Hatul
Contributor
Intuitively I would have thought it better to use EAN on the first dive and air on the second, because there is more nitrogen uptake on the deeper first dive, but that assumes the same length for both dives and air consumption may change this. And John Adsit's table calculation gives the opposite result.
To challenge the premise of the OP, it's not logical to try to save by using air on the second dive. After paying thousands of dollars on the dive trip, trying to save the cost of an extra nitrox tank does not make sense. Just go with nitrox on both dives.
In my Cozumel diving the more common pattern was a deep dive sometimes 100+ or on a wall with no bottom for the first dive, and a shallow <60 ft dive for the second dive. In this pattern I choose air for the first dive and EAN36 for the second. On such a deep first dive with possibly no bottom, air is safer for keeping away from MOD in the event of loss of buoyancy or in a downcurrent.
To challenge the premise of the OP, it's not logical to try to save by using air on the second dive. After paying thousands of dollars on the dive trip, trying to save the cost of an extra nitrox tank does not make sense. Just go with nitrox on both dives.
In my Cozumel diving the more common pattern was a deep dive sometimes 100+ or on a wall with no bottom for the first dive, and a shallow <60 ft dive for the second dive. In this pattern I choose air for the first dive and EAN36 for the second. On such a deep first dive with possibly no bottom, air is safer for keeping away from MOD in the event of loss of buoyancy or in a downcurrent.