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

[Jay]

"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?

As a 'car-guy' let me run with that one first by replying with a different car analogy: and it displays my core point about GTR vs. GTS

The speed limit on the road your travelling is 100km/hr.
Let's say there's 4 cars:
Typical_Car: Car's speedo reads 105km/hr when it's actual speed is 100km/hr. So when it displays 100km/hr it's actual speed is 95km/hr (close enough to). <--conservative
Accurate_Car: Car's speedo reads 100km/hr when it's actual speed is 100km/hr. <--accurate
Sue_Me_Car: Car's speedo reads 90km/hr when it's actual speed is 100km/hr. So when it displays 100km/hr it's actual speed is 110km/hr (close enough to) <--aggressive
She'll_be_Right_Car: Car's speedo reads 95km/hr when it's actual speed is 100km/hr. So when it displays 100km/hr it's actual speed is 105km/hr (close enough to) <--slightly aggressive

All modern cars are actually conservative.

GTR is slightly aggressive.
GTS is accurate.

The "Yr'Honour" paragraph is entertaining and I'll reply to it in a separate post later on (along with some more car stuff), as I don't want to detract from my core 'argument' about GTS & GTR (and my OP got off to a pretty bad start!)

 

[Jay]

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

Thanks for the info, and agreed, and it's designed to be as accurate as possible. Apologies harping on about it (/GTS), but it's the way it should be. Cue the Six Million dollar man lyrics "....we can rebuild him. We have the technology. We have the capability ...to build an accurate GTS computer!!"

Going back to my car analogy above, my ranking preferred ranking would be: accurate, slightly conservative, slightly aggressive, aggressive. Most certainly not having 3rd place in 1st place, that would be illogical. I can see why 1st and 2nd place could be negotiated.

Hi @Jay_Antipodean 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 (for gas. <-- I know you're referring to gas, just don't want to others to read it wider than that with the quote)

As an FYI/PSA : For gas, Mares have TTR. They define it as "the time that a diver can spend at the current depth and current breathing rate before reaching the tank reserve." i.e. nothing to do with the surface. They also have ASC:
Total ascent time, the time it takes to perform the ascent from your current depth to the surface in a decompression dive, including all decompression stops and assuming an ascent rate of 10m/min or 33ft/min. (p.3 Quad Air)

Suunto: "Gas time refers to remaining air (gas) left with current gas mixture, measured in minutes. The time is based on tank pressure value and your current breathing rate. Gas time is also highly dependent on your current depth. For example, all other factors being the same, including breathing rate, tank pressure and tank size, depth affects gas time as follows: ...

(source 9.22 EONCore).

 

[dmaziuk]

Let's say there's 4 cars:

And that is why I hate car analogies.

Let's say there is one computer.

Let's say there is a "no-stop" time limit that a diver is not supposed to go over.

Let's say their one computer does not display that time limit.

if you don't see a fundamental problem with that, here's a little practical implementation issue: you need X amount of gas for a no-stop ascent. You need X+Y amount of gas to make a mandatory deco stop. The way computers work is, the moment you go over NDL, you GTR drops from "have X" to "don't have X + Y". You see problem with that?

 

[Jay]

Let me back things up a little. In my original OP I promoted two things. I deleted/withdrew/etc one of them by post #4 and struck out that text in the OP. The only thing I am now promoting is that an additional form of Gas (to surface) time should be made, and that includes stops. I've been referring to that as GTS (in Rec OC).

My view is that GTS (being an "accurate" number), is superior than GTR (being a "slightly aggressive") number. Hopefully you agree with that?

i.e. nothing above refers to NDL or displays. Hope that clears that up.

Below, is to clean-up / respond to why I think that way re your legal case; (it's not to promote / continue / appear to promote something that's dead).

Let's say there is one computer....

Yes, I agree with your entire statement.

The withdrawn/deleted idea was to do with displaying GTS and NDL. That has several flaws. You thought enough to get sued for, and I didn't agree with that, which led to the 'Yr'Honour' post. Even though I fully agree with your statement above, I think there's some flaws in your legal case that don't make it a slam dunk as part of your statement(s) are different than my OP. That being: you said "... that there was no indication whatsoever?" (post#10) and "Let's say their computer does not display that time limit." (post #13) versus my OP counter "...could have a pictorial icon beside it displaying a tank (if it's driven by gas), or vertical NDL" meaning you (the diver) should know/have seen what it's referring to. You might have read the word "could", as in 'perhaps not have', and for my defence to survive, it needs to be read in as 'has'. A second line of defence would be the immediate notification of a deco obligation from this computer when (actual) NDL hit zero (the one you thought was gas). That should have sent you immediately up (non-deco dive), the deco obligation cleared / would be de-minimis, and you have your reserve gas in tank 1 so no OoA.

 

[dmaziuk]

My view is that GTS (being an "accurate" number), is superior than GTR (being a "slightly aggressive") number. Hopefully you agree with that?

My view is that GTS on a no-stop dive is whatever I have in my lungs. The exact number is entirely irrelevant to a recreational diver.

GTR is a guesstimate of how long I can stay at a current depth before I have to go end the dive, as is the NDL. My view is I rarely dive like that and that on a square-profile dive like that I'll be NDL-limited and running by tables anyway. I'd probably bring the computer just to log the dive, but its calculations will be largely irrelevant.

 

[Jay]

My view is that GTS (being an "accurate" number), is superior than GTR (being a "slightly aggressive") number. Hopefully you agree with that?

My view is that GTS on a no-stop dive is whatever I have in my lungs. The exact number is entirely irrelevant to a recreational diver.

GTR is a guesstimate of how long I can stay at a current depth before I have to go end the dive, as is the NDL. My view is I rarely dive like that and that on a square-profile dive like that I'll be NDL-limited and running by tables anyway. I'd probably bring the computer just to log the dive, but its calculations will be largely irrelevant.

Yeah accuracy and relevance are debatable. It's not at a hair-split stage, but the point I was making is having a number knowingly be more accurate is better than less. One day that 10bar might make all the difference!

I don't get what "whatever I have in my lungs" point means. Are you meaning looking at GTR/S isn't necessary; you just need to know what's in your tank? If so, then for sure that's what the non-AI world is about!

As for square profiles, using GTR/S doesn't dictate those in Rec. You profile could be anything, until you need to being your ascent.

 

[dmaziuk]

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[stuartv]

Jay, I think your ideas here are making things too confusing. Or would, if they were implemented.

I feel like there only needs to be one AI calculation. Call it what you will, but I will call it Gas Time Remaining. What I would like the calculation to be is exactly how Oceanic (et al) do it. It should assume a 30 fpm ascent and it should assume the diver does all stops that the computer will prompt for (mandatory deco or not). If the diver stays at current depth until GTR==0 and then ascends at 30 fpm, doing exactly every stop the computer prompts for, and their breathing stays the same as it has been, they will arrive at the surface with their preconfigured reserve.

Any other terms or ways of calculating GTR add unnecessary confusion to the mix.

The only real question (in my mind) is how the computer should handle GTR if the diver has more than one cylinder.

Personally, I would be perfectly fine with it if the computer only provided GTR on the assumption that the diver will continue to breathe whatever cylinder* they are currently breathing from. This would ignore the possible use of a pony bottle for emergencies. If the diver is carrying deco gases, the computer would ignore them. This actually has a benefit in that if the diver is doing a deco dive, the GTR calculation would be telling the diver how long s/he can stay before losing the ability to deco out on just their back gas. It's a small benefit. Generally, tech divers would simply not use GTR at all when doing a tech dive. But, if GTR were implemented as I described, there would be no reason NOT to have this "benefit" available.


* The only wrinkle I see to any of that is sidemount. But, I think any computer that supports multiple transmitters should simply add a "sidemount" mode to their AI. If used, the computer would assume that 2 transmitters are connected to identical cylinders, filled with the same gas. Details of the implementation on how to configure the computer to know which 2 transmitters are the sidemount transmitters are left up to the computer manufacturer.