Deco cleared "on the go"?

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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.
 
I would recommend to get Subsurface (it's free!) and play a bit with the planner. It will show you the various ceiling and gas loadings and you can adjust your profile (including moving waypoints with the mouse) and it will show how that influences your ceiling.
 
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.

Correct me if I'm wrong, but I thought that GF Lo doesn't control at what point in the dive a ceiling appears, but the depth of the first stop. GF Hi would control when and if a ceiling appears since it controls the maximum amount of decompression stress allowed at any point in the dive (usually on surfacing). Since you are extrapolating what would happen if you ascended directly to the surface at every point in the dive, GF Hi will let you know when your tissue loading has gotten to the point that direct surfacing will result in GF>GF Hi.

Here are some examples from subsurface for 10/80, 30/80 and 50/80 - the ceiling appears at about 17 minutes in each profile:

10_80.png
30_80.png
50_80.png
 
Correct me if I'm wrong, but I thought that GF Lo doesn't control at what point in the dive a ceiling appears, but the depth of the first stop. GF Hi would control when and if a ceiling appears since it controls the maximum amount of decompression stress allowed at any point in the dive (usually on surfacing). Since you are extrapolating what would happen if you ascended directly to the surface at every point in the dive, GF Hi will let you know when your tissue loading has gotten to the point that direct surfacing will result in GF>GF Hi.

Here are some examples from subsurface for 10/80, 30/80 and 50/80 - the ceiling appears at about 17 minutes in each profile:

View attachment 519288 View attachment 519289 View attachment 519290

GF Lo determines when deco would start. A GF 30/70 starts deco assuming you are ascending at that moment and keeps the leading tissue compartment at 30% of M, then on a line between 30 and 70% of M as the dive continues.

Not sure I'm wording it correctly but a lower number would force a deeper stop because the percent of M allowed on the first stop is lower.
 
The rule is that you have a stop if you hit 30%. That is it, GF high doesn’t matter until you have hit the first stop. However that is not a great rule for a computer as it gives unrealistic NDL times as usually the stop is gone or would only be instantaneous.

Instead some computers (and Subsurface whose deco code was based on the OSTC code) use the deepest ceiling, that is quite shallow so depending on how you choose the GF interpolation betweenness lo and hi it might vanish to just GF hi.

I am suggesting that perhaps Garmin do this in a different way with different results.
 
The rule is that you have a stop if you hit 30%. That is it, GF high doesn’t matter until you have hit the first stop. However that is not a great rule for a computer as it gives unrealistic NDL times as usually the stop is gone or would only be instantaneous.

Instead some computers (and Subsurface whose deco code was based on the OSTC code) use the deepest ceiling, that is quite shallow so depending on how you choose the GF interpolation betweenness lo and hi it might vanish to just GF hi.

This is pretty much the same discussion as we had a few weeks ago about which gradient factor determines NDL. There, the upshot was that in the case of a barely existing ceiling, GFhigh and GFlow are almost degenerate and in the end it turns out that GFhigh is more relevant to determine the time at which a ceiling appears while GFlow determines how steep the increase in ceiling with time is. As I said, try it in Subsurface: Change GFhigh and the time of the first ceiling changes, change GFlow and the slope changes (to almost vertical at sufficiently small numbers).

BTW, the deco code of Subsurface initially used the OSTC code to start with but over time there is essentially nothing left and everything is rewritten (to better handle gradient factors, to allow for speed optimisation from caching, to include VPM-B and to include the plan variations and recreational mode).
 
GF Lo determines when deco would start. A GF 30/70 starts deco assuming you are ascending at that moment and keeps the leading tissue compartment at 30% of M, then on a line between 30 and 70% of M as the dive continues.

Not sure I'm wording it correctly but a lower number would force a deeper stop because the percent of M allowed on the first stop is lower.
That's how it works for a dive with a deco obligation. For a no stop dive (<NDL), you may ascend directly to the surface and not exceed the GF high. The GF low does not kick in until you exceed NDL and go into deco. This was extensively discussed in a previous thread On a NDL dive, which computers' NDLs are not affected by GFLo?
 
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GF Lo determines when deco would start. A GF 30/70 starts deco assuming you are ascending at that moment and keeps the leading tissue compartment at 30% of M, then on a line between 30 and 70% of M as the dive continues.

Not sure I'm wording it correctly but a lower number would force a deeper stop because the percent of M allowed on the first stop is lower.

If by "when" you mean at what depth, then yes, it determines the depth of your first stop, by specifying a maximum percentage of the distance between the ambient pressure line and the M value line for the leading compartment. But given a square profile, it doesn't determine at what time in the dive you get a ceiling, that is determined by GF Hi.

GF Hi matters because it specifies the point in the dive when you have enough N2 loading in your leading tissue compartment so that if you were to ascend directly to the surface, you would be greater than GFhi% of the distance between the ambient pressure line and the M-value line, so you are past NDLs and a stop would be generated.

Shearwater fleshes this out with their new SurfGF reading, which is what the GF would be if you surfaced directly at that point in the dive. So you could watch that move slowly upwards as you add bottom time and N2 loading, and when SurfGF > GFHi, a stop will be generated. The GF99 reading tells you what your current GF is. If you ascend and GF99 > GFlo, a stop will be necessary.
 
Here are my NDL times for a variety of GF values. These are calculated as per a planner and I expect that if you try the next whole minute up in Multideco you will find a very short stop.

They are done by trying all the bottom times in 0.1 minute intervals until a ceiling exists.Any ceiling counts, it does not allow for the ascent time. I'd need to spend more than two minutes hacking to do that.

You can see that nobody would buy a computer set this way.

30/70:
NDL at 12m = 15.80 minutes inc descent 16.50
NDL at 18m = 6.00 minutes inc descent 7.00
NDL at 24m = 3.60 minutes inc descent 4.90
NDL at 30m = 2.40 minutes inc descent 4.00
NDL at 36m = 1.60 minutes inc descent 3.50
NDL at 42m = 1.10 minutes inc descent 3.30
NDL at 48m = 0.60 minutes inc descent 3.10
NDL at 54m = 0.20 minutes inc descent 3.00
70/70:
NDL at 12m = 84.10 minutes inc descent 84.80
NDL at 18m = 28.30 minutes inc descent 29.30
NDL at 24m = 13.50 minutes inc descent 14.80
NDL at 30m = 7.60 minutes inc descent 9.20
NDL at 36m = 5.20 minutes inc descent 7.10
NDL at 42m = 3.80 minutes inc descent 6.00
NDL at 48m = 2.80 minutes inc descent 5.30
NDL at 54m = 2.10 minutes inc descent 4.90
NDL at 60m = 1.50 minutes inc descent 4.60
50/85:
NDL at 12m = 45.00 minutes inc descent 45.70
NDL at 18m = 14.30 minutes inc descent 15.30
NDL at 24m = 7.00 minutes inc descent 8.30
NDL at 30m = 4.60 minutes inc descent 6.20
NDL at 36m = 3.20 minutes inc descent 5.10
NDL at 42m = 2.30 minutes inc descent 4.50
NDL at 48m = 1.60 minutes inc descent 4.10
NDL at 54m = 1.10 minutes inc descent 3.90
NDL at 60m = 0.70 minutes inc descent 3.80
100/100:
NDL at 12m = 177.10 minutes inc descent 177.80
NDL at 18m = 58.50 minutes inc descent 59.50
NDL at 24m = 28.00 minutes inc descent 29.30
NDL at 30m = 15.60 minutes inc descent 17.20
NDL at 36m = 10.00 minutes inc descent 11.90
NDL at 42m = 6.90 minutes inc descent 9.10
NDL at 48m = 5.10 minutes inc descent 7.60
NDL at 54m = 3.90 minutes inc descent 6.70
NDL at 60m = 3.10 minutes inc descent 6.20
 
Ok, I was lying about the two minutes hacking - but I have not checked these :) but this is the internet so that is to be expected. (ok I lied about that too, some of the 85/85 numbers are similar to the ones my Perdix gives for 50/85)

30/70:
NDL at 12m = 17.70 minutes inc descent 18.40
NDL at 18m = 6.70 minutes inc descent 7.70
NDL at 24m = 3.50 minutes inc descent 4.80
NDL at 30m = 1.80 minutes inc descent 3.40
NDL at 36m = 0.60 minutes inc descent 2.50
70/70:
NDL at 12m = 85.80 minutes inc descent 86.50
NDL at 18m = 31.50 minutes inc descent 32.50
NDL at 24m = 15.70 minutes inc descent 17.00
NDL at 30m = 9.50 minutes inc descent 11.10
NDL at 36m = 6.40 minutes inc descent 8.30
NDL at 42m = 4.30 minutes inc descent 6.50
NDL at 48m = 2.70 minutes inc descent 5.20
NDL at 54m = 1.40 minutes inc descent 4.20
NDL at 60m = 0.40 minutes inc descent 3.50
40/85:
NDL at 12m = 30.80 minutes inc descent 31.50
NDL at 18m = 11.20 minutes inc descent 12.20
NDL at 24m = 5.60 minutes inc descent 6.90
NDL at 30m = 3.20 minutes inc descent 4.80
NDL at 36m = 1.70 minutes inc descent 3.60
NDL at 42m = 0.60 minutes inc descent 2.80
85/85:
NDL at 12m = 127.50 minutes inc descent 128.20
NDL at 18m = 45.20 minutes inc descent 46.20
NDL at 24m = 22.80 minutes inc descent 24.10
NDL at 30m = 13.70 minutes inc descent 15.30
NDL at 36m = 9.10 minutes inc descent 11.00
NDL at 42m = 6.30 minutes inc descent 8.50
NDL at 48m = 4.40 minutes inc descent 6.90
NDL at 54m = 2.90 minutes inc descent 5.70
NDL at 60m = 1.70 minutes inc descent 4.80
100/100:
NDL at 12m = 181.80 minutes inc descent 182.50
NDL at 18m = 61.00 minutes inc descent 62.00
NDL at 24m = 31.30 minutes inc descent 32.60
NDL at 30m = 18.90 minutes inc descent 20.50
NDL at 36m = 12.60 minutes inc descent 14.50
NDL at 42m = 8.80 minutes inc descent 11.00
NDL at 48m = 6.30 minutes inc descent 8.80
NDL at 54m = 4.50 minutes inc descent 7.30
NDL at 60m = 3.10 minutes inc descent 6.20
 
https://www.shearwater.com/products/swift/

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