is octopus identical to the 2nd stage reg?

[Homunculus]

Hi all,
i'm in the process of buying an octopus (Poseidon Xstream). I've already got the Xstream 2nd stage primary regulator. I've recently heard that octo is not made identically to the primary 2nd stage reg. I've read some comments on the board - not very conclusive and seems to be brand dependent. I guess, my question is specific for Poseidon Xstream.
does anyone know if Poseidon Octo is identical to the primary reg? if not, what are the important differences?

 

[wunat]

I've recently heard that octo is not made identically to the primary 2nd stage reg. I've read some comments on the board - not very conclusive and seems to be brand dependent.

Can't say it for Poseidon but from my understand, the octo (or alternate air source) is just another second stage. They usually (if not all) use the same internal service parts except the external case color. But not all regulator sets has identical second stage for the primary and the alternate air source. For example, some regulator sets comes with a balanced second stage for the primary while the alternate air source is of an unbalanced one. But they don't necessary have to be that way. For some regulator sets, they just come with the same second stages but the alternate gets the bright color casing with a longer hose.

 

[Homunculus]

as it turned out, Poseidon Xstream octo is identical to the "primary" 2nd stage regulator as I was told by the distributer.

 

[MatWill]

be careful when it comes to octos, sometimes if the second stage is overbalanced then you need an overbalanced octo.

a reg that comes to mind is the legend lx which needs an lx or legend lx octo and not an abs octo.. otherwise it could 'leak' at depth

if you were going to go with a completly different brand octo just look for the first stage pressures and find a second stage to suit

 

[DA Aquamaster]

"Over balancing" is a condition where the first stage is advertised to have a greater intermediate pressure, relative to ambient pressure, at depth than at the surface.

I have heard it a lot with regard to Apeks regs but not with regard to the Legend LX, although since Aqualung owns Apeks, it's not a real surprise.

The thing is I have yet to see any actual data to support this claim nor have I seen any design or engineering evidence inside an Apeks or Aqualung first stage that would lead me to conclude that increasing depth would increase the IP beyond what normally occurs in other first stage designs where the IP relative to ambient pressure remains the same as depth increases.

That is a good thing however as if the intermediate pressure did increase in this "overbalanced" manner, it would cause tuning issues with even a balanced first stage. To keep the reg safe in the event of a high pressure seat leak, a balanced poppet still has to have a downstream bias, so that a steady increase in IP will eventually cause the second stage to freeflow slightly to vent the excess pressure - usually at around 170 psi. If this did not occur pressure from a leaking HP seat would eventually cause a hose to ruputure or cause the low pressure inflator to fail open and auto inflate the BC or wing. The effect would be even greater on an unbalanced second stage.

Consequently, to have even a balanced second stage tuned for easy breathing at depth on an "overbalanced" first stage would require that it breathe harder at all but the deepest depth to be acheived and, without the extra IP casued by greater depth and "overbalancing", to breathe much harder near the surface. And again the effect would be much greater with an unbalanced second stage.

"Balancing" a regulator is a very good thing. Overbalancing is not a good thing as you are in essence unbalancing the system again - but as a marketing term it does appeal to the "more must be better" customer mindset.

The only theoretical advantage to "overbalancing" (as the term is defined by Apeks) is that the greater relative IP at depth would help offset the reduced flow caused by increased viscosity of the "thicker" air. This is however not even a real issue at depths shallower than 500 feet and plays no part in recreational , deep air, or mixed gas diving. And in any case, boosting the flow rate in that manner would just be a crutch for a low performance design - there are far easier and more effective ways to get higher flow rates at depth.

So in short, I would not get hung up on the term "overbalanced" as it is basically marketing BS.

What is important twith an octo is to have it tuned to the specific first stage. And where an octo often differs from a primary of the same make and model is in the tuning. Many companies, dealers or techs will set up the octo to have a higher inhalation effort than the primary. At the company level, this detuning to create a higher inhalation effort is convenient as it means that the octo could be put directly on any first stage operating in the normal 120-145 psi IP range and not freeflow. It's convenient for them but will not result in a great breathing octo. At the dealer/tech level, it is usually done with the belief that it will be more freeflow resistant and divers tednt o view this as a good thing. Personally, I want a good performing and easy breathing octo when I need it and the difference in freeflow resistance between a well tuned second stage and a horribly heavy breathing octo is minimal.

 

[Luis H]

The thing is I have yet to see any actual data to support this claim nor have I seen any design or engineering evidence inside an Apeks or Aqualung first stage that would lead me to conclude that increasing depth would increase the IP beyond what normally occurs in other first stage designs where the IP relative to ambient pressure remains the same as depth increases.

The advertised “overbalanced” first stage is easily accomplished with the newer sealed diaphragm first stages. The only thing required is that the effective area of the outer diaphragm be slightly larger that the internal pressure diaphragm.

The above description only applies when a rigid plunger (pushing device) is used to transmit outside pressure to the inner diaphragm. With the old environmental chamber that were fluid filled, the fluid just transmitted the same pressure as exposed to on the outside of the flexible chamber (ignoring any minor stiffness on the rubber chamber or outer diaphragm).

IMHO the overbalanced was accomplished by accident or because it was too difficult for the engineers/ designers to determine the exact effective area and instead of correcting and precisely matching the effective area of the two diaphragms someone decided to advertise it as an improved feature.

Depending on the diaphragm edge clamping condition and the stiffness of the diaphragm itself, it can be difficult to precisely determine the effective area of a high pressure diaphragm. It may require empirical testing, but if the diaphragm is replaced during service it could affect it slightly.

I totally agree that this whole concept of “overbalanced” is a bad idea being used as an advertising gimmick.


The argument about denser air is so overplayed, IMHO it is ridiculous. To explain this comment it would require a lot more time. Let me just mentioned that density and viscosity are not linearly proportional. Also viscosity affects flow and flow drag, not static pressure.


On a related subject: I have being wanting to set up a submersible IP gauge to check how the IP of several regulators actually track with depth. Since I can’t find a waterproof IP gauge (0 to 300 psi) I may just add holes to a gauge and read differential pressure (to surrounding ambient) rather than gauge pressure (were the reference is 1atm).

 

[pescador775]

Thicker air, hah, hah. Yeah, ya gotta love those marketing guys. Another load of hype is the "carbon dioxide buildup". I'll bet half the guys on this board have blamed a headache or foot cramp on CO2. hehhehe

 

[Cheekymonkey]

Ah well in any event you will certainly love your new regs, I have four of them now and I am in love.