Quote: (Originally Posted by
mattarte)
Thanks everybody for the answers. So the mechanical difference is the underwater adjustability of the needle valve. Speaking of the uw adjustable needle valve, unintentional adjustments (eg. sliding against other objects) are a considerable risk?
We put the needle in between the in and out hoses which are of larger diameter than the needle adjustment handle. If you would scrape the whole assembly, say against a cave wall, you would rather get some scratch marks on the hoses than accidentally turn the needle. If you scaped it against something sharp, protruding from somewhere you might, (if you are extreemely unfortunate), turn the needle max half a turn. This would represent a flow shift of maybe 0,3 lpm, so if you check your Po2 every 5 minutes it may have gone up or down a bit or maybe taken out by increased work or vice versa. In short; no big drama.
I also like to wedge the Po2 meter in between the in and out hoses of the needle block. That keeps the block in front of me and keeps the Po2 meter like a rear view mirror so I can check Po2 hands free. The Po2 meter sticks out in front of the needle block and would likely take the first bounce, my forehead the second. Have a look at my avatar and you will get the idea.
If what Giggebo writes is true it seems like the Voyager uses the same system. If they use a low IP like 5 BAR it means you get more flow increase with depth. With an IP of 9 BAR, our orifice and our needle you get 1% massflow increase per meter to 90 msw. After 90 msw you get increasingly less increase per meter as the flow goes subsonic. As temperature comes into play the sum total O2 dosage increase of 1% per meter is about right down to 300 m.
In essence you could use the Pelagian block the same way as the Voyager, meaning you set a surface flow that becomes suitable to your planned max depth and then you dont touch the needle during the dive. Just ad a bit more frequently with the bypass O2 ad valve during the ascent.
This is the way we teach it during training in order to get the basics down. After a few additional dives most students like the idea of being able to adjust during the dive.
All the best,
Andy