ceiling/GF

[Shearwater]

Here it is. The chart is natively drawn in Adobe Illustrator. Feel free to edit.

 

[mcpowell]

I think you mean that you begin your first stop when your leading tissue hits GF low, no?

Well now that is interesting. Is your first stop calculated such that your tissues are 30% of the M value when you begin hovering at depth, or do you ascend to your first stop once your tissues are 30%? From the graph I would say the latter.

You ascend from ambient.

Don't you mean you descend from ambient?

 

[rsingler]

Keep in mind that we're talking percentage of M-value pressures OVER ambient. The nitrogen has been packed in by a descent to some ambient pressure higher than the tissue partial pressure.
On a bounce dive you might have a fast tissue compartment that still hasn't theoretically reached saturation at the deepest part of your dive and is briefly less than that deep ambient before you started back up. On early dives in a repetitive series, you will certainly have slow compartments that remain below ambient until back near the surface. They continue to ONgas until near the surface. But for most dives, you will likely have a leading compartment that is AT ambient when you start up, not counting sawtooth profiles, which are beyond this discussion.
As you start up, the leading tissue partial pressure exceeds ambient, and is some percentage of the M-value OVER your new ambient as you ascend.

So yes, you ascend from the ambient pressure at depth. Ambient pressure implies, "at your current depth". E.g., 1 atm at the surface and 4 atm at 99ft.

Perhaps I should have said you ascend from your deep ambient to some new shallower ambient, and your tissues are offgassing to catch up. Note that the graph shows an "ambient" pressure line all the way from depth to the surface.

 

[doctormike]

Well now that is interesting. Is your first stop calculated such that your tissues are 30% of the M value when you begin hovering at depth, or do you ascend to your first stop once your tissues are 30%? From the graph I would say the latter.

Don't you mean you descend from ambient?

No, ascend. There's ambient pressure at depth as well.

When you are at a constant depth at the deepest portion of your dive, some of your slower compartments may be ongassing, and your faster compartments may be 100% saturated. But they only develop supersaturation (more than 100%) when you ascend. And supersaturation is what allows decompression. The algorithms just give you a reasonable tradeoff between efficient decompression and risk of DCS.

In other words, you only leave that diagonal "ambient pressure" line when you move up in the water column (or switch to a richer gas with less N2). So it doesn't make sense to say that you ascend when you are at 30%, because until you begin your ascent, none of your compartments will ever be above the ambient pressure line. And in most cases, the leading compartment will be saturated (i.e. at ambient pressure), so it will be at a GF of 0%.

 

[KenGordon]

One day we are going to get this right. Here's attempt #3:

Delving Deeper into Deep Stops - Divernet

I suggest you take inspiration from figure 4

 

[mcpowell]

@Shearwater, Thank you for making the file available. Unfortunately my adobe illustrator is one generation of computers removed. In other words, I’d have to crank up a defunct computer that is just sitting on my shelf.

@rsingler, thanks for the clarification. I understand now. I was thinking of ambient as being on the surface, but this makes much more sense.

@doctormike, Okay, your post made me stare at the ceiling for awhile. I’m a bit fuzzy in that I assumed the M value line was independent of the ambient pressure line. For example, if I was slowly descending for my entire dive, wouldn’t I eventually reach a point where my GF low was 30% of the M value, at which point I would need to stop descending and begin the decompression process?

This is a very interesting thread for me. Thank you for taking the time for these explanations.

 

[lermontov]

@Shearwater, Thank you for making the file available. Unfortunately my adobe illustrator is one generation of computers removed. In other words, I’d have to crank up a defunct computer that is just sitting on my shelf.
.

try option click on the pic #15 you should be able to save as to the desktop and open with another app

 

[rsingler]

This is a very interesting thread for me.

You WANT this book. A little dated, and before the NEDU study started to make bubble vs. gradient a little clearer, but as a place to start? Unsurpassed!

Deco for Divers: A Diver's Guide to Decompression Theory and Physiology https://www.amazon.com/dp/1905492294/ref=cm_sw_r_cp_apa_rPSSBbRJ48AV3

 

[doctormike]

@doctormike, Okay, your post made me stare at the ceiling for awhile. I’m a bit fuzzy in that I assumed the M value line was independent of the ambient pressure line. For example, if I was slowly descending for my entire dive, wouldn’t I eventually reach a point where my GF low was 30% of the M value, at which point I would need to stop descending and begin the decompression process?

No, if you continually descend, that number will never be greater than zero.

That number refers to supersaturation. Supersaturation means that the tissue compartment has more dissolved gas in it than is present in the ambient environment. That's why offgassing (decompression) is only possible on ascent.

Let's try an example. You descend to the bottom at 100 feet (4 atmospheres) and sit there, breathing air. The air that you breathe has a partial pressure of N2 in it of 4 x 0.79, or 3.16 ATA. Assuming that you descend like a lawn dart and hit the bottom instantly, your tissues will initiallly have the same PPN2 as they had on the surface, 0.79 ATA. Since this represents is a big pressure gradient (3.16 -> 0.79), your compartments will start to fill up with N2 as the nitrogen moves from the area of greater partial pressure (inhaled gas) to lower partial pressure (your tissues). The faster ones fill more quickly than the slower ones. But they will never be more than 100% until you ascend. Once they reach 3.16 ATA they will be saturated (i.e. same as ambient), and no more tissue loading will happen.

Lets just look at the fastest compartment to make the explanation simple. If you were a hard hat saturation diver on the bottom for a day, eventually all of your compartments would be saturated, but just look at one for now. So once you have spent some time at depth, your tissue (fastest compartment) has a PPN2 of 3.16 ATA, just like the gas you are breathing. It is fully saturated. No matter how long you stay on the bottom, it won't take on any more N2 than that. If you want to plot yourself on that graph, you are on the ambient pressure line, and you are 0% of the distance between the ambient pressure line and the M-value line and that won't change.

Now you start to ascend. You rise to 66 feet (3 ATA). Your tissue PPN2 is still 3.16, but your ambient PPN2 in your breathing gas is 3 x 0.79, or 2.37 ATA. So your tissues are holding MORE nitrogen than is in the ambient gas, they are MORE than saturated, so we call that being SUPERsaturated. And once you have supersaturation, you start to have offgassing (decompression). And if you are running GF of 30/80, then that means that you make your first stop when your supersaturation gets to 30% of the distance between the ambient pressure line and the M-value line.

 

[mcpowell]

You WANT this book. A little dated, and before the NEDU study started to make bubble vs. gradient a little clearer, but as a place to start? Unsurpassed!

Deco for Divers: A Diver's Guide to Decompression Theory and Physiology https://www.amazon.com/dp/1905492294/ref=cm_sw_r_cp_apa_rPSSBbRJ48AV3

I had already downloaded the sample a few nights ago, and am halfway thru it. I’m sure I’ll hit the ‘buy’ button once I get to that point. But for $39, I had to sample first. On another note, my name is also Mark Powell, but there’s no confusing us. He’s a lot more knowledgeable, but it was weird to see people refer to my name the first few times I read it. I was thinking “what the heck?”