ceiling/GF

[KenGordon]

I often find myself arguing with the proponents of GF on SB. Usually this is because they are dismissing Suunto (and other) computers for having a proprietory algorithm. They claim that, somehow, because the is Fortean code for GF it is more predictable and so better in every way. In particular that widely available planning software will give identical plans. Then they will quote a load of sources (the diverite graph being a particular favourite) which are wrong, or at least oversimplified, and go on to demonstrate their own lack of understanding of the small details.

My approach to understanding this was to ask a man who really knows several questions (over a long time) and to write an implementation I could test against commercial ones. There is no where to hide from the numbers, although I do have to go back to the code to check on how stuff works sometimes. Also there are things which a reasonable implementation might choose which are not covered in these discussions.

“Everything should be made as simple as possible, but not simpler.“

People really need to remember that all these algorithms are just models. They are not the real thing, they only try so hard to model stuff and have significant limits.

It is a shame that attempts to improve on these models are met with resistance and distain.

 

[the_ocean]

I am also confused about how GFLo is used on a Shearwater.

My understanding is that the means of anchoring the GF lo is to use the deepest ceiling so far, regardless of the actual ascent. Otherwise what you describe would happen. You can force it in a planner by adding levels just below the stops so as to prevent hitting any stops until nearly at the surface.

The deepest ceiling thing is not nice but better than the alternatives while trying to match a planner.

The interpolation of GF is a nasty thing. Another scheme would have been to use a different slope and initial offset for the compartments. It is a bit hard to support without manned testing through. And he was probably not considering dive computers back then.

Does the Shearwater "anchor" GF lo using the deepest depth, and then interpolate GF between lo and hi based on depth / max depth? If that's the case, then is GF lo what defines the NDL?

According to this post from Shearwater, they say that only GFhi controls the NDL:
Shearwater Perdix AI oddities: GF settings and dive planning
It seems that this is true in a roundabout way. I'm not sure how GF lo fits in with NDLs.

 

[doctormike]

@doctormike, your suggestion to add a line below the ambient pressure line, is illustrated in this post as the "inspired inert pressure" line:
Why plan decompression with a Gf (lo)?

Right. I think that one of the points of confusion is the definition of the term "supersaturation", which seems to be used inconsistently in several published discussions (see above).

When the PPN2 (or partial pressure all inert gases) in a leading tissue compartment is greater than the PPN2 of the breathing gas, you have a gradient, and you have offgassing. But "supersaturation" implies that the PPN2 is greater than ambient pressure (which is relevant to microbubble formation and DCS risk), even though in the space below that, between the ambient line and the inspired inert pressure line, there is offgassing and decompression.

 

[KenGordon]

Saturation is used when equilibrium is meant.

In my book a thing is saturated when it can’t take any more, see Saturation definition and meaning | Collins English Dictionary

So saturation would be the m-line perhaps. ANY supersaturation would mean stuff coming out of solution.

I am not a chemist though.

Who used saturation like this first? Is this a translation error?

 

[KenGordon]

I am also confused about how GFLo is used on a Shearwater.



Does the Shearwater "anchor" GF lo using the deepest depth, and then interpolate GF between lo and hi based on depth / max depth? If that's the case, then is GF lo what defines the NDL?

According to this post from Shearwater, they say that only GFhi controls the NDL:
Shearwater Perdix AI oddities: GF settings and dive planning
It seems that this is true in a roundabout way. I'm not sure how GF lo fits in with NDLs.

Nobody knows, because we can’t read the code. It (almost) certainly doesn’t use the deepest depth, that would be extremely wrong and they claim their plans line up with MultiDeco which properly uses the deepest stop, which is just below the deepest ceiling.

Imagine a 30/70 dive to 60m for 20 minutes. The deepest ceiling is likely to be in the 20s, so GF at 15 will be something like 45. Using max depth it would be 60.

I don’t think that post says they ignore GF lo, just that it works out to never matter for more than an instantaneous stop, which they choose to ignore.

 

[KenGordon]

Saturation is used when equilibrium is meant.

In my book a thing is saturated when it can’t take any more, see Saturation definition and meaning | Collins English Dictionary

So saturation would be the m-line perhaps. ANY supersaturation would mean stuff coming out of solution.

I am not a chemist though.

Who used saturation like this first? Is this a translation error?

Btw, stuff like this is why we have maths.

 

[doctormike]

Btw, stuff like this is why we have maths.

Right, but it's also why definitions of words (which are ambiguous symbols for abstract concepts) are so important. Any two scientists must agree that 1+1=2. But they may have different meanings for the same terms.

In the course of reading other sources for this thread, I have seen the term "supersaturation" referring to the space above the ambient pressure line, where bubble formation begins and DCS risk goes up - p(tis)>p(ambient) - and also to the space above the inspired inert gas pressure line, where an inert gas gradient exists, and offgassing begins - p(tis)>p(insp). I think that the first case is a misuse of the term. A tissue compartment is saturated when it is in Henry's law equilibrium (p(tis)=p(insp)). However that is less important in determining DCS risk than the relationship between p(tis) and ambient pressure.

So saturation would be the m-line perhaps.

I'm pretty sure that the M-line is far beyond saturation.

 

[Jay]

According to this post from Shearwater, they say that only GFhi controls the NDL.

FYI this is correct for NDL dives.

 

[KenGordon]

Right, but it's also why definitions of words (which are ambiguous symbols for abstract concepts) are so important. Any two scientists must agree that 1+1=2. But they may have different meanings for the same terms.

In the course of reading other sources for this thread, I have seen the term "supersaturation" referring to the space above the ambient pressure line, where bubble formation begins and DCS risk goes up - p(tis)>p(ambient) - and also to the space above the inspired inert gas pressure line, where an inert gas gradient exists, and offgassing begins - p(tis)>p(insp). I think that the first case is a misuse of the term. A tissue compartment is saturated when it is in Henry's law equilibrium (p(tis)=p(insp)). However that is less important in determining DCS risk than the relationship between p(tis) and ambient pressure.



I'm pretty sure that the M-line is far beyond saturation.

I am saying that the word “saturation” is misused when talking about deco. You say it above, the special deco speak meaning for the term “saturated” -

A tissue compartment is saturated when it is in Henry's law equilibrium (p(tis)=p(Insp)).

By way of an example, are my lungs currently saturated with He? With N2? No, both are in equilibrium. Neither is saturated, but both match p(‘tis)=p(insp).

 

[doctormike]

By way of an example, are my lungs currently saturated with He? With N2? No, both are in equilibrium. Neither is saturated, but both match p(‘tis)=p(insp).

Tissue compartments do not correspond with any whole organs, they are a theoretical concept to group together tissue that have similar half times for ongassing and offgassing. And lungs would be a particularly confusing organ to use as an example. Do you mean the alveolar space, the vascular space or the supporting soft tissue? The lung certainly aren't a single tissue compartment. It also makes no sense to talk about saturation with respect to gas spaces, since Henry's law refers to a gas-liquid interface.

But for any perfused soft tissue (including the part of the lungs that aren't gas spaces or intravascular), if p(tis)=p(insp) then you have a Henry's law equilibrium and that would be the conventional description of saturation, where there is no gradient for that given gas. Assuming that your p(insp) is 0.79 ATA for N2 and essentially 0 ATA for He right now, I would think that your perfused tissues would have the same partial pressures of those two gases, right? p(tis)=p(insp). So why wouldn't you consider them saturated? Saturated just means that no ongassing or offgassing is happening?