On-gassing and off-gassing question
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On gassing refers to the nitrogen gas your tissues accumlate while breathing compressed air during your dive (at depth). Off gassing refers to that gas in your tissues dissipating (leaving your body) during your safety stop/ascent and while on the surface. It's the reason we have to take some amount of time between diving. The amount of time for your surface interval can be calculated using dive tables, but now most computers do this for you.
If you do not allow enough time for off-gassing (missing your safety stop, or not taking a long enough surface interval) the residual nitrogen in your tissues can cause a lot of issues, the most notable is decompression sickness.
Your body always has gas “absorbed” in its tissues. Currently it is in equilibrium with the air you breathe at one atmosphere.
On gassing will happen when you increase the ambient pressure/depth: your body will absorb more gas until it is saturated for that ambient pressure.
Off gassing happens when the ambient pressure is less than the gas stored in your body and your body will release gas until it equalises with the ambient pressure.
On gassing will happen when you increase the ambient pressure/depth: your body will absorb more gas until it is saturated for that ambient pressure.
Off gassing happens when the ambient pressure is less than the gas stored in your body and your body will release gas until it equalises with the ambient pressure.
Is simplest form, your body wants all its tissues in equilibrium with it's environment. Sitting on the beach we are at 1 atmosphere and our body tissues are at equilibrium (saturated) with a gas at that density (that is what our circulatory system is pushing around inside us). As we now go underwater the pressures increase resulting also in the increase of the density of the gas we are breathing. The body, as said before wants to be in equilibrium, so it begins to transition it's condition changing to having the denser gas in our tissues to be in balance with our new environment (on-gassing). Now, when we decrease pressure (come up), the gas we are breathing becomes less dense, and our body begins to shift over to being in balance with the new pressure condition by off-gassing. This is switching the gasses in the fluids and tissues in the body.
We control the rate we "switch back" so the off-gassing is controlled. We do this so our fluid in our tissues doesn't "fizz" like it does when you open a soda bottle where a rapid release of pressure results in rapid release of gas out of the tissues/fluids.
We control the rate we "switch back" so the off-gassing is controlled. We do this so our fluid in our tissues doesn't "fizz" like it does when you open a soda bottle where a rapid release of pressure results in rapid release of gas out of the tissues/fluids.
Belzelbub
Contributor
What do you mean? This is about as on topic as you can get.
It’s also important to note that different tissues on-gas and off-gas at different rates. Some computers available today will show a graphical representation of the level of saturation for many different tissues.
A dive computer is essentially attempting to predict the level of on-gassing and off-gassing that occurs during a dive. If it feels that a threshold has been breached, which would result in too big of a pressure differential at the surface, it will prescribe a stop to allow more time to off-gas before surfacing. But, it should warn you well before.
The Safety Stop, which you should have learned is to slow down the ascent, and allow tissues a bit more time to off-gas.
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He posted in an accidents and incidents thread. I moved his post to Basic.
Rob9876
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All good information above. Meanwhile, I'm struggling to stop my inner 13 year-old self from making an off-gassing joke about Mexican food. Evidently, there are some topics that I will never be mature about. 
I'm guessing that you learned the principles in your open-water course, but your instructor simply didn't use the terms "on-gassing" and "off-gassing" to describe those principles. So, they are simply new terms for things you almost certainly already know.
Pretty much. Learned the concept, just not the lingo.
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