Deco cleared "on the go"?

[leslietaur]

Hi all, sorry if this sounds obvious but I just can't get my head around as to why the below profiles have the deco cleared before even reaching depth indicated by PDC - I have always thought one will have to reach the range of deco obligation for deco time to count down similar to how a safety stop would work on a PDC? Can someone please shed some light on this, thanks a lot!

Deco obligation started with 1min @ 3m

Deco increased to 17min @ 3m as depth increased

Deco quickly decreased back to 1 min after ascending to 12-13m.

deco cleared closed to 12m somehow?



Other examples I found online (all of which had their deco cleared before reaching deco depth):

 

[tbone1004]

looks normal. Your tissues are ongassing and offgasing constantly based on the tissue pressures each of them is experiencing at that moment as a function of time and pressure. Clearing deco "on-the-go" is ideal really since you are doing something and can shed the deco obligation without the boredom of hanging there. Perks of a dive computer

 

[Graeme Fraser]

Probably due to your slow and gradual ascent enabling the fast tissues to off gas sufficiently to clear the deco obligation. From your GF settings it looks as though your profile allowed you to ascend without GFlo forcing the first stop and to surface with under 75% of maximum super saturation level.

 

[scubadada]

Something is off here, I would suspect the 17 min at 3m. I don't think it would revert back to 1 min with your ascent.

What computer, algorithm, settings, are you using?

 

[doctormike]

Hi all, sorry if this sounds obvious but I just can't get my head around as to why the below profiles have the deco cleared before even reaching depth indicated by PDC - I have always thought one will have to reach the range of deco obligation for deco time to count down similar to how a safety stop would work on a PDC? Can someone please shed some light on this, thanks a lot!

Good question! As others have mentioned, offgassing happens when the partial pressure of inert gas in the tissue (in each compartment) exceeds the partial pressure of inert gas in whatever you are breathing.

The deco obligation bar in those graphs indicated a portion of the dive when, if you ascended directly to the surface without a stop, you would incur more decompression stress than would be considered safe (based on how your computer was set up), and put yourself at risk for DCS. Once you have offgassed to some degree (and you offgas during your ascent even before you get to that first mandatory stop), your tissue inert gas may be reduced to the point when you can now ascend directly to the surface again safely.

Can you tell us, are those profiles from your own computer?

 

[leslietaur]

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?

 

[doctormike]

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?

Great, glad to hear that you aren’t routinely going into deco without training! It’s also great that you took your inadvertent deco dive and made it into a learning experience…

Think of those bars on the profile as a ceiling, rather than a plan. The dive computer is constantly calculating your nitrogen loading and, based on your current depth and breathing mix, letting you know what will happen on the ascent.

The computer will generate a safe ascent profile for you if you inadvertently going into deco (stops and time), for any one instant in the dive. It will tell you what the ceiling is, which is the absolute shallowest depth you can ascend directly to before a mandatory stop.

There is a complex algorithm calculating your nitrogen loading, and what you do on the way up will cause that.profile to be continually recalculated. For example, if you develop a ceiling, start to ascend, the ceiling clears, but then you spend more time at the new depth where that has happened, the ceiling may reappear.

 

[doctormike]

And since you are interested in learning about this (tech-curious?), the closer you get to the ceiling, the more "efficient" your deco is in terms of getting you out of the water. You were wondering about why the profile was so far from the ceiling in those cases. It is correct that while you do decompress below the ceiling, the efficiency of decompression is less.

Your speed of decompression is based on the inert gas (in your case, Nitrogen) gradient between your tissues and the inspired gas. So you can increase that gradient in two ways.

One is to ascend further, and stop just before you get to the ceiling. This will generate the largest safe gradient without putting your overpressure (tissue PPN2 vs ambient pressure) into a zone where bubble formation is more likely. Yes, it's a "bright line through a grey area", but it helps to have some sort of guideline.

The other is to switch to a richer breathing mix. As long as the gas is safe to breathe at your current depth (not over MOD), breathing a gas with less nitrogen will create a greater gradient between your tissues and the gas, and thus more efficient (faster) decompression.

A bit off topic, but still relevant - this is one of the advantages of a rebreather. Generally, open circuit tech divers only carry one or two decompression mixes, but a rebreather will always be generating a mix with the maximum safe O2 for any depth, which means the least possible partial pressure of inert gas and the most efficient gradient for that depth. This means that we can, for example, ascend and stop below our next stop depth if there are operational reasons for doing so (surface current, crowded ascent line, etc..) and still get fairly efficient decompression.

 

[Zef]

With Suunto in particular, the computer initially will give you a mandatory deco stop and typically indicates a deco ceiling of 3m....when you are below 3 meters and at the optimal range for decompresing there will be triangle pointing up with a triangle pointing down below it. If you ascend past 3 meters before your deco oblgation clears the computer will flash and beep warning you and you have 3 minutes to get back below 3 meters before the computer locks out.

If you stay at a depth where your deco obligatin continues to accumulate you will find that the ceiling will shift to a deeper depth and the computer will indicate you are in range at a deeper depth range.

Depending on how much deco obligation you have accumulated will dictate whether or not you immediately start offgassing enough to offset your deco obligation....with a high amount of deco time accumulated, you might find that your obligation continues to grow until you ascend shallow enough that your nitrogen loading is significantly reduced and your offgassing increases....this will most likely happen before you are at the optimal range for decompression.

What computer were you using?

-Z

 

[CptTightPants21]

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?

There are a couple of different factors. Deco will begin to clear as you ascend from your max depth, but it will clear at a slower rate - that 1 min at 10ft might take 2 min to clear after you start your ascent, but before you reach the 10ft stop. Off-gassing occurs when you ascend, but the most efficient off gassing is at the designated stop depth (plus deco gas, but ignore that for now) where the deco time will clear 1 min in 1 min.

That 17 min deco time is wrong. Maybe it is a glitch in the desktop software, idk, but you only went 4m deeper over the course of 4 min. It probably has something to do with the fact that you were just getting barely into deco and your depth changes were messing with its calculations so close to the line. Without a major depth change or having a dive with a major depth profile, deco time accumulates at a steady pace and you can see it tick up 4 min..5min..etc. I am not familiar with the computer, but I doubt your computer ever showed you 17 min stop as it would probably have shown a short 20 ft stop before it showed the 17 min at 10, but that is just my speculation.

Edit: Forgot to add that the deco clears because it is a minor deco obligation on back gas. If you were to do a dive and accrue a 100% O2 deco obligation, it would take an extended time to off gassing on the ascent and would most likely not change your stop time--maybe a minute at most.