Thank you all for the explanations so am I correct in saying that a deco obligation shown by a computer does not necessarily indicate you have to be at that depth (or depth range) for it to count down and ascending slowly will also contribute towards deco obligation and hence possibility of clearing the stop.
But in this particular instance it’s actually not possible to clear this deco at the given depth 3m as it will be cleared before I reach anywhere near this depth? In other words this deco stop given by the computer basically means “1min if at ceiling 3m, but it will change dependent on the following profile”?
The first profile is my personal dive (sorry for the bad profile I was very new and the only time I exceeded NDL..) and the rest 2 are found online.
I used a Garmin descent mk1, setting was just standard with air/single tank and algorithm is Bühlmann ZHL-16c 30/75. Agree 17min to 1min change looks odd or does this indicate something else?
I am going to guess Garmin are doing something interesting as you cross 10m. The NDL line doesn’t look like any curve you want to be seeing on a graph like these, suddenly moving from 0 to lots and then dropping again.
Since there is a GF lo of 30 you will have a ceiling early in the dive. That could be a considerable ceiling even for a short dive to medium depth. Depending on how they choose to decide and present what happens when that GF changes to 75 your apparent ceiling might move a lot.
Have you got any other similar profiles? Can you load that dive into Subsurface and see what it says?
The GF scheme was designed for planning software. Computer manufacturers have to make judgement calls about how various bits should behave in the ‘live’ environment that a dive computer has to deal with. A particular one is how to manage the interpolation between GF lo (your 30% of true Bühlmann ZHL16C) and GF hi (your 75%). If you were to see a ceiling genuinely based on that GF lo (eg for a 30/30 setting) you’d always be doing deco dives and the NDL police would be after you. That is not good for sales, people would be quick to point out that your brand had an NDL of 6 minutes when theirs (not hurt me in 1300 dive, so must be right) gives 60 minutes. As a result there has to be a fudge.
BTW 30/75 is a poor choice.