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Using an MOD limited to a Ppo2 for 1.4 seems excessively conservative with a constant mass flow breather. Beginning during WWII and for at least 40 years ( and perhaps to the present - my information was published in the 1992*) the Royal Navy used a MOD limited to a Ppo2 of 2.0 for the supply gas on its oxy-nitrogen constant mass flow scr's. During that time they never experienced an oxygen toxic event with these rigs. Limiting your Ppo2 to 1.4 is prudent for the oc diver whose Fo2i is set but the beauty of the sc constant mass flow 'breather is that as your metabolic rate increases, and you are more sensitive to oxtox, your Fo2i will fall reducing your risk.. As way of an example, assume that you are doing a dive at 115 using 40% EAN and an injection rate of 12 lpm, if you were in a state of nonswimming relaxation your Pp02 would be 1.8. In such a relaxed state your succeptibility to oxtox would be very low. Now if you started to swim at a rate considered to be as hard as a combat swimmer will volunarily swim i.e. 1.5lpm of o2 consumption your Ppo2 would fall to a conservative 1.4. ( BTW when we are just sport diving with our breather our o2 consumption is around .5 to .8 lpm which would put us at a Ppo2 of 1.6 on the dive shown above.. Barry and I just finished installing the o2 sensor and now have to find the time to go dive it and see how well our calc's jive with the real world : ) Using oc Ppo2 limits for set for OC for SCR's is comparing apples with oranges; and by reducing your Ppo2 unneccesarily you may be trading an vanishingly small risk of oxtox for a very real risk of dcs. Bill * "Oxygen and the Diver" by Kenneth Donald
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