I thought if you had a deco stop that was too deep, your ‘slow’ tissues would continue to on-gas while your ‘fast’ ones would off-gas, which increases your decompression obligation (?) how does it work?
While some slow TC's (tissue compartments) on gas, especially during deep deco stops , they may not on gas to the point of being the CTC (controlling tissue compartment). The amount of on gassing is partly controlled by the TC's half-time. Since TC's 14,15, and 16 have such long ht's they usually don't ongas enough to become CTC's. However, it depends on the profile.
Only the CTC will be used to calculate the ceiling and therefore be the determing factor for deco obligations. As the ascent progresses the CTC will gradually shift from faster TC's to slower ones. From TC1 to TC16 here are the half-times in minutes for Buhlmann ZHL-16C:
5, 8, 12.5, 18.5, 27, 38.3, 54.3, 77, 109, 146, 187, 239, 305, 390, 498, 635.
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From the help page of the dive Excel spreadsheet:
The rate that each TC can on or off-gas is controlled in part by its half-time. Half-times are the amount of time in minutes where half of the existing inert gas in the tissues will be added to (if on-gassing), or removed from (if off-gassing) if the amount of supplied gas is held constant. For example, TC2 has a HT of 8 minutes. If at time zero the TC contains the equivalent inert gas equal to 80 fsw then after 8 minutes of off-gassing it will contain 40 fsw, another 8 minutes later 20 fsw, and another HT 10 fsw and so on. It takes about six HT's for a TC to on or off-gas to 98% of its final value. In this example it would take roughly 6 x 8 or 48 minutes for TC2 to off-gas from 80 fsw to 1.25 fsw. The TC's are ordered from the lowest (1) to the highest (16). The lower the HT the faster the TC will on-gas and off-gas. One consequence of this is that some slower TC's may still be on-gassing after ascending to a shallower depth (the fastest TC's have off-gassed). Their HT's are so long that the tissues have not been exposed long enough at depth to pick up enough inert gas to exceed the inspired pressure at the shallower depth to allow off-gassing.
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My spreadsheet will visually show you the progression in CTC's. You can get my spreadsheet using this
link.