gcbryan:
Dr. Deco,
Is the Rate at which you offgass nitrogen affected by depth? In other words is more nitrogen being offgassed in 15 minutes at 30 fsw than in the first 15 minutes once you reach the surface or is it the same?
I'll take a crack at this question. I'm no expert (to say the least!), and I may be way off-base. No one should hesitate to correct me!
The rates at which various body tissues off-gas is not really known. They are hypothesized by various modelers.
Many models assume off-gassing rates are the same as on-gassing rates, i.e., halftimes. Some other models assume slower off-gassing rates of various theoretical tissues. Pressure gradients (difference between ambient pressure and the pressure of gases in solution within tissues [gas tension]) are not assumed to affect these rates. So, in other words, the rates of off-gassing are assumed to be the same at various depths.
When the gas tension within a tissue is the same as ambient pressure the tissue is saturated. Off-gassing occurs when the tissue is supersaturated, i.e., the gas tension in the tissue is higher than the ambient pressure. When a tissue is supersaturated gas will either migrate to other tissues in which the level of saturation is lower and so stay in solution, or come out of solution and become free-phase gas (bubbles). Gas migrating to the bloodstream and then out through the lungs is off-gassing properly.
The body can tolerate a certain number and size of bubbles, and therefor certain levels of supersaturation of various tissues. By controlling the degree of supersaturation we can control the number and size of bubbles. These are described by Haldane's ratios, Yount's and Wienke's critical radii, etc. And, the degree of supersaturation a tissue can tolerate is thought to vary by depth for a given tissue (Workman's and Buhlmann's M-values).
The objective of decompression strategies are to control the supersaturation of the tissues with the highest tensions (thereby limiting number and size of bubbles) while at the same time minimizing the continued on-gassing of tissues whose tensions are still below saturation.
So, back to the premise of your question, although the rate that gas comes out of solution of a given tissue may be the same at the surface as at 30 feet, the degree of supersaturation must be taken into consideration.