Poseidon Jetstream/Xstream questions/observations

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Michelle Louise

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- Coaxial Jetstream vs. non-coaxial Xstream

To my understanding the consequence of case geometry fault is that the distance from inhale diaphragm to exhaust must be ‘added’ to the initial (unsubmerged) minimum cracking pressure to ensure no free flowing in all positions. Coaxial diaphragm/exhaust as found in the jetstream,d400 etc solve this fault, so why did Poseidon move away from this with the xstream? Additionally the xstreams exhaust is deeper than inhale diaphragm in the normal diving position, I can’t think of any other second stage with this orientation (Maybe Poseidon Triton). What is the thinking behind this design choice?

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- Valve housing nut hole

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This nut that screws onto the main valve body to enclose the condom has two holes on the jetstream and early xstream, later versions had one hole removed with poseidon recommending older xstreams be upgraded.
“For regulators with serial numbers less than 310001 we recommend to offer the customer to replace the old type valve housing nut to the improved version with article number 4547. This will significantly reduce the inhalation effort.”
This nut segregates the inhale and exhale sides of the reg, forcing gas to make a 90° turn towards your mouth. Naturally this gas will bring with it some of the gas from the exhale side, lowering pressure there → venturi boost.
This hole/s in question lets some air through to the other side presumably to decrease the venturi effect.

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So I am wondering, would plugging this remaining hole (maybe partially?) be a way to tune this second stage? You could plug one of them on the jetstream. The Atmosphere full face mask has some kind of a washer blocking this hole/s but I am not sure in what other ways that second stage differs to achieve it's positive pressure function.

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- Reduction of venturi effect at depth

This is an excerpt from the NEDU evaluation of the Jetstream, proposing that venturi effect is reduced at depth due to decreasing pressure differentials between ip and ambient.

“Venturi Action
Figure 5 shows that flow resistance averaged over a breath begins to rise in the fashion of an orifice at ambient pressures of 6 atm absolute or more. We can speculate that the suction created by a second stage venturi is more pronounced at shallow depths than at deeper depths. The Poseidon Odin intermediate pressure was 145 psi above ambient. At 165 fsw, ambient pressure is 88.2 psi absolute. Therefore, intermediate pressure was 145 + 88.2 psi or 233 psi. The expansion of gas across the second stage orifice is therefore affected by a pressure drop from 233 psi to 88 psi, resulting in a 2.6 times expansion at 165 fsw. At the surface, the expansion ratio would be (145 + 14.7 psi) /14.7 psi or 10.9. Therefore, gas expansion is about four times greater near the surface than at depth. To the extent that venturi action is controlled by volume expansion, we would expect the induced venturi to be more vigorous at shallow depths than at deeper depths.”
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I find this interesting, I have played around with intermediate pressures and have found that normal ips (10/11 bar) make my servo regs stutter in the shallows but breathe better 40m+ (cracking maybe a bit higher but subsequent gas delivery increases). I assumed it was more pressure = more gas but it could be increased venturi, maybe both. So I keep my primary first at 10 bar and backup at 8.5 bar and breathe from that in the shallows, usually breathing deco gas anyways. I may now experiment with modifying this small hole when diving resumes.
 
Isn't the Jetstream/Xstream IP 124 psi? I am still trying to come to terms with my Jetstream. The case does not seem particularly intended for Venturi assist. The stutter is because excess air is delivered and it bypasses to the exhale side causing the pilot valve to close, then open and then close. This does go away past about 20 feet. The old TEKNA T2100 behaved similarly but perhaps worse so.
 
Isn't the Jetstream/Xstream IP 124 psi? I am still trying to come to terms with my Jetstream. The case does not seem particularly intended for Venturi assist. The stutter is because excess air is delivered and it bypasses to the exhale side causing the pilot valve to close, then open and then close. This does go away past about 20 feet. The old TEKNA T2100 behaved similarly but perhaps worse so.
10 bar used to be recommended
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Manual from the 90's
 
@rsingler is your best resource for these regulators. Also @tbone1004 is guru status :wink:.

I think the mass flow (density+volume) more equally matches the diver's exact demand deeper and is excessive above 20 or so feet.
 
@rsingler is your best resource for these regulators. Also @tbone1004 is guru status :wink:.

I think the mass flow (density+volume) more equally matches the diver's exact demand deeper and is excessive above 20 or so feet.
Yes, probably the minimum that the servo can be opened results to more air than needed at shallow depth
 
10 bar used to be recommended
"Recommended" is not the term I would choose.
One has to inspect the manual very closely to discern what Poseidon intended. They have relatively poor translations of the subtleties, and occasionally frank errors that have taken years to correct.
For the Jetstream, 10bar/145psi is an absolute maximum. Breathing characteristics are poor that high, and a stutter in shallow water can be one consequence.
When one looks at v2.0 of the 3960/ Jetstream manual, the instructions can be confusing.
The Intermediate Pressure Quick Reference Table is misleading. It lists IP at low tank (300 psi) as 108‐145 psi, and IP at high tank as 123psi.
There are better instructions above the table:
1) At low tank, make a rough setting of 145psi. Okay, who cares? But then it says that a 14.5 psi IP Drift is acceptable before lockup. Unfortunately 159.5 psi is unarguably out of spec for this regulator. What were they thinking? But forget about that for a minute.
2) Now we get to the one reasonable step. Increase tank pressure to 3000/4350 psi, and set the IP at 123 psi. NOW, you're maybe done.

And now the table makes sense. At high tank, there is one listed value. 123 psi.
At low tank, since this is a diaphragm, one expects IP to rise. And if you have a ridiculous IP shift from full to empty tank of 22 psi (!!), you'll reach the max IP of 145 psi. More likely, you have an acceptable shift of 7 psi from full to empty tank, and an allowable drift of 14.5 psi, and hit 144.5 psi with a marginally sealing valve. To Poseidon (not to me), that's acceptable. But that's the ONLY way you'll get to 145psi.
Instead, I look at the allowable range of IP's in the table, of 108-145 psi. And I run my regs at about 118psi at a full tank, which may shift up to 125psi when the tank's almost empty (I don't accept IP drift - I repolish the valve piston and replace the HP seat). Those IP's (118-125) are within the allowable range, and my Jetstreams breathe much smoother. I don't have stutter.
I don't have ANY problem with setting my personal reg's full tank IP at 110, shifting up to 117 when empty.

For the XStream, it's even more confusing. Section 9 (Settings and Adjustments) in the Mk3 (XStream 1st stage & Jetstream 2nd) manual has a table with a list of properties (P1, P2, P3, P4@P1, R, Q, Internal Leak) that are meaningless unless you saw their definitions in a completely different manual (Appendix C of the XStream manual with same first stage, but XStream second stage)! There is no Appendix C in the Mk3/Jetstream manual. Arrrgh!

That's a subject for a completely different thread.
 
The Atmosphere full face mask has some kind of a washer blocking this hole but I am not sure in what other ways that second stage differs to achieve it's positive pressure function.
That so-called "reverse" or "check valve" 2952 (a silicone washer) was intended, so I was told by an old Poseidon tech, to prevent "back-flow of gas into the full face mask"; though, my earliest Atmosphere models somehow lacked them and I had never seen a difference in the performance of either arrangement, that couldn't be attributed to poor tuning. The "Jetstream PP" was set to lightly free-flow into the FFM; and there was no other difference between it and a conventional XStream valve.

Those Poseidon manuals had always been a bone of contention and, not surprisingly, were the only brand where Vance Harlow in his 1999 SCUBA Regulator Maintenance and Repair, actually encouraged taking the manufacturer's tech course.

When the Jetstream was first introduced, more than four decades ago, as the Cyklon Maximum, its recommended IP had originally been set to a maximum of 9 bar (128 psi), within spitting distance of its current 8.5 setting; but it was then cranked, for whatever reason, to 10 (not to eleven, heh, heh) for years, and was still kept within that range by the time of Harlow's book.

To his credit, he did lower the Cyklon 300's original IP from a max of 13 to 11.6 bar; though most run those at about 10 -10.5 bar nowadays.

As an aside, here is an insight from an old manual illustrating why Poseidon decided to add that "sensitivity" switch to the Jetstream . . .
 

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