Deco cleared "on the go"?

[KenGordon]

And turning back to the instant ascent version it is shorter than the physical dive computer so I think I can say the Perdix does what people claim, assume a normal ascent and explicitly ignore the short lived ceiling or GF lo.

How does the Garmin planner compare?

85/85:
NDL at 12m = 122.90 minutes inc descent 123.60
NDL at 18m = 42.20 minutes inc descent 43.20
NDL at 24m = 19.60 minutes inc descent 20.90
NDL at 30m = 11.30 minutes inc descent 12.90
NDL at 36m = 7.20 minutes inc descent 9.10
NDL at 42m = 5.20 minutes inc descent 7.40
NDL at 48m = 3.90 minutes inc descent 6.40
NDL at 54m = 3.00 minutes inc descent 5.80
NDL at 60m = 2.30 minutes inc descent 5.40

 

[leslietaur]

Hey all, thank you so so much for all your input you guys are amazing and I think I do now understand the theory behind it, it's very interesting to know!

Regarding the deco dive I did - agree the numbers indicated seems very odd, here's some background to the dive:

Computer used: Garmin Descent MK1
Software used: Macdive
Algorithm used: Bühlmann ZHL16C 35/75 (Very sorry I misread previously, it should be 35/75 and it's one of the default GFs on Garmin)
Gas: standard single tank with air

- I got curious about tech / deco so I looked back at my previous dives and this was the only deco dive I've ever had so was trying to look into it and learn from there. I was very new (still am...) and could not recall the time the computer showed during the deco back then.
- Macdive indeed shows 17min. Deco started with 1min @ 3m; 5min later deco jumped to 17min; another 2 min later deco cleared with 31min NDL (note that "deco schedule" on Macdive still shows 1min @ 3m as the red box indicated). I have messaged Macdive and their response is that all related figures come from the computer itself.
- Have also imported the dive into Subsurface, essentially the same but subsurface doesn't have a dynamic time during the deco, please see the below screen cap.

Would it be possible that it's some other calculations other than the actual deco time?
By the way, is it usual to have quite a bit of NDL immediately after the deco is cleared?

Thanks again!

@KenGordon

I have compared threeof the GF you've listed. Kindly see below, Garmin NDL in brackets: (no descent times are included in Garmin's planner) Unsure why the very small change between the two on Garmin between 40/85 and 85/85 tho...

40/85:
NDL at 12m = 30.80 minutes inc descent 31.50 (122:48)
NDL at 18m = 11.20 minutes inc descent 12.20 (42:54)
NDL at 24m = 5.60 minutes inc descent 6.90 (20:05)
NDL at 30m = 3.20 minutes inc descent 4.80 (12:06)
NDL at 36m = 1.70 minutes inc descent 3.60 (8:10)
NDL at 42m = 0.60 minutes inc descent 2.80 (6:16)

85/85
NDL at 12m = 122.90 minutes inc descent 123.60 (122:54)
NDL at 18m = 42.20 minutes inc descent 43.20 (42:56)
NDL at 24m = 19.60 minutes inc descent 20.90 (20:05)
NDL at 30m = 11.30 minutes inc descent 12.90 (12:06)
NDL at 36m = 7.20 minutes inc descent 9.10 (8:10)
NDL at 42m = 5.20 minutes inc descent 7.40 (6:16)
NDL at 48m = 3.90 minutes inc descent 6.40 (5:07)
NDL at 54m = 3.00 minutes inc descent 5.80 (4:2)
NDL at 60m = 2.30 minutes inc descent 5.40 (3:46)

100/100:
NDL at 12m = 177.10 minutes inc descent 177.80 (177:51)
NDL at 18m = 58.50 minutes inc descent 59.50 (58:56)
NDL at 24m = 28.00 minutes inc descent 29.30 (28:26)
NDL at 30m = 15.60 minutes inc descent 17.20 (16:18)
NDL at 36m = 10.00 minutes inc descent 11.90 (10:57)
NDL at 42m = 6.90 minutes inc descent 9.10 (7:59)
NDL at 48m = 5.10 minutes inc descent 7.60 (6:22)
NDL at 54m = 3.90 minutes inc descent 6.70 (5:19)
NDL at 60m = 3.10 minutes inc descent 6.20 (4:34)

 

[ChuckP]

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. - I'm not so sure about that, I understand GF Lo determines start of the ceiling, GF Hi determines length of the ceiling - GF Hi is what you will be at when you clear deco and can surface. When your dive computer tells you at some moment in time that NDL is zero and your ceiling is xx' for x minutes - it is making the assumption you will ascend to that depth at some rate (I assume 33'/min), it is also saying that the leading tissue compartment ambient pressure will be GF Lo% of M when you get there. You certainly don't have to ascend then but staying at depth or gradually going shallower will change that initial stop point and total length of deco or conversely, you could ascend and thru off gassing, you could easily clear deco before you get there.

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. - Again, GF Hi is the number you chose to surface with, when deco is cleared, your body will be = or < the GF Hi number. Yes, SurfGF shows you this - during deco, your surfGF will be higher than GF Hi, it will be equal when you clear and it'll be lower than GF Hi if you hang out watching the fish.

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.

This is all based on Shearwater computers - see this video
GF99 still confuses me.........

 

[atdotde]

- Macdive indeed shows 17min. Deco started with 1min @ 3m; 5min later deco jumped to 17min; another 2 min later deco cleared with 31min NDL (note that "deco schedule" on Macdive still shows 1min @ 3m as the red box indicated). I have messaged Macdive and their response is that all related figures come from the computer itself.
- Have also imported the dive into Subsurface, essentially the same but subsurface doesn't have a dynamic time during the deco, please see the below screen cap.

Would it be possible that it's some other calculations other than the actual deco time?
By the way, is it usual to have quite a bit of NDL immediately after the deco is cleared?

I am not really sure I fully understand what you are trying to say. Subsurface has a lot of time dependent info in the little box in the top left corner (if it's in the way you can move it around) including all the ceilings, the time to surface (TTS) and potential stops as well as the surface GF (maybe not yet in the version you are using but the development version has that already) and also the Shearwater tissue plot described in the video above (the red yellow green thingy).

And to understand the influence of the deco parameters, I would recommend using the planner (command-L) rather than looking at a logged dive: There you have all the controls that you want to dial. And final hint: Toggle the third icon from the top to get a smooth ceiling rather than the one with 3m/10ft steps.

 

[taimen]

Macdive indeed shows 17min. Deco started with 1min @ 3m; 5min later deco jumped to 17min; another 2 min later deco cleared with 31min NDL (note that "deco schedule" on Macdive still shows 1min @ 3m as the red box indicated). I have messaged Macdive and their response is that all related figures come from the computer itself.
- Have also imported the dive into Subsurface, essentially the same but subsurface doesn't have a dynamic time during the deco, please see the below screen cap.

Subsurface has also an option to display the deco ceiling calculated and displayed by the computer during the dive. Not sure if it works with Garmin (I'd expect it does), with Shearwaters it works perfectly. With this option you can compare the ceilings computed by your dive computer and subsurface. With Shearwater they match.
Dive computer ceiling will be displayed in red and the Subsurface calculated in green. The TTS calculation in the info box may have to be turned on from icons on the left.
One more hint, you can "edit dive in dive planner" if you want to compare different deco parameters for your real dive profile. A meaningful way to do this would be delete all the data points of your ascent and let subsurface planner calculate new ascent profiles.

 

[doctormike]

GF99 still confuses me.........

Decompression stress (i.e. the risk of developing DCS) is proportional to how far partial pressure of inert gas (call this PPN2 for simplicity) in your leading tissue compartment is above ambient pressure. Offgassing, on the other hand, is determined by how far your tissue PPN2 is from your inspired PPN2 - the inert gas gradient. You can instantly and dramatically increase your inert gas gradient in all of your compartments (and increase offgassing) by switching to 100% O2 (or any richer mix above MOD), but this doesn't increase your DCS risk because the relevant baseline for that is ambient pressure.

Gradient factor is a way of assigning a number to that stress - it's the distance between the ambient pressure line and the M-value line. The M-value line is the amount of supersaturation that is (somewhat arbitrarily) considered to be high risk for DCS. The closer you are to the M-value line (or the further over it), the greater your risk. So these decompression algorithms use gradient factors to let you adjust how close you want to get to that line on your first stop and on surfacing.

GF99 is your current GF, SurfGF is what your GF would be if you surfaced directly with no stop. So if you watched SurfGF on a recreational dive, it would rise as your N2 loading progressed, and when it reached your GF Hi, you would have reached NDL=0. If you stay at depth longer than that, your SurfGF will go above GF Hi, and you can no longer ascend directly without an unacceptable risk of DCS, which means that a ceiling will be generated. The depth of that ceiling will be determined by GF Lo.

GF99 and SurfGF are useful since they give you an idea of your current GF loading, in case you need to trade higher DCS risk for getting out the water sooner for operational reasons.

 

[leslietaur]

I am not really sure I fully understand what you are trying to say. Subsurface has a lot of time dependent info in the little box in the top left corner (if it's in the way you can move it around) including all the ceilings, the time to surface (TTS) and potential stops as well as the surface GF (maybe not yet in the version you are using but the development version has that already) and also the Shearwater tissue plot described in the video above (the red yellow green thingy).

Subsurface has also an option to display the deco ceiling calculated and displayed by the computer during the dive. Not sure if it works with Garmin (I'd expect it does), with Shearwaters it works perfectly. With this option you can compare the ceilings computed by your dive computer and subsurface. With Shearwater they match.
Dive computer ceiling will be displayed in red and the Subsurface calculated in green. The TTS calculation in the info box may have to be turned on from icons on the left.
One more hint, you can "edit dive in dive planner" if you want to compare different deco parameters for your real dive profile. A meaningful way to do this would be delete all the data points of your ascent and let subsurface planner calculate new ascent profiles.

Sorry I didn't make this clearer. Understand subsurface has a lot of time dependent info too including the "deco schedule" IE 1min @ 3m. However, the deco time shown on Macdive is very different. Please see the below graphs from Subsurface and Macdive respectively - both showing info at the same time. Both has the deco schedule of 1min @ 3m but Macdive has 17min (started from 1min; 5min later deco jumped to 17min; another 2 min later deco cleared with 31min NDL). The developer of Macdive advised that this was extracted directly from the computer. Doubt this is "true" deco but no idea what it is.... (To make it clearer I have attached again the start and end of the deco time frame from Macdive) And I am not so sure about the reasoning behind the high NDL when the deco is cleared?

Deco Start

Deco End

 

[jgttrey]

I cannot begin to explain that goofy result from MacDive or whatever it is.

For one thing, look at the second image. It purports to say you have a 17 minute stop at 3M -- But your "TTS" or "time to surface" is 2 minutes. That makes zero sense. Usually TTS is calculated based on your programmed ascent rate (usually 10M/min) and the software takes into account how long it will take to get to the surface, including any required stops, if you use that rate. If you had a real 17 minute stop obligation, your TTS would be more like 18 minutes.

Your "Deco Start" image (#3) is a good example of this. You are at 19M. Yes, it shows deco of 1 min at 3M, but a TTS of 2 minutes. From 19M if you ascend at 10M/min, it will take 2 minutes to get to the surface even without a stop. Thus, even though the computer is showing a 1 minute stop at 3M, if the TTS is accurate, it is simply showing you that what is shown as a 1 minute stop at 3M will clear as you get close to it, or only last a few seconds. Nothing unusual about that.

That's why the 17 minute thing makes no sense -- you're at the same 19M and your TTS is shown as 2 minutes. There's no way in hell you'd clear a REAL 17 minute stop obligation in the couple of minutes you'd be ascending to 3M. Something is funky about that software.

As to your question about why you get a large NDL pop back as soon as deco clears, yes, that's normal. As you are seeing, decompression is a complicated topic. It is tempting, but wrong, to think of a continuum where you're NDL slowly drops to 0, then you have deco, and as you clear it, the NDL starts counting back up from 1. It doesn't work that way. It is much more complicated. That misconception does not take into account the fact you are changing depth or the fact that you are actually tracking multiple tissue compartments that are ongassing or ongassing at different rates and sometimes some are doing one while others are doing the other, etc.

In this case, you're clearing deco because you have shallowed up and you are offgassing. Your deco "clears" when you have offgassed sufficiently that you can surface safely (given whatever level of conservatism/surfacing gradient you have programmed). If you're a 3M when that happens, your NDL is unlimited because you could stay at 3M for a month and, even if fully saturated, go to the surface without DCS because the pressure gradient is insufficient to cause a problem. Thus, when you "clear deco" at your last stop and "get rid" of enough of that N2 you accumulated earlier and deeper, your NDL will be large. On a Shearwater, once you clear deco on the last stop, your NDL jumps to 99, which is the maximum.

 

[taimen]

Sorry I didn't make this clearer. Understand subsurface has a lot of time dependent info too including the "deco schedule" IE 1min @ 3m. However, the deco time shown on Macdive is very different.

Yes, you made it very clear. We were just trying to give tips for using Subsurface for solving the mystery.
None of us can explain the 17 minute ceiling/stop for that profile as it doesn't seem to be a result for any known pure decompression algoritm. Certainly not Buhlmann 30/75.
Now the first question is where did this number actually come from? Did you see it during the dive on your Garmin?

The developer of Macdive advised that this was extracted directly from the computer.

Apparently it is produced by the Garmin and not calculated in Macdive.
In this case you should be able to display this 17 min stop also in Subsurface by using "Toggle DC reported ceiling" icon next to graph.
Now you should be able to see the "computer reported ceiling" and Subsurface calculated ceiling overlayed. If you can display the 17 min stop (in red) overlayed with shorter ceiling (in green) in Subsurface, I think we have at least narrowed it is originally form Garmin and not Macdive/Subsurface.
Next, try to edit dive in planner and try to fiddle with deco algoritm setting if you can recreate the 17 minute ceiling. I doubt you can, at least with anything resembling GF 30/57, but it will be a great learning experience.

Doubt this is "true" deco but no idea what it is....

Agreed. Could it be result of an adaptive safety stop added to pad the "real deco" deco obligation?

 

[dmaziuk]

Could it be result of an adaptive safety stop added to pad the "real deco" deco obligation?

Well if the result of your "safety stop" is to add 500% to your deco obligation, I'd say your developers, coders, QA, and your legal department, have all failed the sanity test, and your company should've been class-actioned into oblivion long ago.

That said, where is the "deco? All I see is a ceiling. You need to have an actual stop when you get there, to be in deco. (And even then at 30/75 you've a bit to go before you hit that dark box on the RDP that is still "not deco" but "optional means mandatory here" .)