My new G250Vs and ramblings

[Couv]

I think it's time for a recap. After 250 post and 25 pages we have learned that Rob is older than Couv. AND the VIVA flow vane, for maximum venturi effect, should have the crescent facing the diaphragm.

 

[buddhasummer]

And that a non FF G250V, all other things being equal will not negatively affect in water performance. I'll confirm this by days end after a very chilly morning dive.

 

[1967Goat]

Sorry, looks like i am late to the party. Simon, did you still have a question on how you configured the 250V's? All 4 of mine, plus the 2 extra vanes I have, all have the crescent. The crescent is pointing toward the mouthpiece.

Mine got wet back in late Aug when I went to Coz. Heading back (Playa del Carmen) Feb 4 for 11 days. My boy took the SSI Open water class 2 weeks ago, and takes his OW while we are in PDC. All 4 will get quite wet next month.

 

[buddhasummer]

Just out, both performed just fine. Same as a G250, a G260 is nicer, but not much in it.

I can now confirm myself that a G260 exhaust valve, despite being fractionally smaller, works just fine in a G250V which means it will also work in a G250.

 

[buddhasummer]

Sorry, looks like i am late to the party. Simon, did you still have a question on how you configured the 250V's? All 4 of mine, plus the 2 extra vanes I have, all have the crescent. The crescent is pointing toward the mouthpiece.

Mine got wet back in late Aug when I went to Coz. Heading back (Playa del Carmen) Feb 4 for 11 days. My boy took the SSI Open water class 2 weeks ago, and takes his OW while we are in PDC. All 4 will get quite wet next month.

View attachment 635259

Interested to know if they all VFF mouth piece off, viva in max position, pretty sure they do/did/will. Thanks.

Take care
Simon

***added: still got the bone in my mouth.

 

[buddhasummer]

And just coz I couldn't help myself, after them settling after my dive today, I retuned them both. Just past leak point. Again both will VFF if I play with adj knob a fraction. Flow vane in as per manual position, crescent facing mouthpiece.

Now I'm really done playing.

Yeah, right.

 

[halocline]

A 156 when tuned properly (or a little lower than specs) will definitely VFF. I'll have to double check the 109. But Rob brings up an interesting point. The mechanical spring is stronger in the 109, but if we add the pneumatic force to the mechanical spring in a 156 it stands to reason the total force must be the same-else the IP would overcome the low spring pressure.

It would be the same when the valve is closed, but when the valve opens, IP drops and with it a portion of the total force trying to close the valve. So the balanced poppet should be somewhat more susceptible to free flows than the duro poppet, all other things equal.

 

[Geo7]

It would be the same when the valve is closed, but when the valve opens, IP drops and with it a portion of the total force trying to close the valve. So the balanced poppet should be somewhat more susceptible to free flows than the duro poppet, all other things equal.

Interesting! So the IP drop increases freeflow in a balanced 2nd. And what if you combine this with the initial IP drop of a balanced piston 1st stage (which you have described in previous threads), before the piston moves and fully opens? Might that make a further difference to keep a balanced 2nd stage poppet open for a fraction of a second?

The idea being: Lever gets depressed and overcomes spring and pneumatic force. Pneumatic force drops because of poppet separation. IP drops noticeably* just before piston moves (inertia, sticking forces), but once piston opens there is plenty of flow, and it takes a fraction of a second before the pneumatic force is re-established and the poppet closes. End result a boost of air rush.

Does that make any sense?

Perhaps that phenomenon is why the G500 was praised in the past. And I wonder if one vs two o-rings on the poppet have an influence... And if co-axial balancing is free of this effect.


*(Edit: this is refering to an IP drop observed in balanced piston 1st stages that appears to contradict the idea that pistons have more stable IP due to hugher flow. The IP drop referred to here is very short and before the piston opens. Once open, the IP of a balanced piston drop is less likely to drop due to high demand in volume, unlike 1st stage designs with lower flow rate.

 

[rsingler]

Interested to know if they all VFF mouth piece off, viva in max position, pretty sure they do/did/will.

I just couldn't quite let this go yet. @Open Ocean Diver already did the study, and posted the numbers above. But for the visual folks, here's a picture.
I finally found an old first model G250HP in parts in my to-do box.

Essentially Simon's G250V with a primitive micro adjust. Most important, it has a crescent in the VIVA vane. So I threw it together and ran a Reg Savvy dynamic flow test.
First off, tuned to 1.1" cracking effort with a first stage IP of 135 and the VIVA vane on max (Dive), banging on the purge valve caused a freeflow with or without a mouthpiece, with the vane crescent in either position (facing mouthpiece or facing diaphragm). So that means that any effects on dynamic flow are going to be more subtle than that.
I plotted out the results here (sorry, my rotameter only goes to 15 CFM). These results are without a mouthpiece restriction, so will show slightly more flow than in the water.

With the vane crescent facing the diaphragm,

you can clearly see the reg heading toward freeflow at very high flow rates. In fact, during a trial run when I ran the flow rate up too quickly, the reg freeflowed.
With the crescent facing the mouthpiece, those effects were still present, but to a much lesser degree.

What's the practical impact of this whole vane crescent discussion? Well, at moderate/heavy effort, respiratory flow can be around 4 CFM during the inspiration phase (instantaneous flow can be double that during a sudden inhale). So at 4atm (99') with this regulator, you could experience rough regulator performance under a high workload with the crescent facing the diaphragm. It might try to crossover and "blow" at you with each breath. On the other hand, ease of inhalation improves at high workload at flows short of freeflow. Either way, the effects are very subtle with this regulator.

As an aside, I was actually disappointed in the inhalation resistance of this reg, compared with past G250's. Here's a G250V I serviced in 2013 that was more like the performance I recall:

As you can see, right after cracking at 1.1", inhalation resistance drops significantly to the 0.6" range, rather than what we saw above. Just goes to show you the effects of subtle case design changes. And as a further aside, twisting the VIVA vane to PreDive only raises inhalation effort about 0.4", as you can see at that 8CFM data point. Pretty good SP design!
Here's the other extreme: a plain downstream reg of a different design (Poseidon Cyklon) with the Venturi collar too far out of position. With higher flow, breathing just gets harder and harder:

EDIT: @Kupu reminds me that this has been discussed before. And the flow test I was really looking for is here: WOB questions
This better allows you to see the effects of G250 vane position, if not vane crescent cutout position.

 

[Kupu]

After seeing the sad chart (venturi collar out of position) for the Cyklon above, I had a look at the old NEDU test charts to reassure myself that I was not dreaming regarding how much I have enjoyed a metal Cyklon second recently added to my menagerie of in use regulators. Well tuned and in recreational use, it is a nice breathing regulator.

As a lurker in this thread, thanks to all for the enlightening nuggets of information on SP regulators.