve the problem first.
