Q re NDL, GTR and TTS - on AI computers - for Rec Divers (OC-Rec)

[Jay]

For Rec divers that use AI computers, why isn't there a simpler acronym/word to represent key data? By 'simpler' I something a concise and clear.

Aside from Dive Time, Depth, and Tank Pressure, AI computers display NDL, GTR and TTS (generally speaking in Rec mode or by default).

TTS includes stops, GTR doesn't. This post is not about changing those definitions.

If we define GTS=GTR but to include stops, EDIT (reason in post #4) and also define TIME to be the minimum of GTS and NDL. Then your home-screen has only Dive Time, Depth, Tank Pressure and TIME.

TIME could have a pictorial icon beside it displaying a tank (if it's driven by gas), or vertical NDL, which would save space and keep things informative.

Then, to me, it makes a very user friendly display for REC using AI. It saves having (a redundant) TTS and replaces GTR with an accurate/correct GTS [EDIT: bold has been added]

It also utilises the computers power to do all calculations.

My post is more about KISS, and do it once, DIR.

Makes sense?

p.s. This post is not about changing anything in the Tec world. Nor is it about allowing one's brain to got into zombie mode.

 

[scubadada]

Oceanic, I assume all PPS computers, define air time remaining (ATR) as ascent and all stops. I don't know exactly why Shearwater decided to leave out the stops. Especially for rec diving, the difference is trivial, less than 100 psi for the safety stop. How hard is it to include deco stops, apparently, not very.

 

[dmaziuk]

Presumably Shearwater lets you set your gas reserve for GTR just like everyone else, so set it to extra 100 psi to account for your safety stop, problem solved.

There are a few reasons you don't want to do min( GTR, NDL ), the big one is "non-stop" divers will land in mandatory deco and then lawyers will arrive on your doorstep to explain the error of your ways.

 

[Jay]

Presumably Shearwater lets you set your gas reserve for GTR just like everyone else, so set it to extra 100 psi to account for your safety stop, problem solved.

Agreed, it solves the problem in that way, but ... (below)

There are a few reasons you don't want to do min( GTR, NDL ), the big one is "non-stop" divers will land in mandatory deco and then lawyers will arrive on your doorstep to explain the error of your ways.

That single number will tick down to zero, with a reason displayed, warnings set etc etc , so I don't agree that it's a potential legal issue. I think it's more of a legal problem by having GTR knowingly being wrong by the known stop(s) time(s). I'm sure a good lawyer would have a field day with that one. I'm not about to propose changing the GTR methodology/calc.

My post is more about KISS, and do it once, DIR.

One reason that I didn't properly consider, which I think is what you might also being saying, is that 'you' (being a Rec diver) would want to run your GTS down to zero, yet you probably don't want to run your NDL down to zero, and being represented by a single number ... that's a problem - so scrap that TIME (minimum) display idea.

So that means instead of saving two pieces of data my proposal saves one (TTS can be done away with) with GTS.

I don't know exactly why Shearwater decided to leave out the stops. Especially for rec diving, the difference is trivial, less than 100 psi for the safety stop. How hard is it to include deco stops, apparently, not very.

It seems (from speed-reading 1/2doz of the AI manuals) that it's only Oceanic which includes stops. The error is a small number, but small known errors shouldn't be embedded into a device. Sure, 3-5 min of SS, call it 100psi-10bar max, isn't a trivial % . Again, KISS, and do it once, DIR. Obviously this 'error' is embedded into the industry and there's little to be done about it unless there's a mass tec revolt. But with more tec computers going rec (I'm thinking Teric here) it's a chance to right the wrong with GTS.

If a Rec diver does mess up and go into deco, then s/he needs to remember their GTR doesn't include that deco stop. Whilst that's a given no-brainer for a Tec, I wonder for a typical (lets say non SB) Rec person if that will be at the forefront of their mind at that moment.

So, does GTS makes sense? Would it be used by a Rec Person?

 

[scubadada]

I've been using an Oceanic VT3 for 8 years with ATR including stops. I would have no problem using a Shearwater that does not include stops. My SS requires less than 100 psi, my short deco stops are easy to estimate. I like the included stops with my Oceanic, but I could easily adapt to Shearwater, and will likely do so in the future.

 

[JohnnyC]

Gas time and NDL, while both are dependent on depth, are entirely different mechanisms. It's entirely possible to run out of gas time while having plenty of NDL left (say a 500psi GTR while in 6m of water with functionally unlimited NDL). It's also possible to run out of NDL while having plenty of GTR (a deep bounce dive where NDL time is measured in fingers on one hand). Again, NDL is dynamic. GTR is dynamic. They are not related in any manner other than that depth reduces both. And because GTR is a variable set by the diver, there's no guarantee that two divers will have similar GTR's, even if their SAC rates are the same and they are diving the same profile. You could say the same about NDL, by setting different levels of conservatism.

Having TTS with or without stops included isn't a big deal, and if you're making the argument that a diver has to remember that if they go into deco that their GTR doesn't include stops, you can make the same argument for the Shearwater iteration of TTS, and a diver needing to remember that it DOESN'T include stops. TTS really only exists because it's useful for technical divers to have an dynamic overall total runtime. Recreationally, it's pretty useless anyway since the answer is always going to be dependent on your ascent rate. And since it's no-stop diving (i.e. a safety stop not required), anything shallower than 120 feet/30 meters it doesn't really matter if it says 2 minutes or 3 minutes. And if all of a sudden it decides to say 4 minutes because it wants a safety stop, the diver has to decide if it's saying 4 minutes because the computer has added a safety stop, or because they're deeper and the computer decided that the chosen ascent rate will mean that it's just going to take a little more time, or if it's because the computer has decided that they don't have enough gas left, or if they actually have a decompression obligation. Well now the diver has to make a significant decision, and you've already maintained that a diver isn't going be be able to parse the difference between TTS, GTR, and NDL. So now you've stripped all relevant meaning out of all of those categories.

Either way you can't abdicate a diver needing to remember one type of data but not another. And because GTR and NDL are unrelated, combining them into some nebulous unifying time requirement is pretty pointless. Especially when you can drastically change the figures displayed in either category with a simple settings change.

If you really want to simplify things, you need to have the diver use whatever gas time remaining as a countdown timer to their hard deck time. Set a reasonable amount of gas for the ascent, plus a reasonable reserve, and when the tank pressure reaches that, the GTR will read zero, and the diver MUST start their ascent. But people bust NDL's all the time because it clears on the way up. Who's to say they will respect their GTR? And that's actual gas, not an ascent to a point where their NDL will top out back to unmeasurable and they can piddle around in the shallows until they need to pop their head up.

I get simplifying things, but trying to take a set of unrelated numbers and shoehorn them into one unifying time scale isn't the way to do it. It certainly isn't KISS, it certainly isn't doing it once, and it certainly isn't "DIR." The bottom line is that the diver needs to understand what each of those terms means. If they refuse to do that, that's on them, and it's not up to the computer manufacturers to provide enough colors of crayons to explain it to them. They HAVE to know what an NDL is. They HAVE to know what GTR is, regardless of whether or not in includes stops. The idea that a TTS is "wrong" because it doesn't include stops in NO-STOP DIVING is fallacious. A safety stop is a choice, one that each diver is free to make on their own. That's why TTS doesn't include stops before a deco obligation. That's why GTR including stops isn't an accurate representation UNTIL the computer calls for a decompression obligation where it is actually an accurate measure. And in that case, you sure as hell want to know the difference between your GTR and your TTS.

 

[Jay]

@JohnnyC if you could please re-read my OP again, and take note of the struck-out text and reference to post #4 (edited several hours prior to your post).

the Shearwater iteration of TTS, and a diver needing to remember that it DOESN'T include stops. ......

The idea that a TTS is "wrong" because it doesn't include stops in NO-STOP DIVING is fallacious.

TTS does include stops. Here's the definition (pic below - from Teric's manual). That error aside, I've not said TTS is "wrong", I've only used that word when discussing GTR versus GTS (purely in a Rec setting).

A safety stop is a choice, one that each diver is free to make on their own. .... That's why GTR including stops isn't an accurate representation UNTIL the computer calls for a decompression obligation where it is actually an accurate measure. And in that case, you sure as hell want to know the difference between your GTR and your TTS.

By definition (see OP), GTS (being GTR including stops (as we've both termed it)) would be accurate in all cases, regardless of stop type or timing. e.g. you have your Teric set for a 3min SS, and at depth of 35ft+ that SS would be triggered. The GTS just prior to 35ft the would be correct, and just deeper than 35ft, it would suddenly and decrease, and still be correct. Same logic applies to an unexpected deco obligation. If the dive only went to 34ft and thus not triggering the SS then GTR=GTS - both being correct.

I agree, for sure you want to know the difference between GTR and TTS.

But perhaps this discussion highlights the need for GTS in a Rec sense.

Imagine (in a Rec sense, with AI) if GTR and TTS had never existed and the only acronym was GTS which is the 'manual' would define as "the time in minutes that can be spent at the current depth until a direct ascent to the surface once the required stop(s) have been made at a rate of 33feet/min (~10m/min) which would result in surfacing with the Reserve Pressure". In this case, we both would not be having a similar conversation about GTS.

I think it's clear that GTS would be preferred over GTR in a Rec+AI sense.

p.s. 30m=98ft, not 120ft.

 

[scubadada]

...It seems (from speed-reading 1/2doz of the AI manuals) that it's only Oceanic which includes stops...

Hi @Jay_Antipodean

I am under the impression that all Pelagic Pressure Systems computers calculate air or gas time remaining as a normal ascent with all stops. This is certainly true for Oceanic, Aeris, Hollis, Sherwood, and Tusa. Aqua Lung does not include enough information in their manuals for me to confirm this.

Scubapro also includes all stops in their remaining bottom time calculation.

I don't know what Mares, Suunto, Ratio, Liquivision, or Heinrichs Weikamp do, but you probably do from your reading.

We all know that Shearwater does not include any stops.

 

[scubadada]

My end pressure is set at 500 psi. I have overstayed my ATR=0 many times when drift diving in SE Florida. ATR remains at 0 and my computer tells me to surface. Eventually I do, and end with a pressure less than 500 psi.

I have actually never tried this but know the result. If one dived the ATR down to 0, or near 0, and then went into deco, with an obligation of greater than 3 min (same as the SS), one would also surface with an end pressure of less than the set pressure. One would have to be quite lame to violate this to such a degree to risk OOA.

I have over 1,500 dives on Oceanic computers with ATR, I am extremely familiar with their operation. The calculation is remarkably accurate.

 

[dmaziuk]

That single number will tick down to zero, with a reason displayed, warnings set etc etc , so I don't agree that it's a potential legal issue.

"You see Yr'Honour, all the time I believed it was counting my gas time, but I carry this Al63 pony for which I have no transmitter, so I had plenty of gas for a safe no-stop ascent. How was I to know it was counting my no-stop time when there was no indication of that whatsoever? And when it counted me into mandatory deco, it turned out my Al63 did not have enough gas for all the mandatory stops, so... I got bent. And now I fully expect @Jay_Antipodean to cover the costs of my lost diving vacation, my helo ride to the chamber, my chamber rides, and all the pain and suffering inflicted on me solely by an incompetent design of his dive computer's user interface."

PS. I hate car analogies, but do you really want your car's speedo to show its current speed or the current speed limit, whichever's smaller?