Doing a dive as deco dive with air doesn’t make sense when you could probably do the same dive with nitrox without deco.
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Doing deco dives with air as backgas and deco gas doesn’t make sense for me. You can probably do that dive without deco when using the right gas.
Remember it is also a question of logistics. By preference [1], we would select an optimum Nitrox mix, in the case this is not possible, a 'standard' gas, failing that air.
Nitrox is not always available.
We managed to squeeze a trip to Scotland in between lockdowns. There was limited O2, which wasn't guaranteed. (I took a spare twinset to swap out my CCR, just incase.)
Other than the gas we arrived with, we dived air. Decompressed on air (or the limited deco gas we arrived with).
We run trips to Scapa Flow every five years or so.
The first time I went, only 1 boat would provide Nitrox, by prior arrangement. My buddy and I where the only two on our boat using twinsets, we also took 3 litre cylinders. I used to get up early, to visit the boat to get the 3 litres filled with 50%. We dived air, and decompressed on 50% (on air tables). The longest stop was 45 minutes.
The second time, O2 clean twinsets, + 3 litre cylinder (50%). Optimum mix each dive. (Nitrox was now more available)
Now, O2 clean twinsets + deco cylinders. 36% mix on the second dive. Air top for the following morning first dive (but ensuring PO2 will be 1.4 or less). Even then, we still run stops on the first dives, often on the second.
If Nitrox is logistically difficult. It's easy to take a few stages with deco' mixes and use them over the week and use air as the diving gas.
I've been on trips where the O2 and Helium start to run low. You have to manage the available gas and modify the dive plans appropriately, or stop diving.
We've also taken our own J's of O2 to blend our own. But transporting O2 is complicated, especially when you put it on a ferry.
Gareth J
[1] Note a predominantly dive CCR these days