I currently dive back double LP85s. A shop guy told me yesterday I would be shocked by how much my air consumption would decrease if I switched to double LP50s. He said that the streamlining of 50s is so much better that I would move effortlessly through the water and therefore consume much less air. I don't have a set to try this experiment on but is he correct? An 85 has about 70% more air than a 50 at the same pressure. How much more time would I get out of 85s vs 50s (if any)? I assume based on his comments the additional time would be much less than 70%?
in cave diving which is where this really matters, the difference in gas size and penetration distance is not linear. As you said, an 85 has 70% more gas than a 50, so if you can get 1000ft on a set of 50's, you'd think you would be able to get to 1700ft on a set of 85's. In reality, you'll probably only get to 1500ft or so. Numbers completely made up, but it's close enough. It's far enough away from linear to matter, but enough of an improvement to be worthwhile. Same concept applies with stages.
I disagree with the argument about being streamlined since bottles are round and the drag profile between a bottle with a 5.5" diameter and a 7" diameter at the speeds that we move is functionally irrelevant. Someone like
@victorzamora is qualified to actually speak on that since that's what he does for a living, but as slow as we move, if the only difference is a set of double 50's vs a set of double 85's, then the difference in drag profile is practically irrelevant. Even less so if you are sidemounting.
What does matter though is mass. The 85's are ~65% heavier than 50's with a total increase of roughly 30lbs when full. It takes a lot more effort to get that mass moving. This can be proven if you log the mpg's in your car. If you log with just you driving, then log with the car fully loaded with people/stuff, you'll see a change in fuel economy. Same applies to diving.
