Very good point there. As for HUD options, they can be added to any system. Currently I use Shearwater Pursuit with one of the original KISS displays (as a back up to the Shearwater) for PPO2 monitoring. If/when I choose too, I can easily jack in a Shearwater's HUD system with no difficulty at all.

As to the question how often us Kiss owners actually have to inject 02 or visually monitor your p02 readout.

Visually monitoring our p02 is the same as with any other rebreather out there. What's the mantra "know your PO2 at all times or you will die." Yeah, sounds harsh, but it keeps you thinking.

As for how often us Kiss owners actually have to inject 02 into the system, that really depends on depth and activity level.

The KISS works by bleeding O2 into the system at a rate of .06 lpm (just below the average human being's metabolic rate), with a manual bypass button punch a quick or long shot of O2 back into the loop.

I can't speak for everyone (KISS Divers) here, but not very often as I have somewhat low SAC rate (close to .5) and can maintain very efficient breathing rate, even when task loaded. Dive the system as long as have, tapping the button is as automatic and second nature as stroking your fingers through your hair or scratching your chin.

When I doing something like shooting macro, or simply laying in wait for something, to maintain a setpoint of 1.2, the drop is so low, at a depth of 40 - 50 feet the flow rate is dam near equal to my metabolic rate. At 60 feet and deeper, it actually moves in the other direction, ticking up faster with each increase in depth, requiring I have pay very close attention to not letting it get too high.