airsix wrote:
There's a lot of pressure change in a foot or two when your at the 10ft depth I was doing this, but it still left me thinking I'd be frustrated trying to get neutral in a CCR.
You'll find that there's one other complication here. The volume is
approximately constant, but not exactly. Your loop volume actually diminishes over time as you metabolize oxygen, and then will "spike" a little bit as the solenoid replenishes the O2 with a little bit of overshoot to try to average out the PO2 to whatever your setpoint is.
With a manual unit, there's a constant trickle of O2, but you hand-fly the addition of O2 to play "human solenoid", if you will. It helps that the trickle is there, if the unit is properly adjusted, to reduce those spikes to a minimum, but they're not non-existent -- and the shallows are the trickiest part of it all, from a buoyancy and PO2 maintenance point of view.
As you ascend, even in a normal situation, you are forced to add (or the computer adds) quite a bit of O2 to make up for the fact that as the absolute pressure on you, the diver, decreases, so does the PO2, which, on a CCR, is designed to be held constant. In doing so, the counterlungs will also be expanding, so you end up adding O2, moving things around the loop by continuing to breathe, and exhaling through your nose, or, alternatively, by opening the BC-style dump valve on the lungs, if you have one.
The reality is that this is all second nature after you've been diving them for a while.