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I'm sorry for you you couldn't join us for the inwater part because of your illness. You prooved you were well prepared and showed us some nice teaching. Nice to meet you too and I hope I will soon make some nice dives with you and Flavio at your place. Keep in touch... and keep wet!
Just to make sure I understand your question, is it correct that the dive time in your example is only 20 minutes? This is the only time I can see in your message. In our 1:1 ratio using 25/25 and O2 for deco, a 100'/30m dive is a fairly benign dive - as a matter of fact truly an non-decompression dive because the NDL is 30min. This is what is confusing me a bit about your question.
Let's say that you have a 45min dive at 100'/30m using the same ratio - this now results in 20min of deco. Again you are correct by allocating 10min in the 70'/21m to 30'/9m segment and the remaining 10min in the 20'/6m to the surface segment. Now, the first segment is conducted on backgas and therefore exponential in shape. Again you are correct by stating that the deco starts at 50% of the max. depth, i.e. 50'/15m. Your curve could look something like this:
The O2 segment could be like this:
20'/6m 5min or 20'/6m 7min
up for 5min up for 3min
Now the only thong left is the deep stops starting at 75% of max depth. You are only 15min over your NDL, therefore the times are ones. So the final profile could look like this:
up for 3min
Now, i understand that this does not answer your question entirely. Even this dive is a fairly easy dive - only 20min of deco. Is it worth to carry an extra bottle of O2 which you - I certainly don't think so. (Also let me just throw the official disclaimer in that one must be trained to carry 2 deco gases.) The drag versus the benefits are certainly not worth it in my mind. Also, keep in mind that the O2 window is not exactly huge between 25/25 and EAN50. Both have still 50% N2 as well. Not really ideal.
But, let's assume that you are doing a 90min dive at 100'/30m then I would certainly consider adding an EAN50 bottle. I would not necessarily give myself deco credit for the additional bottle (depending on the circumstances) but it helps maintain a reasonably safe rock bottom. Your dive partner will be happy for that...
As you can see, there is not a real black and white answer to most decompression questions. We as think divers have to look at the end result, such as coming out of the water as clean as possible versus 'I have to get out because of an emergency', and make our decisions as a team.
Hi and welcome. I'm so glad you decided to join us here at Unified Team Diving!
My name is James Mott and I am one of the UTD instructors based in the Great Lakes area. If you have any questions or comments, please feel free to contact me or any of the other instructors, we would be more than happy to help you.
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