kramynot2000:
I sure hope this question doesn't cause too much controversy, but I have always been curious as to why stops in a decompression profile seem to get longer when you get shallower. For instance, you may get something like this (I just made this up):
70 1 50%
60 1
50 2
40 5
30 7
20 10 O2
10 15
It seems to my very, very, very non-scientific mind that it should be sorta inverse with the most time spent at the gas change depths since this is where the gradient is greatest, so more time at 70 and more at 20. So I must be missing something.
Can one of you deco experts provide me with a dummy'd down explanation of what I'm missing?
Thanks
Let's see if I can get this across without making a mess of it...
The deeper part of the ascent is a balancing act. Go too fast and you'll bubble and go too slow and you will either not off-gas or even continue to on-gas. Both bubbling and taking on extra gas will actually increase your deco obligation because bubbles are harder for the body to eliminate than gas in dissolved form and extra gas is.... well.... extra.
I think Your confusion is stemming from the thought that the gradient is greatest at your deep stops. In fact the gradient for offgassing increases as you go shallower or when you change to a breathing gas with a lower percentage of the inert element in it.
So....Stops get longer as you go shallower because (a) you need to either go shallow or change your mix or both in order to keep the gradient increasing so you can keep the offgassing running and (b) you need to avoid further on-gassing, which you can only do shallow.
That's the main line of thinking.
To my way of thinking the deep stops are not there for deco purposes per se. It's more of a mechanism to keep the ascent slow so bubbles don't start to form (or that the ones that do form stay small). The bubble models cook the deep stops right into the recipe so it looks like part of the deco but it's really part of the ascent leading up to the deco..... If you get my drift.
Having said that your instinct that you can extend a deep stop in combination with a gas-switch is correct within limits. The gas-switch gives you a little spike in the gradient that you can use to stay at the stop a little longer. However it's not something you can just do arbitrarily because the gradient spike is short and if you wait too long your offgassing will slow again and you'll end up with extra deco in the shallow zone. On the other hand, a little extension of the 70ft stop after the switch may allow you to actually shave a few min off the deco later on in the curve. To my way of thinking, however, it's not worth trying unless you really know your stuff because of the risk of extra ongassing. It's better to just follow the ascent rate you calculated with and extend a shallow stop instead. It's harder to screw up extending a shallow stop and a little more deco on the shallow stops can't hurt.
Does that make any sense?
R..