Can someone please explain how Shearwater computers do deco?

[johndiver999]

Hey guys! I have now done about 40 dives on my Tern TX and went into deco (?) with it for the first time. Well at least that's what the computer said during the dive. I didn't have to do an extended deco stop though as it cleared up during the ascent. In the beginning it stated 2 minutes at 6 meters though. Can someone please explain how exactly the algorithm works once i enter deco? I also dive with surface GF on my screen so I monitor where I am at anyways, but I somehow expected the computer to just stop me at a certain depth without taking into account any off gassing I did during the ascent. It seems like Shearwater computers are smarter than I originally expected and I can adjust how I plan my dives with the NDL of the computer accordingly.

I can adjust how I plan my dives with the NDL of the computer accordingly. I'm curious, what does this statement mean? How do you think your computer is smarter?

Also, I think the answers you got already are correct, but perhaps are reinforcing a misconception - I think most (maybe all?) computers will do the exact same thing - in other words, if you do an "extra' slow ascent the deco ceiling will dissolve during the ascent and possibly at a depth well below the original deco stop. Assuming the required deco stop is relatively short and your ascent rate is within a certain range; neither too slow or too fast.

 

[Cio]

There is a manual written all about it.

 

[lizardland]

I used to write deco software.

Computers work by sampling the depth at a particular time frequency. The depth and what it believes you are breathing are used to load the 16 theoretical tissues of the model (assuming the SW is using ZHL16, which I think it does). Each tissue will generate a depth that it is safe to ascend to before bubbles would (in theory) start to form in that tissue. Whichever tissue generates the deepest ceiling then that is your deepest deco stop (most computers round to 3m/10ft intervals). The computer calculates how long you will have to spend at that stop in order for all the tissues to have their next ceiling 3m or more shallower. And then it does that for every step of the way and generates a predicted time to surface.

But that is only a prediction, it is not taking into account live data. If you start ascending then the ppN2 and ppHe that each tissue is exposed to starts to drop. This means the tissues get less loading and some may even start to offgas. In the right circumstances, say a 3m or 6m stop with a short required stop and a slow ascent, then the max allowable depth ceilings in every tissue will decrease to 0 i.e. you can surface safely with no deco.

Obviously I have no idea what is going on inside a Shearwater but that is generally how you go about calculating a deco schedule. Or at least how I would.

 

[scubadada]

So @bencaha

I'm sure you have realized that you should learn more about decompression, the Buhlmann algorithm, GF factors, and gas consumption before you pursue additional decompression dives. Since you use a Shearwater computer, you can learn a lot from your Shearwater Cloud download. Here is an example of a light back gas deco dive. There was 1 min of deco at 10 ft at about 38 min. The deco obligation was satisfied at about 43 min during the ascent, well below the safety stop. The end surface GF was 77. This particular dive had a max depth of 100 feet, an avg depth of 71 feet, and 31% nitrox. Did you download the dive you reported to the Shearwater Cloud, we'd like to see it.

Best of luck in your dives.

 

[MB NZ]

Hey guys! I have now done about 40 dives on my Tern TX and went into deco (?) with it for the first time. Well at least that's what the computer said during the dive. I didn't have to do an extended deco stop though as it cleared up during the ascent. In the beginning it stated 2 minutes at 6 meters though. Can someone please explain how exactly the algorithm works once i enter deco? I also dive with surface GF on my screen so I monitor where I am at anyways, but I somehow expected the computer to just stop me at a certain depth without taking into account any off gassing I did during the ascent. It seems like Shearwater computers are smarter than I originally expected and I can adjust how I plan my dives with the NDL of the computer accordingly.

I think this is pretty normal across a range of computer algorithms. Maybe it's because my first dive computer did this that I consider it to be desirable and normal. When I got a computer that didn't clear deco (and actually racked up deco) during the ascent, I didn't like it at all! Note this is personal preference rather than anything more scientific.

Do you not have dive planning software specific to your computer which will allow you to anticipate how a dive is going to go?

 

[bencaha]

I think this is pretty normal across a range of computer algorithms. Maybe it's because my first dive computer did this that I consider it to be desirable and normal. When I got a computer that didn't clear deco (and actually racked up deco) during the ascent, I didn't like it at all! Note this is personal preference rather than anything more scientific.

Do you not have dive planning software specific to your computer which will allow you to anticipate how a dive is going to go?

This is exactly why I asked my question. For some reason I expected the Deco to not clear up during ascent and me having to do the stop to clear the time, which left me confused when I started my ascent as planned and the deco disappeared.

I only use the Shearwater cloud app. Does it have a feature where I can pre-plan and model dives?

 

[bencaha]

So @bencaha

I'm sure you have realized that you should learn more about decompression, the Buhlmann algorithm, GF factors, and gas consumption before you pursue additional decompression dives. Since you use a Shearwater computer, you can learn a lot from your Shearwater Cloud download. Here is an example of a light back gas deco dive. There was 1 min of deco at 10 ft at about 38 min. The deco obligation was satisfied at about 43 min during the ascent, well below the safety stop. The end surface GF was 77. This particular dive had a max depth of 100 feet, an avg depth of 71 feet, and 31% nitrox. Did you download the dive you reported to the Shearwater Cloud, we'd like to see it.

Best of luck in your dives.

View attachment 918811

There you go. This is the dive that had me ask questions about my computer. I had another dive with more extended planned deco today where now, everything I saw on my computer made perfect sense again.

 

[bencaha]

I can adjust how I plan my dives with the NDL of the computer accordingly. I'm curious, what does this statement mean? How do you think your computer is smarter?

Also, I think the answers you got already are correct, but perhaps are reinforcing a misconception - I think most (maybe all?) computers will do the exact same thing - in other words, if you do an "extra' slow ascent the deco ceiling will dissolve during the ascent and possibly at a depth well below the original deco stop. Assuming the required deco stop is relatively short and your ascent rate is within a certain range; neither too slow or too fast.

I was simply confused as I expected the behavior of the software to be: you went over your NDL, now make a mandatory stop at x meters for y minutes. So i expected to stay at 6m for the stated duration and planned accordingly, but by the time I got there, the computer alredy calculated that I had fullfilled all my deco obligations.

 

[scubadada]

...I only use the Shearwater cloud app. Does it have a feature where I can pre-plan and model dives?

Your Tern TX has a deco planner and a NDL planner under dive tools in the menu. They are described on pages 40 and 41 of the user's manual. The deco planner will also give you your gas usage, assuming you know your RMV. Are you using your computer with a transmitter and AI?

 

[bencaha]

Your Tern TX has a deco planner and a NDL planner under dive tools in the menu. They are described on pages 40 and 41 of the user's manual. The deco planner will also give you your gas usage, assuming you know your RMV. Are you using your computer with a transmitter and AI?

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View attachment 918860 View attachment 918862

Thanks! I never messed around in the dive tools menu too much except for the compass. Guess I should deep dive the manual once I have some time on my hands!