Been diving for many years now, and finally decided to go into the advanced specialties. Master Diver and beyond. Problem is, my local SSI dive shop doesn’t have much demand for the advanced courses, so I’m at a loss when I have random questions. I was able to get into nitrox class, but it will be hit or miss if people sign up for anything after. Question is nitrox related. What is the practicality of using ean32 as a travel gas down to about 100ft, switching to air for the dive, then going back to nitrox for the ascent?
as someone who works casually with a small local shop, i understand the frustration when trying to advance your skills and you cannot get a course organized. trust me, it is just as frustrating for the instructors. quite frankly i would love to do less open water diver training and do a lot more specialty training. but our small customer base, along with the lack of advertising and organization by the shop makes that vurtually impossible.
i believe i understand what your original post is referring to. you are asking about using one gas as a travel gas ((getting from surface to bottom (or close to bottom) then again from bottom to surface)) and a different gas for the bottom phase of the dive.
this is not uncommon but it is outside of the scope of this forum. as others have pointed out i am sure, this is far more advanced then the average recreational diver would ever consider.
there are many factors to consider. what is the best mix for the travel gas ? what is the best mix for the bottom gas ? are we using trimix for bottom gas ? can you plan a proper dive profile considering the gas switches etc ? do you have the proper equip to plan and execute the dive ? are trained how to safely complete the required gas switches ? etc etc
typically for deeper recreational dives (down to 130 feet) most divers will choose a gas mix that is optimal for the deepest point of the dive (obviously assuming they are diving nitrox) and will breath that gas for the entire dive. at 130 feet it is usually 28%. if they want more gas for redundancy, they will add a second, smaller tank.
since that gas is just a back up, they may choose to carry air in that tank so it will be safe in all recreational diving scenarios, whether you end up using yourself or if you need to share with someone else. but often is the same as their main tank. my only caution in doing this, is that if you were planning for a deepest depth of 100 feet on day one, and then on day two you do 130 feet, the gas in your back up tank may be too rich for the deepest part of the dive. so you may have to empty it and refill it with a leaner gas. so why not just keep it filled with air or 28% ?
