SP G250 Graphite as backup 2nd

[Lobzilla]

I am in the process of changing to a bungeed 2nd stage.

1st stage is MK17 and primary 2nd a G250

Is it wise to use another existing G250 for the backup 2nd.

If yes, I assume it is breathing effort screw 'out' and VIVA to MIN for the backup.

 

[DA Aquamaster]

It will be perfect for a backup.

 

[Lobzilla]

Thanks.

 

[Couv]

.....but I would screw in the knob to increase cracking pressure in order to avoid free flow.

 

[Lobzilla]

.....but I would screw in the knob to increase cracking pressure in order to avoid free flow.

Thanks for catching that. I was thinking about increasing the spring pressure and cracking pressure but typed the wrong thing. Righty tighty, lefty loosy.

But what is the deal with the VIVA setting? According to ScubaPro's advertisement this is supposed to assist inhalation. In shallower water I cannot detect any WOB difference between the MIN and MAX setting. Does that become effective when the air density increases in deeper water?

I guess, if I increase cracking pressure by turning the screw IN I could leave the VIVA at MAX and not have to be concerned about freeflow. The VIVA setting is pretty hard to change under water, especially when the reg hangs around your neck.

Come to think about it, once the backup reg is filled with water it should be pretty tame anyway. And if it goes nuts on entry, big deal.

Is there no owners manual on the ScubaPro site or did I just not look in the right places?

 

[LeadTurn_SD]

Thanks for catching that. I was thinking about increasing the spring pressure and cracking pressure but typed the wrong thing. Righty tighty, lefty loosy.

But what is the deal with the VIVA setting? According to ScubaPro's advertisement this is supposed to assist inhalation. In shallower water I cannot detect any WOB difference between the MIN and MAX setting. Does that become effective when the air density increases in deeper water?

I guess, if I increase cracking pressure by turning the screw IN I could leave the VIVA at MAX and not have to be concerned about freeflow. The VIVA setting is pretty hard to change under water, especially when the reg hangs around your neck.

Come to think about it, once the backup reg is filled with water it should be pretty tame anyway. And if it goes nuts on entry, big deal.

Is there no owners manual on the ScubaPro site or did I just not look in the right places?

Disclaimer: I don't own a G-250. I do however use a Scubapro R-109 Adjustabe as a backup, stage reg, pony reg.... ditto with a couple Zeagle ZX (these are also balanced, adjustable barrel poppet regs).

What I do when using these regs as "octo's" is to slightly "detune" them with the adjuster knobs just enough so that I can't make them freeflow by sharply tapping the purge.... I have these regs tuned so that they will freeflow strongly on dry land at their easiest setting if I tap the purge quickly (they have a strong venturi effect, coupled with low cracking pressure = easy freeflow if not handled carefully).

Like you said, once full of water they are nice and stable. Detuning with the adjuster just makes them a bit less touchy on the surface.

With my non-externally-adjustable "octos", I just handle them more carefully (I do not detune my non-adjustable octo's... I tune them about the same as my primary 2nd stages).

Best wishes.

 

[PfcAJ]

The backup (ideally) needs to be a non-balanced reg. Simple, reliable, doesn't freeflow, and not finicky at all.

I've got a bucket full of g250v's, g250hp's, s600's, 390s and 190s. Time and time again, its the 190/390s that last the longest without issue. This is an ideal quality to have in a backup reg.

 

[halocline]

The backup (ideally) needs to be a non-balanced reg. Simple, reliable, doesn't freeflow, and not finicky at all.

This is simply not true; it's an older DIR idea, based on the mistaken idea that unbalanced 2nds are somehow more reliable than balanced ones. There is a theoretical advantage to having an unbalanced alternate in the sense that it will be more sensitive to IP changes, and as such, would theoretically free flow first in the event of an IP spike. However, serious IP spikes in high quality 1st stages that are serviced properly are extremely rare.

The regulator that is the subject of this thread, i.e. G250, is every bit as reliable, in fact more stable IME, than the R190. Seat wear/imprint is less of an issue because the spring is so much lighter. It's also adjustable, which means a diver can compensate for slight IP fluctuations, seat wear, or conditions.

 

[Zung]

+1
And let's make it real simple: have it, use it.
The G250 is one of the greatest things on earth, and it's the only 2nd we use, even though I'm messing with a R109 as a continuing research project.

 

[Couv]

Hello Lobzilla,

The adjustment knob and flow vane both change breathing resistance, but do so in different ways.

The knob experts pressure on the poppet spring which increases the cracking pressure. Cracking pressure is the force required to allow an opening between a soft seat and hard seat to let air into the second stage box. Cracking pressure is usually measured in inches of water. Most second stages are tuned to have a cracking pressure of approximately 1.5 inches give or take a little. One of the advantages of a regulator like a G250 is that is can be tuned at service time for minimal inhalation resistance, but "de-tuned" by the diver with the adjustment knob.

The vane (I'll let Luis explain the Venturi/Bernoulli effect and principles) provides an obstruction to the flow when it's perpendicular to the air flow. This causes a slight back pressure in the box and "fools" the air delivery mechanism into "thinking" there is slightly less demand. This is helpful in preventing freeflow both above and below the water. When the vane is in line with the airflow, the velocity of the air creates a low pressure area in the box and assist the muscle (diaphragm) push on the demand lever lowering the breathing effort.

The G250 will serve you well either as a primary or backup because it is so versatile.

Clear as mud?

Couv