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I should explain why you want trimix to be surface breathable (FO2 >= 0.16). There's really no magic here. As a background note, gases with PO2 < 0.16 ATA are hypoxic; you risk losing consciousness if you breathe them. If the FO2 in your mix is < 0.16, as Wrolf points out, you can't breathe it at the surface, but can use your intermediate deco mix (EAN50 or whatever) to breath from the surface down to the the minimum operating depth of the mix (that at which PO2 >= 0.16 ATA). Sometimes this is necessary, as when diving below 255 fsw. If you don't need your FO2 to be < .16 because of depth concerns, you should use more O2 for number of reasons. Probably the best reason is that if you run your O2 level as high as possible, you minimize your decompression obligation. The extra N2 in PMM not only make you narced, it adds unecessary hang time. Just as with nitrox, you want to run your bottom PO2 at 1.4 - 1.6 ATA, depending on your expected exertion level and bottom time. For a working dive to the second class china in the Doria (220 fsw) you want something like 18% O2 in your mix.
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