I have no aversion to air on shallow, minimally repetitive dives (a 2-dive boat in Hawaii? Air is fine), but I NEVER fill my tanks with air in anticipation of that. I always fill my own tanks with 32% (who knows where you will next dive?), and don't stress it if my dive happens to only be 40ft.
"close to your MOD of 111 fsw with a standard 1.4Po2"
I don't understand this statement, could you dumb it down or should I wait to take the class and they'll explain it?
I got the 111 feet salt water part.
One of the important considerations of what your maximum depth should be is the physiological effect of the partial pressure of a gas, in this case, oxygen. As you descend from the surface, which is 1 atmosphere (1ATA), to 33ft (2ATA), 66ft (3ATA), etc., the air becomes denser, and the amount of each gas you take in is higher for each breath. Oxygen is toxic at high enough concentrations/exposure, if you're exposed to too much oxygen for too long you can develop a bunch of nasty symptoms, including convulsions and loss of control (resulting in drowning) - that's oxygen toxicity.
Air is 21% oxygen at the surface. That's a partial pressure of 0.21. At 33ft, or double the atmospheres, it's still 21% of the total mix, but the mix is twice as dense, so the partial pressure doubles to 0.42. At 66ft, ppO2 goes to 0.63, etc.
With nitrox, the fraction of oxygen is higher than air, and so the ppO2 at any pressure is also higher. For 32% nitrox, it's 0.32 at the surface, 0.64 at 33ft and 0.96 at 66ft. Once you get above ~ppO2 1.4, the risks of developing these toxicity symptoms increase--the risk is still small, but the consequences if it happens are severe. So one aspect of determining the maximum operating depth of a gas is in keeping the ppO2 below this risky level. Most people use a ppO2 of 1.4, which is about 111fsw for 32% nitrox. I keep it 1.2, which is a little less than 100fsw.