100% O2 is 100% o2 regardless of what depth you are at, the depth is there to keep the bubble size down, but the gradient that the o2 is fighting to push the nitrogen out should theoretically be the same regardless of depth because there is no other gas being used.
Okay, I got an A in college chemistry. I've read Deco for Divers. I knew that. It's been a few months and I had to think it through again to remind myself exactly why that is and I've got it now.
To answer my own earlier question, the difference between 50/60 feet matters where 10/20 doesn't because at 50/60 you will have some amount of N2 in your deco gas. And any N2 in the deco gas means the depth affects the partial pressure of N2 in that gas. A deeper depth means a higher ppN2 in the gas which means deco is slower. But, once you get to 20' and switch to 100% O2, the ppN2 is 0, no matter what depth you're at (0 times anything is still 0), which means N2 off-gasses at the same rate no matter the depth.
So, any idea why these different dive plan tools show a faster deco (shorter run-time) when you ascend to 10' versus hanging at 20' (on 100%)?
---------- Post added January 13th, 2016 at 05:35 PM ----------
So, any idea why these different dive plan tools show a faster deco (shorter run-time) when you ascend to 10' versus hanging at 20' (on 100%)?
I think I have figured it out. And I note now that the MultiDeco plan actually has the deco that includes the 10' stop taking 1 minute longer than with the last stop at 20'.
I think it is because the software only does stops in full 1 minute increments.
So, with a last stop at 20', the program calculates > 1 minute of deco required, but less than 1:40. So, you stop at 20' for 1 minute, then complete the rest of the deco during the 0:40 ascent from 20' to the surface.
But, when you have a last stop setting of 10', the program stops you at 20' for 1 minute, then has you ascent to 10' to complete your deco. You only need 40 seconds (or less, but probably more than 20 seconds), so it wants you to stop at 10' to complete it. It only allows whole minute stops, so you have to stop for 1 whole minute even though you don't need a whole minute to finish deco. And then it shows 1 more minute to get to the surface because, obviously, it takes > 0 time to go from 10' to the surface.
I'm guessing that if the app allowed me and I set it to generate deco stops down to the second, instead of in whole minutes, then the plans would be the same.
---------- Post added January 13th, 2016 at 05:43 PM ----------
I'm slated to receive the H3 I ordered tomorrow. I have a plan in mind to try and test what Victor said, without getting in the water and ruining my ability to return the H3 for a refund.
The latest H3 firmware has a dive planning tool that supposedly allows you to enter multi level dives.
So, my thinking is to plan a dive that results in a 20 minute (more or less) deco stop at 10' on 100%.
If I then add a second level to my dive of 20' for 20 minutes on 100%, the resulting dive plan in the simulator should yield the same run-time. If the H3 works like it should.
If the current firmware still works the way Victor described, then my dive simulation should result in a dive that ascends to 20' for 20 minutes and then stops at 10' for another 20 minutes.
Right?
