Downstream vs. Upstream Regulator Design

[rsingler]

In the case of the old Poseidon Cyklon I picked up, it had a downstream second, a 109 as the alternate. I heard Poseidon's don't play well with others

Oops! A Jetstream works just fine with a 109. Similar IP's. But combining a Cyklon that needs 160 psi with a 109 that expects 145 psi maximum and perhaps there's another reason that things don't go smoothly. That 109 had to be tuned so stiffly to not leak, that it was indeed a pig. Or if the 109 was perfect, then the Cyklon valve probably didn't crack before 3" of water. Another undeserved hit on Poseidon.

But like I said, few are neutral on the issue.

 

[tbone1004]

I think @tbone1004 mentioned he has a dive buddy that said something to the effect of he would “rather drown” than have to breath off one of those servo type regs. In tbone’s case I think he’s talking about Poseidon’s.

that would be @victorzamora and yes, it's about the Jetstream in particular.

@rsingler The 500se is downstream which is why it doesn't whoosh when you pressurize it.

@Bob DBF the Poseidon OPV is actually to maintain normal breathing and prevent damage to the second stage. Long before the hoses burst or anything else fails, the condom that seals the regulator will actually extrude out through the small holes and give an uncontrollable freeflow of LOTS of gas. The OPV lets you breathe relatively normally *WOB will increase due to the unbalanced design, but not any worse than a rental MK2*. They should ALL have an OPV of some variety though which is just a downstream valve.
The Cyklon is the only one that really doesn't play nice due to the 165psi IP. The 123psi IP the Jetstream and Xstream want can be used with most all balanced second stages, though an unbalanced like the 109 won't play quite as nice. The Jetstream/Xstream can be tuned to work at 135psi though but when tuned to 135psi they tend to whoosh more for the diver which is why a lot of people complain about them, techs will tune them to 135-145 and that's a big difference in how they breathe

 

[CharlieDontDive]

The only advantage to an upstream second stage that I can imagine is that in the event of a second stage failure it fails shut and you don’t lose a bunch of critically needed gas. Other than that I think they seem creepy.

yeah that was my thought too. just wasn't sure how well-informed it was....

 

[АлександрД]

I do like upstream regulators (2nd stages). I use "oldstyle" regs, mass produced in 1960-1970`s. Soviet regs from Ukraine-2 scuba.
Main reason for me - it is simplicity of this type of regulator.
It is not servo-valve, but most simple design - just "nail on the seat". It is skewing on the seat and opens.

It has few details, and could be opened without instruments for cleaning. Also it is very lightweight.
on my avatar picture this types of 2nd stages

Also we have AVM-5 (also -9, -12) 2nd stages, that use similar scheme with little more details

Another great advantage of this types of 2nd stages - it is freeflow proof and could be used for diving in the very cold water.
Getting Into Cold Water Diving - DIVE.in

here AVM-12 2nd stage


As I know, only this company has such type of regulator in mass production ("mass"... )
Divex Sweden

Divex The 960 MIL

Divex The 970 MIL

They offer it as regulator for extreme cold water, with extreme durability

Another advantage of this upstream regulators (withOUT servo valve) - it could work (and works well) with intermediate pressure around 4-6 atm (60-90 psi).
I use my 2nd stages from Ukraine-2 with IP around 6 atm (90 psi)
Also nice thing - when IP decrease - your breathing effort became easier

The upstream seconds are usually used with an over pressure relief,...

small correction - not usually, but IT MUST BE used only with over-pressure relief valve OR with downstream octopus.

So - upstream 2nd stages has rights to be alive. But you MUST to use overpressure valve.

 

[Bob DBF]

I do like upstream regulators (2nd stages). I use "oldstyle" regs, mass produced in 1960-1970`s. Soviet regs from Ukraine-2 scuba.
Main reason for me - it is simplicity of this type of regulator.
It is not servo-valve, but most simple design - just "nail on the seat". It is skewing on the seat and opens.

I have an old Nimrod from that era with a similar valve. Unfortunately it is a sealed unit and I have found no way to dismantle nondestructively, and since it works with a very slight leak I see no reason to destroy it at this time. I think the valve was made to replace as a unit rather than repaired.


Bob