deco myths

[Angelo Farina]

Unless they agree with you.

From the video: "All dives we compress and we decompress, therefore we can view every dive as a decompression dive."

From a search: "Every scuba dive involves the compression and decompression of nitrogen in a diver's body, and can be viewed as a decompression dive."

It's not rocket science, it's not saying anything that isn't true, and it's not wrong.

What IS wrong is the inference (from the video) that you can do a one hour surface interval if you do a safety stop but you'd need to do a 1.5-2 hour surface interval following the same exact dive if you include a safety stop. What sort of nonsense is this?

I entirely agree with Mark Powell. Every dive is a deco dive.
And planning (end equipping, training, etc.) for deco stops is SAFER than planning for staying "just within" NDL.
I find strange that @tursiops thinks the opposite. An NDL dive is reasonably safe only if we stay very far from the NDL limit: as the diving profile approaches (rides) the NDL, the risk increases significantly, if the diver is not trained, equipped and planned for deco stops: any minor inconvenience can cause exceeding the NDL, and the diver finds himself in a truly dangerous "emergency deco" case.
Consider instead the case of a perfectly trained, equipped and planned deco diver, which conducts a dive just BEYOND NDL. If the dive becomes a bit longer than planned, nothing dramatic happens, just few more minutes of already-planned stops.
So the second dive with planned deco is generally safer than the first dive, with no planned deco, but significant risk of requiring some unplanned deco stops.
In my opinion, non-deco dives should always be conducted with a huge safety margin, let say always 10 minutes away from NDL. And this not considering the "safety factor" obtained with various algorithm (GF, etc.).
Teaching recreational divers to "ride the NDL", and perhaps "tuning the GF" so that the planned dive fits "just exactly" in the NDL, is truly much more dangerous than teaching them to plan and perform deco stops, as we teach in CMAS and BSAC recreational courses.

 

[BlueTrin]

I understand there are limitations in recreational diving that tech divers are not subject to including what you posted and of course the ability to dive deeper reefs and wrecks. I'm just saying to me, the additional time spent on a hang, and the time, expense and additional gear is simply not worth the trouble.

Some people may have different interests, but surely you already knew that

 

[Lorenzoid]

Along the lines of what @Angelo Farina said, I would sum it up by saying that if a diver anticipates diving to the edge of so-called no-deco limits, the diver should get trained in deco theory and procedures and otherwise gain a thorough understanding of what is going on. The diver is then in a better position to decide how to do those dives in a way that satisfies their personal risk tolerance. And in a better position to use whatever terminology the diver wishes to describe that dive. It's not the terminology that's important!

 

[admikar]

In my opinion, non-deco dives should always be conducted with a huge safety margin, let say always 10 minutes away from NDL. And this not considering the "safety factor" obtained with various algorithm (GF, etc.).

And now we again go back to what is safer: 10 minutes away from true NDL of GF 100/100 or 5 minutes away from NDL at GF 45/70?
AFAIK, NDL tables already have safety factor included (and I am guessing a lot of the dive computers).
How much safety is too much?

 

[Angelo Farina]

And now we again go back to what is safer: 10 minutes away from true NDL of GF 100/100 or 5 minutes away from NDL at GF 45/70?
AFAIK, NDL tables already have safety factor included (and I am guessing a lot of the dive computers).
How much safety is too much?

For me the second: you setup your computer with GF values you find suitable for your body. 100/100 are values proper for a 25 y.o. Navy Seal.
At my age and health condition I find appropriate some additional conservatorism, so using your proposed 45/70 is appropriate.
At thay point the computer "knows" the best estimate of my real personal NDL limit.
If I want to dive with no-deco equipment and planning, I ascend at least 10 minutes before such personal NDL limit.
If I want to dive longer, I plan and equip for a deco dive, with additional deco tanks, two independent regs on the main tank, etc...
Of course I can do this as my recreational certification includes deco procedures and a max depth of 50m.
People without such training and equipment and planning capability should simply avoid to come close to their personal NDL.

 

[scubadada]

With SurfGF, many of us have the ability to surface with the GF of our choice. Though my GF high is set higher, I have not surfaced with a GF greater than 80 since I bought my Teric a little more than 4 years ago.

 

[L13]

And now we again go back to what is safer: 10 minutes away from true NDL of GF 100/100 or 5 minutes away from NDL at GF 45/70?
AFAIK, NDL tables already have safety factor included (and I am guessing a lot of the dive computers).
How much safety is too much?

You are using the wrong metric. Time from NDL is not the appropriate measure for safety. SurfGF is much more useful, It is the actual inert gas loading you will have if you surface right away.

On an 37m dive at 100/100, 10 min from NDL is SurfGF = 7%. At 45/75, 5 min from NDL is SurfGF = 35%. 10 min form NDL on 100/100 wins. (actual surfacing GF's will be 14% and 35% due to the fact that you will pick up more N2 during the ascent, so not as big a win as it looks at first)

On an 20m dive at 100/100, 10 min from NDL is SurfGF =90%. At 45/75, 5 min from NDL is SurfGF = 70%. 5 min form NDL on 45/75 wins. (actual surfacing GF's will be 87% and 64% due to off gassing during the ascent)

(all examples done with SubSurface)

 

[Angelo Farina]

You are using the wrong metric. Time from NDL is not the appropriate measure for safety. SurfGF is much more useful, It is the actual inert gas loading you will have if you surface right away.

On an 37m dive at 100/100, 10 min from NDL is SurfGF = 7%. At 45/75, 5 min from NDL is SurfGF = 35%. 10 min form NDL on 100/100 wins. (actual surfacing GF's will be 14% and 35% due to the fact that you will pick up more N2 during the ascent, so not as big a win as it looks at first)

On an 20m dive at 100/100, 10 min from NDL is SurfGF =90%. At 45/75, 5 min from NDL is SurfGF = 70%. 5 min form NDL on 45/75 wins. (actual surfacing GF's will be 87% and 64% due to off gassing during the ascent)

(all examples done with SubSurface)

Possibly I did not explain eell enough the reason for which I suggested a safety boundary of 10 minutes fron NDL.
First of all, for me setting values of GF smaller than 100/100 means tuning my computer on my body.
Once tuned, I must trust it and not worry anymore.
If I did set GF high to 70%, if I am at 70% at surface it means that I am "on the edge of my personal NDL", not that I have a 30% safety margin..
I have NO safety margin at that point...
10 minutes is for me a reasonable safety buffer for inconveniences that can cause the need of delaying my ascent.
This number is perhaps related to the distance travelled horizontally, to depth, to the presence of obstacles and currents. When many of these factors occurs, then perhaps I need a 15-minutes safety margin.
But it has nothing to do with the amount of nitrogen in my body: that is already correctly bounded by my personal NDL, which I thrust my tuned computer for being evaluated properly.
So that is the NDL, and for being safe of not needing deco stops I must plan to end the dive at least 10 minutes before, so, even if some delaying event occurs, I will respect it.

 

[L13]

Possibly I did not explain well enough the reason for which I suggested a safety boundary of 10 minutes from NDL.

I was responding to @admikar . I understood what you were saying. I mostly wanted to point out that "X minutes" is not the same physiological safety margin for different dives , and with different GF High's, there could be counter intuitive safety implications.

 

[Angelo Farina]

I was responding to @admikar . I understood what you were saying. I mostly wanted to point out that "X minutes" is not the same physiological safety margin for different dives , and with different GF High's, there could be counter intuitive safety implications.

On this I entirely agree.
But, as said, I suggested 10 minutes as a time buffer accounting from some adverse circumstances causing a delayed ascent.
It is part of dive planning, it is not "deco conservatorism", which has akready been addressed with a proper choice of GF values (or, depending on the computer, with other conservatorism factors).
One should not confuse dive planning with deco conservatorism, so I thank you for making the distinction clear.
As said, I was not clear enough in my previous post, making @admikar confusing the two things.