Decompression vs Off-gassing

[DaleC]

As stated before, you neither compress nor decompress-the human body, for the most part, is not compressible. Gas in your body, most notably nitrogen, is what is compressed and decompressed into and out of tissue. Because, in diving, we are specifically talking about gas compressing and decompressing we can also use the terms on gassing and off gassing as these also refer to excess gas being stored or released into and from tissue.

 

[dfx]

As stated before, you neither compress nor decompress-the human body, for the most part, is not compressible.

That's not really true however. Liquids and solids can be compressed and thus can also be decompressed. Of course the change in volume and density will be minscule even under extremely high pressures, but they still compress.

Or you can simply think of "compression" as "increase in pressure" even if the volume/density changes are minimal.

 

[dumpsterDiver]

What would the difference be?... de-saturating to surface pressure.... off-gassing to surface pressure

In either case:

1. The diver is not completely free of excess (1ata) nitrogen upon leaving the water - only sufficiently so to prevent risk of DCS. Saturated nitrogen continues to leave the body in line with tissue compartment half-times during surface interval/post-diving.

2. You are still 'saturated' after a dive ends... just not super-saturated.

3. De-saturating focuses on gas coming out of saturation within body liquids. Off-gassing focuses on gas leaving the body entirely. (?)

I don't think you understand the term super saturation. It is used to represent any differential in partial pressures between say a liquid and it's surroundings. The comments in #1 are correct, but what you say in # 2 is NOT correct.

Before a dive we are all saturated. After, a dive and upon reaching the surface we are all super- saturated. The degree of supersaturation is represented by our repetative group number on the tables.

Our goal is to hit the surface in a supersaturated condition, but to ensure that the degree of supersaturation in each type of tissue does not exceed a deleterious amount (as you say in number #1).

I think that the understanding of what supersaturation means is really critical to the understanding for even the most basic scuba diver. The concept of supersaturation is the cornerstone of understanding diving theory in my mind. It is exactly how I begin to explain it to someone.. So I have a hard time understanding why you would make comment number 2?

As for the OP's instructor: maybe he was trying to emphasisize that rec divers do not do DECOMPRESSION dives so they don't "decompress"... but recreational divers do "offgass" on ascent, but they are involved in "no-deco" diving... I find the distinction trivial.

 

[DaleC]

That's not really true however. Liquids and solids can be compressed and thus can also be decompressed. Of course the change in volume and density will be minscule even under extremely high pressures, but they still compress.

Or you can simply think of "compression" as "increase in pressure" even if the volume/density changes are minimal.

No, in diving it is true.

One needs to keep things in the practical working sense, as they are being applied to the subject, and not devolve into theoretical abstract semantics or one will become horribly confused.

For example compression can evoke a completely different discussion in regards to spinal fractures. On gassing way mean something else when fueling a jetliner. Try using the same theories when comparing fluid hydraulic braking systems and diving decompression theory (though both may use the same words).

In diving decompression we are specifically talking about the effects of gas on tissue so the terms and the conceptual constructs apply.

KISS.

 

[dfx]

In diving decompression we are specifically talking about the effects of gas on tissue so the terms and the conceptual constructs apply.

That's exactly why it's important to realize that your whole body compresses and decompresses during a dive. Not only the gases do, everything does. You start taking up more N2 not because the N2 is compressed, but because your body fluids and tissues are compressed, which allows them to dissolve more gases. Once the N2 is dissolved, there's no N2 gas any more - it's in solution. The N2 comes out of solution because your body gets decompressed too fast and not because the gas in your blood and tissues gets decompressed too fast (because there is no gas there).

 

[PfcAJ]

Off-gassing is what your body is doing during the decompression process (while on ascent to the surface) and while on the surface until your dissolved gases are at equilibrium with the current atmosphere.

PS: Recompression is what you do if you botch the above process DAN loves to help with that!

 

[agilis]

PfcAJ is exactly correct. Clear and concise.

 

[DaleC]

Yes, AJ is correct in what he is saying (as a statement) but some of us have been trying to address the OP's question.

"In a class recently, while discussing the reasons why a technical diver might use a nitrox mixture higher than 40%, the teacher told us higher mixtures are used while ascending to decrease the deco time. I said, "So you switch to the higher mixture to decompress", and the teacher corrected me by saying, "To off-gas". Is there a difference?"

In this case, there is no practical difference between the two.

That's exactly why it's important to realize that your whole body compresses and decompresses during a dive. Not only the gases do, everything does. You start taking up more N2 not because the N2 is compressed, but because your body fluids and tissues are compressed, which allows them to dissolve more gases. Once the N2 is dissolved, there's no N2 gas any more - it's in solution. The N2 comes out of solution because your body gets decompressed too fast and not because the gas in your blood and tissues gets decompressed too fast (because there is no gas there).

I don't understand this at all?

 

[PfcAJ]

You're already decompressing when you switch to your deco gas, the deco gas just speeds things along since there's less inert gas in it compared to what you were previously breathing. Its the relative lack of inert gas that makes it happen.

In an ideal world, we could switch to oxygen immediately (since it has no inert gas at all), but it is dangerous to breath o2 below about 20ft. Other deco gases are used to fill in the gaps between the bottom and the surface. For instance, I recently did a dive with 15% oxygen on the bottom, and switched to 35%, 50%, then finally 100% oxygen throughout the ascent. Deco gas use is determined by how much reserve gas you need (going from say 200ft to 20ft would require a ridiculous amount of reserve gas), how much time each additional gas shaves off from your ascent time, and the bulk and added complexity of adding an additional tank (or 2, or 3).

Its a bit complicated and requires some serious training to be undertaken safely.

 

[dfx]

I don't understand this at all?

Take a look at your morning coffee. You dump a spoonful of sugar in it, stir, and the sugar seems to disappear. Coffee is a liquid, sugar is a solid, but what happens is that the sugar goes into solution: it ceases to be a solid and becomes part of the liquid.

The same happens with water and air. Leave a puddle of water around and eventually it will disappear. The water goes into solution with the air, it ceases to be a liquid and becomes a gas (water vapour). We call this evaporation.

Ditto with nitrogen and your blood stream. Nitrogen goes into solution, it ceases to be a gas and becomes part of the liquid. In all of those cases, there's certain factors that determine how much of a substance another substance can dissolve. Pressure is one factor, temperature is another. When a substance has dissolved as much of another substance as it can, we call that a saturated solution.

With a decrease in pressure or a decrease in temperature, a saturated solution can't be sustained any more. Whatever is dissolved wants to come out of solution and go back to its original phase. That's where clouds and fog comes from: the dissolved water vapour in the air can't be sustained any more because air pressure dropped (low pressure system moving in) or the temperature dropped. Water comes out of solution, forms water droplets, you get clouds and fog.

The same happens with nitrogen in your blood. It's not a gas that gets decompressed and thus becomes problematic, because there is no gas. The nitrogen is in solution and part of the liquid blood. The blood gets decompressed and that's what causes the nitrogen to want to become gaseous again due to the loss in pressure.

Luckily that doesn't happen right away. Substances come out of solution easily when there's a nucleation point around, something that the substance can attach to. In air, there's lots of tiny dust particles that the water vapour can attach to and that's where it forms fog and clouds. In a carbonated drink (there's CO2 gas dissolved in a liquid) the nucleation points are usually on the surface of the containing can or glass. But not all of the substance comes out of solution right away. A carbonated drink is still sparkly when you drink it. With a lack of suitable nucleation points, the substance may not come out of solution at all. What you get is a super-saturated solution: more of the substance is dissolved than what the solvent would actually be able to hold under the current conditions.

That's what happens upon decompression from a dive in your blood. There's more nitrogen dissolved than your blood would normally be able to hold. It's a super-saturated solution. Fortunately there's no good nucleation points around, so the nitrogen stays in solution. The only points where it can come out of solution is in your lungs: you exhale it. That's off-gassing. However, if you decompress too quickly, the amount of super-saturation becomes too great and nitrogen starts coming out of solution without proper nucleation points. You've got DCS.