Why do people add a few minutes to their last deco stop?

[crofrog]

I have read them. They don't say even close to what you say. Maybe you need to re-reed them if you have forgotten.

In short, they don't say "we know nothing." Instead they say "we only know this much, and these are practical implementations of what we do know."

ok so how much safer is 80/80 over 70/70?

 

[L13]

Yes. It was measure of VGE, but the time chosen for the stop was arbitrary as best I can tell, and both of them showed a reduction in VGE, but is 5 minutes better than 3, is 10 better than 5? is 20ft better than 15? Those parameters where not tested.

Ahhh, so true. People might have started getting DCS and dropping dead again at 10 min. Since they didn't test it we will never have a clue. /s

You know that interpolating data between test points and extrapolating beyond test points is legitimate, as long as you stay consistent with know mechanisms and don't extrapolate to far? right?

There are an infinite number of possible test cases, it would take infinite time to test them all. But we don't need to, just design a reasonable sample across the space and you still have real knowledge.

 

[L13]

ok so how much safer is 80/80 over 70/70?

Actually 70/70 is safer than 80/80. Do you have a cite, or any other evidence, even anecdotal, that suggests it isn't safer?

How much safer is 100/100 over 8080? If you can't find a number, are you going to dive 100/100 and save the deco time and gas bill?

 

[inquis]

@L13 - I cannot find any source that uses your definition of solubility. Here are the ones I checked (in addition to Powell's book), and they all agree as to the dependence on partial pressure rather than total pressure:

• Young, in The Solubility of Gases in Liquids, available from nist.gov: A generally used formulation of Henry's Law may be expressed as P (g) = KH * x(g) where KH is the Henry's Law constant and x(g) the mole fraction solubility. P(g) was earlier stated to be the "partial pressure of gas".
• Petrucci, General Chemistry Principles and Modern Applications (Solubility of Gases): For many gaseous solutes, the relation between solubility, Cg, and partial pressure, Pg, is a proportional one, Cg=kPg, where k is a proportionality constant that depends on the identities of the gaseous solute and solvent, and on the solution temperature.
• McKibbin, in Transport Phenomena in Porous Media (Henry's law): Non-condensible gas solubility in the liquid phase may be expressed in terms of Henry's law, which gives a relationship between the partial pressure of a component in the gas phase and its molar fraction in the liquid, see Perry et al. [13]
• The layman's reference seems to mirror that of the peer-reviewed papers and textbooks: Wikipedia (Solubility): The solubility of a gas in a solvent is directly proportional to the partial pressure of that gas above the solvent.

Would you kindly point me to a resource that discusses solubility within a total pressure context?

 

[harvalen]

Again, the part I bolded is completely wrong. A simple rebuttal: for someone at sea level, nitrogen saturation is 0.79 atm precisely because the partial pressure of nitrogen in the inspired air is 0.79 atm. If you breath EAN50 long enough while sitting on the couch, your tissues will be saturated at 0.5 atm. All this without changing the total ambient pressure.

Assuming this definition. If we take a diver with a 6m/20ft ceiling. If this diver switches to oxygen and stays at 6m then the increase in supersaturation would both be quicker and larger than if the diver swam to the surface as quickly as possible breathing air. I'm pretty sure the risk of DCS would be much lower when switching to oxygen and staying at depth, so that definition of saturation and supersaturation seems to have very little to do with DCS.

 

[inquis]

that definition of saturation and supersaturation seems to have very little to do with DCS

The discussion where supersaturation depends on partial pressure was about "solubility" and "saturation". Inert gas can form bubbles even when inert tissue pressure is below ambient pressure -- fortunately, they don't last very long, as @L13 described previously in the thread. I agree that DCS is more concerned about the ratio of inert tissue pressure to total ambient pressure.

 

[inquis]

I agree that DCS is more concerned about the ratio of inert tissue pressure to total ambient pressure.

I will add that the meaning of "supersaturation" is usually clear from context, whether that's above inspired partial pressure or above total ambient pressure. Since the pedantic flag was thrown where words have to have a single/specific meaning, I would side with the inspired definition if there is to be only one -- the DCS context came later, after all. However, like the DCS researchers, I'll continue to use both meanings.

 

[L13]

I will add that the meaning of "supersaturation" is usually clear from context, whether that's above inspired partial pressure or above total ambient pressure. Since the pedantic flag was thrown where words have to have a single/specific meaning, I would side with the inspired definition if there is to be only one -- the DCS context came later, after all. However, like the DCS researchers, I'll continue to use both meanings.

I must admit error here. I threw the pedantic flag, and I was wrong.

Several months ago I was reading several papers on gas bubble formation in liquids, and it's similarities to solids precipitating out of solutions as atoms initial aggregate in solution. In at least one of those papers, they used the words around saturation as I have above, which closely parallels the use of those terms in dealing with dissolved solids and precipitation. That is non-standard usage, but was convenient for thinking about that phenomenon.

While in solution, the difference chemically speaking between a gas solution, a liquid solution, and solid solution is negligible. All three are just atoms in suspension between atoms of the solvent. The difference only occurs as the atoms start to aggregate. For liquids and solids, solubility is defined as I defined it above in this thread. For gases, the normal convention is as everyone else was defining it in this thread.

But:
While my language was wrong. My point to @Germie that DCS does not occur when the tissue partial pressure of inert gas is below total ambient pressure, while still having enough delta between the tissue partial pressure of inert gas and the inspired partial pressure of gas to result in not insignificant off-gassing. While not practical, it would not take days. For example, a dive to 20m(65ft) for 20 min would have a first stop at 12m and a total deco time of 67 min with ZERO risk of DCS, even though the peak SurfGF was 65%.

 

[dylanfromwinnipeg]

While my language was wrong. My point to @Germie that DCS does not occur when the tissue partial pressure of inert gas is below total ambient pressure, while still having enough delta between the tissue partial pressure of inert gas and the inspired partial pressure of gas to result in not insignificant off-gassing. While not practical, it would not take days. For example, a dive to 20m(65ft) for 20 min would have a first stop at 12m and a total deco time of 67 min with ZERO risk of DCS, even though the peak SurfGF was 65%.

Is this effectively a GF 0/0 profile?

 

[L13]

Is this effectively a GF 0/0 profile?

yes. The example I gave was essentially the same as ascending to ~GF99 = 0 at 12m and then riding the GF99 = 0 curve to the surface. (I did not include discrete stops in the equation, they make it somewhat longer). Though I did not derive it that way.