Decompression vs Off-gassing

[DevonDiver]

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.

Nope.

As DaleC suggested, I'd suspect he was being pedantic about terminology for no good reason, potentially to appear more knowledgeable that he actually was.

There's no practical difference between 'decompression' and 'off-gassing'. Both terms simply refer to the process of removing saturated gas from the body.

 

[rongoodman]

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.

No, every time you do a recreational dive, with no special deco mix, your tissues eventually off-gas all the extra inerts they picked up under pressure, and return to their normal surface saturation(ppN2=.79atm). If you then got into an unpressurized plane and ascended to 10,000', they would begin the process again. If you were quickly transported to the summit of Everest, the process would likely be so fast as to cause injury, if the hypoxia didn't kill you first.

 

[RTee]

Those two terms are only synonymous when off gasing occurs as part of the deco profile. Anytime a diver conducts a well planned multi level dive, he or she will, at some point in time be off gasing in the shallower areas even though he is still well within NDL and has not incurred any deco obligation.

 

[Tigerman]

No, every time you do a recreational dive, with no special deco mix, your tissues eventually off-gas all the extra inerts they picked up under pressure, and return to their normal surface saturation(ppN2=.79atm). If you then got into an unpressurized plane and ascended to 10,000', they would begin the process again. If you were quickly transported to the summit of Everest, the process would likely be so fast as to cause injury, if the hypoxia didn't kill you first.

Not while in water? Even on shallow dives you have a nitrogen loading above "surface standard", which means you dont fully offgass untill some time on the surface?
On a deep dive I will be MORE saturated at the end of my deep part of the dive than after my safety stop, but I will still be saturated above "surface standard", no?

 

[rongoodman]

Rebreather divers run head-long into this phenomenon--while they're trying to keep their closed breathing loop at a certain ppO2 to optimize their deco, they're constantly seeing the ppO2 dip as the loop is flooded with the inert gases leaving their bodies, required flushing the loop and adding more O2. As OC divers, we just blow them off into the water as we exhale and don't have to worry about it.

 

[DevonDiver]

Those two terms are only synonymous when off gasing occurs as part of the deco profile. Anytime a diver conducts a well planned multi level dive, he or she will, at some point in time be off gasing in the shallower areas even though he is still well within NDL and has not incurred any deco obligation.

Is it necessary to differentiate between formal/stage/mandatory decompression stops... and the fact that every diver decompresses upon ascent - through the process of adhering to a maximum ascent speed? After all, a mandatory stop is nothing more than the process of (significantly) reducing overall ascent speed to counter increased nitrogen saturation (from longer/deeper dives).

A recreational diver ascending at 18m per minute (or slower) is decompressing - but no formal stops are needed, due to the saturation of nitrogen being controlled by slower tissue compartments. With greater saturation (depth vs time), slower compartments begin to dictate the rate of desaturation, requiring a much slower...or staged... ascent.

Decompression, as a term, is typically only used in respect of formal decompression (staged mandatory stops)... wheras 'off-gassing' is term that is typically used to describe any decrease of saturated gas from the body, such as (as RTee mentions) on a multi-level dive.

Thus, the difference between the terms is not one of definition/meaning, but rather the common usage of the word/s.

 

[Tigerman]

Yes, but will we ever be completely desaturated to surface level while dreathing anything with a ppN > 0,79ish which is what we normally breathe?
Granted the nitrogen load you get at 15ft is very low, it still IS?

 

[DaleC]

Technically yes, but we need to keep it in the practical sense or the whole discussion devolves into a semantic debate.

While diving at 15 feet we are gradually on gassing or "compressing" gas within our tissues but the time it takes to reach a level where attaining the surface directly will injure us is very very long. More than even an unreasonable recreational dive would be. So we can say this is not of practical consequence

All decompression theory has the basic goal of rendering a diver able to attain the surface without risk of DCS symptoms, not 100% free of N. That is why the safety stop is optional for some depths and times but mandatory for others. Where it is mandatory, or where formal decompression tables begin, the risk of DCS symptoms have a reasonable chance of occurring.

 

[Tigerman]

But thats why I find the term "off-gassing" to be misleading. You still have plenty of nitrogen in your system after a deep dive, even with a safety stop and I would assume even more so after a deco dive?
Wouldnt "de-saturating" be a more precise term?

 

[DevonDiver]

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. (?)