USD Conshelf 21S 2nd stage.

[jbrians]

Thanks for the help with putting the 1st stage together. I used mineral oil (baby oil) for the environment chamber and it seems OK. I had it down to 60ft on the weekend and it worked as it should.
My next question concerns the 2nd. It's set up to start flowing at about 1"...maybe as low as 3/4" and I thought it was working well at the surface. At depth, however, I was noticing I was starting to get tired pulling air thru it after a few minutes (60ffw). I took a couple of breathes off my buddy's new Oceanic at depth and it was a gale of air compared to mine.
As i said, cracking pressure is nice but the flow isn't what I'd like. Is there anything about flow rates that I can tweak?

 

[Luis H]

Based on what you are saying, my guess is that the first stage IP is not compensating for the increasing depth. The ambient pressure compensating chamber is probably not completely full of oil (there is probably an air bubble trapped somewhere).

I am not particularly familiar with the Conshelf 21, but my guess is that the second stage it is a basic down stream demand valve. Therefore, if the IP does not follow the ambient pressure, it will breathe harder at depth.

The first thing I would try is removing the outer diaphragm on the first stage and just diving it with the compensating chamber open to the water. If it works well, you know what is wrong. An environmentally sealed Conshelf is nice, but not essential.

 

[jbrians]

Thanks for that idea, Luis.
It is a simple downstream 2nd stage but I was thinking it was backing up with depth and I couldn't figure out why. I'll take it to tobermory with me this weekend and try it with the enviro chamber open. 100+ft and 42F water. It'll be dry suit diving for sure.