Many caves have roller coaster bottom profiles that roll up and down over subtle and dramatic depth changes. When air diluent is used, it means that gas is frequently dumped to adjust for buoyancy changes. Soon after, the diver drops again, diluent is added to make up for the pressure increase, thus diluting the mix, and causing oxygen to be injected in the system. Volume increases result in more gas dumping and so on. Buoynacy becomes more challenging and gas use increases significantly.
If the diver uses Nitrox in their diluent tank, they can choose a mix that closely resembles their planned PO2 for maximum depth. I like to choose a diluent mix around1.0 and sometimes as high as 1.2. I run my rebreather at a set point of 1.2. In this situation, my gas use decreases, buoyancy control is easier and I still have adequate ability to flush down hyperoxic scenarios, flush up more effectively from hypoxic situations and flush with diluent to confirm oxygen cell readings.
Still others will criticize me for selecting a diluent as high as 1.2 arguing that I cannot flush down hyperoxic situations very well. Every dive has a specific risk assessment. Most caves I dive have hard bottoms where I am unlikely to be capable of exceeding a particular depth. I feel my risk of hypoxia is greater than hyperoxia. I have enough room in the water column to shift up and down in depth to confirm cell verification flushes.
I also challenge every diver to see just how much diluent it takes to flush up or down to relieve hyperoxic or hypoxic scenarios and make them safe to breathe. The moment you discover a dangerous mix, you should be bailing to safe offboard open circuit gas supplies until the problem can either be solved or you abort the dive. In the event that the problem is fixable, such as an inadvertently closed slider valve, then the loop can be flushed appropriately with whatever gas is onboard. Jill Heinerth