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

[L13]

This is not how the buhlmann algorithm is implemented in all modern deco software and dive computers.

Solubility and saturation are fundamental chemical principles not determined by the Buhlmann algorithm.

Assuming you consider supersaturation and off-gassing as synonymous (as I and others do),

Then you are using the term wrong.

then supersaturation (and the rate of off-gassing) is determined by calculating the pressure gradient between partial pressures in breathing gas vs tissues.

You are correct about the rate of off-gassing, but not about supersaturation (since you are using the term unscientifically)

Ambient pressure doesn't come into the equation (it does when calculating M-values, but not when calculating off-gassing/saturation).

Correct with regard to off-gassing, but not saturation.

However, Ambient pressure does come into the equation when determining solubility, supersaturation, and gas bubble formation.

 

[inquis]

Saturation, is when the tissue is holding as much of a gas as it can stably hold at a given pressure. It is independent of the partial pressure of that same gas in the inspired air.

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.

@L13 -- I'd like to hear your thoughts on the above, not about a sealed container.

 

[L13]

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.

Your tissues are not saturated in that condition, they are just at the maximum saturation they can get (equilibrium) in that environment. They are at ~.79 saturation (ignoring water vapor).

Calling it saturation is a colloquial use of the word, not a scientific use of the word.

 

[boulderjohn]

Yes, to an extent. Go look at the primary research on safety stops.

The 15 foot safety stop was based on an actual study from about 40 years ago. I don't have a link, and I am about to hit the road for 5 days. @tursiops can probably supply it.

Regardless of its origin, it is a good depth, since a study of saturation dives showed that those divers saturated with air at that depth are able to ascend safely. That means that a diver doing a safety stop has any unsafe tissues are moving toward a safely level, and any tissues still on-gassing cannot exceed a safe level.

 

[tursiops]

The 15 foot safety stop was based on an actual study from about 40 years ago. I don't have a link, and I am about to hit the road for 5 days. @tursiops can probably supply it.

Regardless of its origin, it is a good depth, since a study of saturation dives showed that those divers saturated with air at that depth are able to ascend safely. That means that a diver doing a safety stop has any unsafe tissues are moving toward a safely level, and any tissues still on-gassing cannot exceed a safe level.

Info - History of the Safety Stop

There have a number of threads recently on ScubaBoard that refer to a Safety Stop, and some assertions as to how it came about and when. Here is what I can find on this topic; it would be nice to flesh it out so myths and speculation can be minimized! And always remember the research has been...

scubaboard.com

 

[L13]

Everyone needs to realize that even the experts often use imprecise language when discussing scientific phenomenon. but that imprecision, combined with their expertise, does not redefine the word. Often when you call them on it the will admit the more precise usage.

I personally am guilty of imprecisely using scientific language. I am not that much of an expert. Often it is close enough to get the job done, and doesn't mater. But when delving into the finer details of a technical discussion the precision become important.

 

[crofrog]

So how would you edit those words to match your actual position.


Try these:

Fraedrich DS, Validation of algorithms used in commercial off-the-shelf dive computers, Diving and Hyperbaric Medicine, V48 No 4 , 2018
Doolette DJ, Gerth WA, Gault KA. Redistribution of decompression stop time from shallow to deep stops increases incidence of decompression sickness in air decompression dives. NEDU TR 11-06, Panama City FL; 2011.
Howie, LE, Weber PW, Hada E, Vann RD, Denoble PJ, The probability and severity of decompression sickness, PLoS ONE 12(3): e0172665,2017
Balestra C. Dive computer use in recreational diving: Insights from the DAN-DSC database. In: Blogg SL, Lang MA, Møllerløken A, editors. Proceedings of validation of dive computers workshop. 2011 Aug 24 Gdansk. Trondheim: Norwegian University of Science and Technology; 2012. p. 99–102.
Van Liew HD, Flynn ET. A simple probabilistic model for standard air dives that is focused on total decompression time. Undersea Hyperb Med.2005;32:199–213.

Do you have a single citation that even suggests that the difference between the two dives above (GF's=80/65 and GF's = 60/50) might not be significant?

You should read those. They're great. I think you'll find that what I'm saying is pretty well supported in them.

 

[L13]

The 15 foot safety stop was based on an actual study from about 40 years ago. I don't have a link, and I am about to hit the road for 5 days. @tursiops can probably supply it.

The whole "give me a cite", without giving a countervailing cite bugs me. You don't ow them doing their research for them. If they had research to back them up, they would provide their cite with their request for your cite.

 

[L13]

You should read those. They're great. I think you'll find that what I'm saying is pretty well supported in them.

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."

 

[crofrog]

The 15 foot safety stop was based on an actual study from about 40 years ago. I don't have a link, and I am about to hit the road for 5 days. @tursiops can probably supply it.

Regardless of its origin, it is a good depth, since a study of saturation dives showed that those divers saturated with air at that depth are able to ascend safely. That means that a diver doing a safety stop has any unsafe tissues are moving toward a safely level, and any tissues still on-gassing cannot exceed a safe level.

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 were not tested.