Quote: (Originally Posted by
divelermentov)
because it is not about getting a bigger gas flow,
the issue is to keep the orifice at sonic speed so you get a constant gas flow.
The orifice will only stay sonic if the upstream pressure is roughly 2.2 times the ambient pressure, therefore 7 bar will limit you to somehow 30 meters.
my first kiss dives attempts were done on the same orifice then the first gordon smith orifice, the IP for 0.8 lt/minute was 7 bars !!!
I dived it down to 50m without a shadow of a problem,
of course due to the fact that the orifice was not sonic anymore I had to face a flow rate cut but ok It still worked fine.
like gordon was saying "for the time one spend in 60m even a dil flush would be enough to maintain the ppo2 level"
what I mean is that the dogma of the sonic flow is too limiting.
if you read the jacques vettier book about rebreathers you will find out tests and charts showing that getting a sonic flow is nice but that the flow rate drop is not so dramatic when you get a bit under the 2.00 ratio.
the sonic depth is not an absolute limit under wich you will not have o2 anymore, it is more subtile then that, it can be exceeded.
personly I use an hydrogom with 11 bars for 0.8 lt/mn and dive it down to 65-70 m and I have never noticed any need for injecting manualy more often.
last thing, swagelock website provides a flow rate simulator where one can tests all of that, orifices and needlevalve have different reactions
regards
jean mi