Abstract: The invention provides, in one aspect, a head up display unit for a re-circulating gas self-contained breathing apparatus of the type having at least one sensor for determining the oxygen partial pressure of the re-circulating gas. At least one optical fiber (32) is provided for transmitting a light signal or signals from a respective light emitter, positioned outside the wearer's field of view and electrically connected to the at least one sensor, to a display (26) in the wearer's field of view to provide the wearer with a visual indication of the light emitter signal or signals.

Abstract: A Self Contained Re-breathing apparatus with a single bank of oxygen sensors for signaling oxygen contact back to a display and oxygen valve control, where the signal is set via two identical and independent sets of signal control circuitry which are interconnected in a primary and secondary relationship. The primary circuitry controlling the valve and the secondary circuitry sending a signal to a display, wherein the two identical and independent sets of circuitry can perform each role and serve to confirm the satisfactory operation of a master signal processing circuit.