The ZX 50-D
Yoke shown, also available with DIN connection 350-2110 (Yoke) 350-2121 (DIN with Yoke converter)

The Performance Graph shown above has a lot of information, but all of that information boils down to the final "EXT WORK OF BREATHING" number. In this case it is 1.05 joules per litre. That means that it took 1.05 joules of energy at the mouthpiece to move 1 litre of air in and out. The US Navy considers a regulator that has a work of breathing LESS than 1.4 J/L at this depth and breathing rate, as a class "A" regulator. This istheir top ranking. The US Navy Tests are done at 1500 psi inlet pressure. Note that this test was done with less than half that inlet pressure (727 psi) and yet theEnvoywas still able to meet the Class "A" rating level.A real regulator value!