No stop limit vs. No stop time remaining

[Outbound]

Do you use GTR instead of using remaining no stop time or do you use both?

Your remaining NDL will always show. But even if you could turn it off you wouldn’t want to. It’s critical information that you ALWAYS need to keep an eye on.

 

[tursiops]

Do you use GTR instead of using remaining no stop time or do you use both?

Whichever is smaller.

 

[Outbound]

Whichever is smaller.

Ok, I think I misunderstood what was being asked.

 

[Belzelbub]

Do you use GTR instead of using remaining no stop time or do you use both?

You should use both. Whichever is shortest is what controls the dive time. My current computer (Shearwater Perdix AI) displays both, if I choose. GTR can be displayed if chosen, NDL is required.

My previous dive computer had a different measure called DTR (Dive Time Remaining). This would use both NDL and GTR and display whichever is shortest. While I prefer having both NDL and GTR displayed separately, the way DTR was calculated is exactly how GTR and NDL should be used.

 

[EFX]

One other thing about how they are used. The "no-stop limit" is an estimate of the maximum amount of time you can dive to a given depth, assuming a square profile. That is, assuming you descend directly to your bottom depth, hang out down there for X number of minutes, and then ascend at a certain rate, possibly with a safety stop built into the definition.

No stop limit (NSL) and no decompression limit (NDL) are the same. I prefer NSL as that term conveys the idea more accurately. In the calculation of NDL there are no assumptions, estimates or guesses. The calculation is based on the amount of inert gas the CTC (controlling tissue compartment) contains at that moment in the dive and compares that value to the %GFHi of the m-value at that depth. As the depth and dive time change the NDL is updated for every iteration of the dive program.

One consequence of the fact that NDL is based on the CTC is that as you go shallower the NDL increases in a non-linear fashion due to the CTC shifting to a slower half-time compartment. The slower HT compartment has not absorbed as much inert gas as the previous faster compartment. That is why NDL increases so rapidly the shallower you get.

 

[EFX]

One consequence of the fact that NDL is based on the CTC is that as you go shallower the NDL increases in a non-linear fashion due to the CTC shifting to a slower half-time compartment. The slower HT compartment has not absorbed as much inert gas as the previous faster compartment. That is why NDL increases so rapidly the shallower you get.

During your OW/AOW course your instructor (should have) taught you to check your gas remaining and NDL time remaining frequently especially as you go deeper. A very low NDL is not a concern for panic. For example let's say you get distracted and check your gas and NDL. Gas remaining is good but your NDL is now 2 minutes. Do NOT panic! Immediately, get your buddy's attention and alert them you need to go shallower now. Ascend at a safe rate of 30 ft (10 m) / min to a shallower depth. Then, recheck your SPG and NDL. You'll be surprised on how much NDL you have at the new depth.

To get some appreciation for this I entered a dive to 90 ft on air for 14 minutes at medium conservatism (GFHi of 85%), a descent rate of 60 ft/min, and an ascent rate of 30 ft/min in salt water. The NDL time was 2 minutes. Ascending immediately to 65 ft, and after 1 minute gave an NDL of 8 minutes. Eight minutes is not a lot of time but it is enough to decide what to do for the rest of the dive, assuming gas is not an issue. Ascending to 50 ft after 1 minute gave a new NDL of 32 minutes. Ascending just 10 ft shallower to 40 ft, more than doubled the NDL to 70 minutes.

 

[dmaziuk]

... In the calculation of NDL there are no assumptions, estimates or guesses...

You do realize, of course, that the calculated NDL is based on the assumption that the diver remains at their current depth -- exactly as you explain in the subsequent paragraph.

 

[EFX]

Yes. However, you must realize that you are applying a human trait to a computer program that cannot do any thinking. The dive program simply processes real-time data based on sensor input, stored dive profile parameters and past data. An assumption implies that there is a possibility of another course of action that the computer cannot possibly know.

From a human perspective it appears that the computer is making some sort of assumption and therefore is basing some calculation on that but, there are no calculations based on assumptions -- just calculations based on hard data in real-time. See Bret Hatch's comment in post #15 for context.

 

[dmaziuk]

Yes. However, you must realize that you are applying a human trait to a computer program that cannot do any thinking. The dive program simply processes real-time data based on sensor input, stored dive profile parameters and past data. An assumption implies that there is a possibility of another course of action that the computer cannot possibly know.

That's where we differ: "the human trait" here is that what we want to know the future, and the future is unwritten. It will remain an assumption until/unless it comes to pass.
NDL assumes you're going to stay at your current depth and are not getting caught in a downcurrent, GTR assumes no great white shark coming to taste you and change your breathing rate by doing so. DTR is based on both (IRL you can get them into a race condition if they're sufficiently close). Your car's remaining range (if it's new enough to display it) is based on the assumption you're not about to start a race, an airplane's landing distance is based on the assumption the pilot will engage reverse thrust when they're supposed to, and so on.

All of it has a possibility of a different course of action that the computer cannot possibly know, anyone who thinks their computer *knows* if they're getting bent or not, is delusional.

PS there is a more-or-less high probability the future will happen as predicted, but it's never an absolute certainty.

 

[tursiops]

That's where we differ: "the human trait" here is that what we want to know the future, and the future is unwritten. It will remain an assumption until/unless it comes to pass.
NDL assumes you're going to stay at your current depth and are not getting caught in a downcurrent, GTR assumes no great white shark coming to taste you and change your breathing rate by doing so. DTR is based on both (IRL you can get them into a race condition if they're sufficiently close). Your car's remaining range (if it's new enough to display it) is based on the assumption you're not about to start a race, an airplane's landing distance is based on the assumption the pilot will engage reverse thrust when they're supposed to, and so on.

All of it has a possibility of a different course of action that the computer cannot possibly know, anyone who thinks their computer *knows* if they're getting bent or not, is delusional.

PS there is a more-or-less high probability the future will happen as predicted, but it's never an absolute certainty.

What is the point you are trying to make? The sun might nor rise tomorrow, either, but we press with fhat assumption. Do you have a better suggestion?