GFs & Algos - Rec <-> Tec/Deco. A disconnect?

[Jay]

If I stack up the M-lines of various algos:

DSAT
PZ+
ZHL-16C
RGBM

(from relatively aggressive at the top)

and consider these two cases:

Case 1. A Rec (non-deco) diver named Rebecca Rec who's been happily* using her Oceanic Computer (on DSAT algo) for many years doing a variety of dives (multiple dives per day, and at times multi days on dive trips - all on air for simplicity). She decides to change her computer to a Garmin Descent (ZHL-16C) and to (solely) dive with that. What does she set her GFs to on her Garmin?

*happily = always inside NDLs, never bent or signs of.

Case 2. A Tec (bounce) diver named Tommy Tec and his sole computer is a Perdix (ZHL-16C) which he solely relies on (let's simplify it by taking tec tables / multi-computers out of the equation). He's doing a few days of diving with his buddy Rebecca Rec (all inside Rec limits, inside NDLs, on air, a few dives per day - they will do their 3min SSs). What (if any) adjustments might Tommy make to his GFs on his Perdix? (from his previous Tec dive setup) and he ignores Rebecca's computer / setup for the purposes of this q.

1/ For Rebecca Rec I would go 100/100 as that's still relatively less aggressive than what she's been using. (i.e. she is not closer to the DSAT M-line using ZHL-16C (with any choice of GFs)).

EDIT 26 AUG 2018: re 100/100 - this is incorrect. Please see Shearwater Gradient Factors=DSAT? esp posts #10 &29

2/ I'm GUESSING Tommy Tec would make his GFs more aggressive?

/2a. Do Tec (bounce) divers adjust their GFs for relatively shallower depths (ceteris paribus)

My ultimate question is 3/ Rebecca and Tommy seem to be miles apart wrt to GFs when they team up and moves their dive worlds closer (one the same computer algo - let's assume the algo and GFs work exactly the same way across those two brands). Yet it's the same ocean, same physics. I'm aware in that some tec divers are using less conservative GFs more recently. Has Rebecca been historically too aggressive or just lucky? Does depth played a bigger role? Or put another way, are the slopes of some of the M-lines 'wrong'? Is there a disconnect?

I understand that:
a/ GFs were introduced to add some degree of conservatism (by moving away from the M-line).
b/ my above ranking is very generic and crude and I'm reflecting/simplifying it as static, and not as dynamic process both across a single dive and multi dives, but ...
c/ this crude stacking seems to hold true for Rec (or at least I've not seen evidence / theory to the contrary) and I've merely listed it to demonstrate what I see as a disconnect (rightly or wrongly) - which I'll guess I'll soon find out / have a better idea if my thought has merit / errors with my stacking.
d/ there's some degree of 'we don't know', and there won't be a definitive answer to any of my questions.
e/ FYI this post is written for learning (and as you can tell my Tec knowledge is near zip - is it Tec, Tek, Tech??!! apart from some posts re GF and contributions by Tec people ).
f/ (almost) nothing in this thread is about deep stops, NEDU study, etc. Let's try not add it please.

 

[Blasto]

The reason NDL diving deco algos are set more aggressive than tec (deco) diving algos is that their role is different. In NDL diving, your deco profile is a set of no-exceed limits; in deco diving, it's the target profile.

So, in your example, Rebecca is supposed to dive freely until she sees a few minutes left on her NDL, then go to the shallow portion of her dive, then do a safety stop, all while well within her NDL. At no point in this profile, taught in rec diving, does she come close to her set GF.

Meanwhile, Tommy and his team have a specific ascent plan, which follows their GF. Minor deviations from the plan can bring them to either side of their target line, as a +/- tolerance, but generally they deco as close to their target GF as practical.

There are only two ways Rebecca will see her target GF. One is staying at depth until her NDL is 0, then surfacing immediately, skipping the safety stop. This isn't normal rec protocol, only a response to emergencies (including buddy separation). The other is overstaying her NDL, riding her ceiling up, and skipping her safety stop; then again her tissue supersaturation will match her set GF. If she does that on purpose, on normal dives, it's neither rec nor tec, but stupid diving, i.e. outside of either protocol.

The reason rec diving algos are set to higher GF is that deco in rec gear is somewhat of an emergency. There's no redundancy past the buddy system and no dedicated air reserve for deco. So the safest plan of action is to get them out of out of deco ASAP, back into the safe zone, where they can surface at any time. A 100/100 profile is much safer than running out of air on 30/70 and turning it into 30/200 on forced ascent.


If Rebecca and Tommy team up, they would follow one of their protocols, not something in between, and definitely not mix and match. Either Rebecca's high GF limits with guidelines that dictate staying away from them, or Tommy's low GF with full individual redundancy and gas planning.

 

[KenGordon]

Jay, I do not think your mental model of how DSAT relates to ZHL is correct. I suspect it is only based on the NDL values. According to the link I posted previously DSAT has a fixed surfacing limit rather than an M value line. I think that means that once you get deeper the ZHL limits will allow you to get shallower faster since they have similar surfacing limits plus a bit * depth.

I don’t claim to be an expert on DSAT at all.

As to your two divers. They should dive as usual and follow the more conservative computer on the day.

 

[Diver0001]

If I stack up the M-lines of various algos:

DSAT
PZ+
ZHL-16C
RGBM

(from relatively aggressive at the top)

I actually don't agree that this is true in all contexts. It's clear to me at this point that RGBM among others calculate (much) more aggressive profiles than Buhlmann with respect to decompression diving. You have followed other threads in which I mentioned that bubble models generally at this point have a calibration error that needs fixing. For decompression diving I would describe models that calculate deep stops as *more* aggressive, not less.

For diving within the NDL's I don't think a 1 minute deep stop here or there is going to really shake the foundations of deco theory. In that context I actually don't personally think it matters a whole lot which algorithm you use.

I guess the deep stop thing is really the crux of this post. Rebecca could set her computer to 30/70 or even 20/80 and make perfectly safe dives within the NDL's. In the big picture she might want to even choose 80/80 provided she had good control of her ascent speed and applied good habits to her diving. The only real function GFlo would have in a recreational context would be to slow the ascent.

Tommy, on the other hand, would need to limit the time calculated for deep stops so he would probably want a higher GFlo (like 50 or higher) in order to get him off the bottom and decompressing sooner rather than later. This would result in a more efficient and cleaner deco in general if you believe the research. Rebecca probably doesn't understand this stuff and really doesn't have to because her context is so different.

R..

 

[tbone1004]

@Jay_Antipodean

though I refuse to be called Tommy, I'll play.

1-she shouldn't do that, gf-hi of 95 roughly correlates to DSAT and is the most aggressive setting on the Perdix. Unsure what the descents defaults are, and gf-lo shouldn't matter to her so let it pick a default.

2-I do not make adjustments to my gf based on who I'm diving with. I base it on my personal acceptance of risk based on the dive that I'm doing. I typically dive 50/80 and would unlikely change it. As a tec diver, I don't care about deco, so I'll dive as long as my dive buddy wants to and then my safety stop may be a mandatory deco stop, or may have an extra couple minutes on it, but oh well.

2a-I don't know any that do.


summary:
you dive with personal acceptance of risk for DCS and that is personal. If you are diving with others who have different opinions than you have to make a choice. @victorzamora and I have a buddy who dives with BAUE and he runs more aggressive GF's than we do. In caves where we dive together, he has the option to leave or stay with us until we are done. He usually stays.
As a tech diver, we should know a lot more about what we are doing with GF adjustments and once you have your personal choices for that dive, you have to have a discussion with your buddy. That either goes one of two ways. You are 99% going to have the shorter NDL due to lower GF-hi, so you can cut the dive short when you hit NDL, or you can just go into deco and if they are going to do the safety stop, your deco obligation should be done by then.

100ft dive for 30mins on EAN32. No deco dive on 50/95 *well in DecoPlanner it's a 1min stop at 20/10ft but those can't be removed*, 6 minutes deco on 50/80. I'm going to do the deco vs. cut that dive short if my buddy is fine with it. I have a mandatory deco stop, they're doing a safety stop. Perdix on that would kick that dive up to a 5min safety stop, so they'll clear at the same ish time

 

[dmaziuk]

I base (GF) on my personal acceptance of risk based on the dive that I'm doing.

Since this is fresh in my copy-paste buffer:

It is important to keep in mind that although these ascent-limiting M-values are handled as specific "hard" numbers, the experience behind them is that they belong to a spectrum of gradually increasing risk.

-- R.W. Hamilton et al, The DSAT Recreational Dive Planner, Development and validation of no-stop decompression procedures for recreational diving, 1994 Feb 28, p. 14. section III.B.3.