Hi everyone,
I believe the NITROGEN half time for the slowest tissue compartment 16 on the Buhlmann ZH16 algorithm is 635 minutes.
Thus 6 x 635 = 3,810 minutes or 63.5 hours (two and a half days)
Also the equivalent helium half time is 240 minutes.
Thus 6 x 240= 1,440 minutes or 24 hours.
This is obviously why Trimix diving (with helium) gives different deco times.
On gassing and off-gassing are thought to be exponential, like radioactice decay or the cooling of a liquid. The smaller the differential the slower the rate of gas transfer, the lower the rate of radioactive discharge or the change in temperature. This can be expressed mathematically by a differential equation but I am no mathematician and do not know how this is written (I did not study maths at "A" level.)
This is a good practical rule of thumb, but as Dr deco rightly says the body does not consist of 16 tissue types and some ill-perfused tissues will have significantly longer half times (that do not figure in DCI.)
While the slowest compartment 16 will be 98.438% saturated with nitrogen after 63.5 hours of exposure to the new pp N2 it will take much, much longer to be completely 100% saturated. (ie. in stable equilibrium with the external environment). How long does it take for your cup of tea to cool to exactly room temperature?
I understand that it is now believed that the half times for on-gassing and off-gassing differ, or at least an allowance is made in the newer deco programmes to account for a theoretical diffence (possibly due to bubble dynamics.)
If this is the case and the off-gassing half time is 1.2 times the on-gassing half time it will take 24 hours to effectively saturate the body with helium but nearly 29 hours to return to the original steady state prior to the dive.
Is that right madmole/Dr D?