Decompression vs Off-gassing

[FPDocMatt]

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?

To me, the term "off-gas" makes more sense, since it states exactly what you're doing. The term "decompress" is a bit confusing, since you're not becoming less compressed when you stay at 50 feet for 10 minutes.

But I thought they referred to the same thing. That is, when you make a stop to off-gas, it's called a decompression stop.

Or am I missing something?

 

[waterpirate]

Decompression is what it is called, off gassing is what is happening. By increaseing the 02 you simply are " speeding up" the off gassing proccess.

Decompression is a black art. It is not an exact science. There are tools available add nauseam to help divers manage the risk, but at the end of the day we are all takeing part in our own field trial of those mathematical tools.

Hope that helps
Eric

 

[DevonDiver]

Formal definitions will probably vary, however IMHO:

Off-gas - specifically focuses on the removal of saturated gases from the body.

Decompress - is a broader term, covering the removal of saturated gases from the body in conjunction with pressure differentials on ascent.



With regards to rich nitrox for technical decompression, just Google 'Oxygen Window'.

 

[Tigerman]

I know decompression theory and compartments and all that is complex THEORY, but dont you actually have to use o2-rich/nitrogen lean gases to off-gass? Id think that as long as your ppN is above what we breathe on the surface you cant actually off-gas as you keep introducing more nitrogen to your body than what youre "supposed" to or is the ppN for continues aturation higher than the surface ppN?
While decompression you can do on any (breathable) mix?

 

[FPDocMatt]

I know decompression theory and compartments and all that is complex THEORY, but dont you actually have to use o2-rich/nitrogen lean gases to off-gas?

Aha! So the instructor was using "off-gassing" as a way of describing what a technical diver does when switching to a higher oxygen percentage on ascent, beyond simple decompression.

 

[rongoodman]

As long as the ppN2(ppHe) dissolved in the tissues is higher than that in the alveoli and circulation, the inert gases will diffuse out of the tissues into the circulation, and then to the lungs. You get that effect automatically just by decreasing your depth. Reducing the inerts in the inspired deco gas, easiest to do by replacing them with O2, just speeds along the process, thus "accelerated decompression".

 

[DaleC]

When you decompress it's not your body that's decompressing - it's the gas (most notably N) which has been compressed by increasing water pressure and stored in your tissues. As you ascend those gasses "decompress" and come out of solution to become a gas again.

Technically, you need to breath out those gasses to "off gas", like vapor from a jug of gasoline, but that is a fine point. Decompressing and off gassing in the practical sense are the same thing.

You do not need a rich mix to off gas or decompress; you just need to move upwards in the water column. The decrease in water pressure will allow more N to come out of solution. But your blood stream can only carry so much N at one time so not introducing more (by breathing a mix with less N in it) allows more N to be transported by the bloodstream to the lungs and out. It speeds the process up but the process will occur regardless. It's not the increase of O2 in the mix, but the decrease of N, that is important.

So, a diver both decompresses and off gasses no matter what mix they use.

I think your instructor may have been trying to appear "smart" by playing pretty with the terms.

Semantically though, one could say that a diver, holding his breath from say 300' could decompress (explosively) without off gassing - though I'm sure some would escape out his ears, eyes and butt hole.

Ron, we posted the same thing at the same time... Jinx

 

[Tigerman]

I guess what I meant to say was to get it down to surface-levels, dont you have to use rich mixes. I find "off gassing" to have an inherent "get rid of all of it" meaning..
Then again, Ive not studied deco stuff for long, I just tend to go looking for information thats pretty useless to me

 

[DevonDiver]

In respect to terminology:

Decompression is the safe release of saturated gases from the body on ascent. Safety being dictated by the absence of DCS occurrence, due to maintaining a gradual, calculated decrease in ambient pressure on ascent - in line with pre-formulated ascent speed, inc. pauses/stops based on hypothetical (but statistically 'proven') algorithms.

Accelerated Decompression is the same as above, but involves the use of higher % O2 breathing gases to make use of the 'oxygen window', which has the effect of creating more efficient/rapid off-gasing - and thus, shorter decompression times (where formal 'stops' are required).

 

[danvolker]

Also, a "planned decompression" that lasts 2 hours in constantly changing ocean conditions, and where if a diver in the group had a problem, time to get him help is a real issue....is usually less desirable.....Planning would be better aimed at a one hour decompression ....in other words, purposely doing an air only deco when a big obligation exists, would not be good planning, in my way of thinking.