What is a primary pp02 HUD?
Most recreational rebreathers have a Primary gauge for observing the partial pressure of oxygen. The Primary gauge is the main means for doing this. Most also have secondary pp02 displays in case the Primary has a problem. Some rebreathers also have a Head Up Display (HUD) indicating pp02. If these have a single LED (usually bi-/tri-colour LEDs in a single package with two actual LEDs) then they are not primary displays, as they have single failure points in the hardware, software and displays, and so should not be relied upon for such life-critical information.
If they have more than one LED they must have been specifically designed as a Primary HUD if they are to be used as the Primary method of observing pp02. It is not good enough to just duplicate and isolate electronics without ensuring that all possible failure modes are taken into consideration such that there is no way that both or either could fail without alerting the user that there is a problem.
The important issue is whether all failure modes can be shown to fail safe and alert the user. If they can then the HUD can be safely used as a Primary pp02 gauge.
Another important issue is the independence of the electronics used for monitoring the rebreather from that used to control the rebreather. If the same electronics are used for both, then if the control side fails, it is likely that the monitoring side will fail as well. In this case, one can lose both control and monitoring at the same time.
Most rebreather manufacturers and rebreather training agencies insist that the diver monitors their pp02 gauges regularly. If they could show that the control electronics were reliable and failsafe enough, this would not be necessary.
By separating monitoring electronics from control electronics and using the same oxygen sensors for both purposes and by means of an architecture (patent applied for) that can be shown to be both fault tolerant and fail safe, a safe and effective Primary HUD can be built.