ceiling/GF

[KenGordon]

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?

Only in deco speak. Which is why it is confusing. Really my lungs/blood/bones are in equilibrium with p(insp) (ignoring h20, co2 etc). I can go up hills and no gas will come out of solution. If I were genuinely saturated bubbles would form.

Consider a damp sponge on a humid day. It reaches equilibrium by getting damper or less damp. When at equilibrium can you pour water on it and expect it to absorb it? Was it saturated?

 

[doctormike]

Really my lungs/blood/bones are in equilibrium with p(insp) (ignoring h20, co2 etc).

I would avoid talking about whole organs in a Henry's law context, since they are physical collections of a variety of tissues, each with different half times, etc... But you could certainly say that your various tissue compartments are in equilibrium with p(insp), i.e. p(tis)=p(insp). That is, they are saturated. If you are sitting at sea level for any significant length of time, the PPN2 in your various tissue compartments will be 0.79 ATA, same as p(insp) in the atmosphere.

I can go up hills and no gas will come out of solution. If I were genuinely saturated bubbles would form.

Actually, although we are talking about extremely small changes in ambient pressure, gas certainly would come out of your tissue compartments, if you could measure such small changes. If you go up a hill, you change ambient pressure and therefore change PPN2 in your inspired gas, and your tissues will offgas until they reach a new equilibrium.

Whether or not bubbles form may be a question below the limits of clinical detection. The idea of a slow safe ascent is to keep the overpressure gradient below some arbitrary threshold (usually calculated as a percentage of the m-value) where clinical DCS is more likely. However, doppler studies show bubbles in asymptomatic divers who have done normal ascents. So are there tiny bubbles formed when you go up a hill slowly enough to allow for safe offgassing? Who knows, but certainly this isn't a DCS risk if you are not ascending rapidly to altitude with significant inert gas loading (e.g. flying after diving).

Consider a damp sponge on a humid day. It reaches equilibrium by getting damper or less damp. When at equilibrium can you pour water on it and expect it to absorb it? Was it saturated?

.

Different use of the term saturation - the sponge isn't a solution in which gasses dissolve, it's a matrix that hangs on to water droplets.

 

[rjack321]

Only in deco speak. Which is why it is confusing. Really my lungs/blood/bones are in equilibrium with p(insp) (ignoring h20, co2 etc). I can go up hills and no gas will come out of solution. If I were genuinely saturated bubbles would form.

They are saturated - for the pressure you are currently at, like sea level. If you ascend to Katmandu Nepal you will also offgas N2 until you reach a new saturated equilibrium. The pressure difference is not that huge so you are not likely to bubble. If you ascend fast enough and up a big enough hill you will bubble though.This is why astronauts prebreathe 100% o2 before spacewalks, the exterior pressure is low enough and the airlock opens fast enough that they would bubble.

 

[Cichlidgoob]

This was a fantastic thread to read. I recently read Deep Into Deco and a couple of Erik Baker’s papers and I had questions about the pressure graphs that led me to this thread. Maybe I should have started here. Thanks to all who spent valuable time contributing to this thread. It’s also fantastic to see how engaged @Shearwater is with customers and the diving community. Thanks to all of you!

 

[rsingler]

Glad to see from the "like" in the post above that @Shearwater is still tracking this thread.
Any chance that you can resolve our ongoing question about how Shearwater determines/anchors GFLo?

 

[doctormike]

Glad to see from the"like" in the post above that @Shearwater is still tracking this thread.
Any chance that you can resolve our ongoing question about how Shearwater determines/anchors GFLo?

Forget that. I want them to figure out an good adhesive that sticks to Delrin!

 

[Shearwater]

We anchor the GF Low where it would occur with a desktop planner. On a desktop planner a simulated ascent occurs until the leading tissue tension crosses the GF Low line. Then you round back to the nearest deeper stop. This is the first stop depth, and also the GF Low starting point. On a dive computer, after you have anchored GF Low at this point, the GF Line creates a relaxing tension limit as it moves towards GF High at the surface. So this means the next stop (and often the first couple of stops) are “instantly clear”. So the first stop on the dive computer will be shallower than this GF Low anchor point. On a desktop planner, the convention is to enforce a minimum stop time (typically 1 minute) at each stop depth, regardless of whether already clear to the next stop. But on a real-time computer, it doesn’t make sense to time arbitrary stops when the algorithm indicates clear to the next stop. Also, within a single dive, we only allow the GF Low anchor point to move deeper, and never shallower.

 

[scubadada]

@Shearwater

I assume there are no GF low stops for an NDL dive where you will surface below GF hi, is this correct? It seems like your Shearwater computers work this way, as does my Dive Rite Nitek Q. Multideco does not work this way and GF lo will kick in whenever applicable.

 

[Shearwater]

Correct. GF low is only relevant for dives that require mandatory decompression stops.

 

[rsingler]

We anchor the GF Low where it would occur with a desktop planner...
...On a dive computer, after you have anchored GF Low at this point, the GF Line creates a relaxing tension limit as it moves towards GF High at the surface. So this means the next stop (and often the first couple of stops) are “instantly clear”. So the first stop on the dive computer will be shallower than this GF Low anchor point...
...Also, within a single dive, we only allow the GF Low anchor point to move deeper, and never shallower.

So in the case of a diver that ascends to just below the first stop depth, but doesn't trigger the stop, I can't tell what will still be indicated on the screen if the requirement disappears due to offgassing just below the stop depth.

Will the stop requirement remain (with what duration?), but clear instantly once that ceiling is crossed on the way up?

Or will the stop disappear during the hover below stop depth? I presume that the incrementally rising GF determining the next stop (if any) will persist, based upon the anchored GFLo.
I'm guessing that this latter is what happens on the screen, based on your comment that

within a single dive, we only allow the GF Low anchor point to move deeper, and never shallower

Did I get that right?