than the conventional flexible bags. The approach was intended to use tank pressure to compensate for the o-ring friction and inertia of the displacing water. Although the o-ring friction in air was acceptable, the model could not handle the inertia of the necessary water displacement. The prototype suffered from excessive work of breathing and did not pass test of surface breathing while submerged for even a few minutes. It got him started however, and nevertheless had the dubious description of "looking like R2D2 making love to an octopus" according to Gordon. His next prototype was a semi-closed design again, but this time with more conventional flexible counterlungs. This worked very successfully, and was dove for many months. However, with the experience he gained with the semi-closed designs, he saw that the fully closed system was certainly feasible and much more desirable in terms of performance. He quickly decided upon the merit of including an oxygen sensor due to the risk of inadvertent hypoxia if required to exercise at the surface. One of his innovations early on was the bailout-integrated mouthpiece, switching immediately to diluent for short term open-circuit operation in an emergency or even just upon entering the water. He also incorporated a regulator to prevent negative loop pressure should the counterlungs become fully deflated upon descent. Gordon devised his fully closed rebreather system in 1998. One of his key contributions was to realize that the formidable electronics engineering required to automate control of the loop partial pressure of oxygen was neither required nor necessarily the best way to minimise the risk of hypoxia or hyperoxia. Instead, a constant mass flow orifice injected oxygen continually at a rate slightly under the divers basal metabolic rate - requiring only periodic top-ups by the diver. This was the KISS principle, the philosophy by which the diver was counter-intuitively made safer by removing electronic protections. Specifically, design philosophy was that the knowledge that the diver was the one and only thing controlling the loop oxygen level meant that attention was required and would be applied. This avoided the human tendency to eventually neglect to monitor the machine-controlled process outputs, which work seamlessly until there is a failure or mistake (whether software bug, miscalibration or other issue) and then cause an accident. In practice, the constant mass flow of oxygen meant that partial pressures changed so slowly that this form of manual control was