SP G250 Graphite as backup 2nd

[Lobzilla]

Thanks everyone for the helpful input.

I got the G250s because they are well liked and respected.

What concerned me was that GI3 mentioned on an earlier DIR tape that lower performance, unbalanced regulators should be used as backups to prevent free flow.

I rented a ScubaPro R-series regulator and it could not hold a candle to the G250 even in shallow water.

My hope was that the G250 can be 'desensitized' enough to fit the role of a backup/stage/deco regulator.

Thanks for verifying this.

 

[redacted]

If unbalanced is really important, you could replace the balanced "guts' with unbalanced components, same as the G200 and G200B, and you would be good to go. The performance difference would be difficult to detect, and there are a number of small advantages such as a simpler design and less expensive service parts.

 

[Lobzilla]

If unbalanced is really important, you could replace the balanced "guts' with unbalanced components, same as the G200 and G200B, and you would be good to go. The performance difference would be difficult to detect, and there are a number of small advantages such as a simpler design and less expensive service parts.

The way I understand the balanced system is that the supply pressure acts on both sides of the valve. Therefore, any change in supply pressure will cancel out.

That is a huge advantage for 1st stages as the supply pressure goes from >3500psi to <100psi in the worst case.

However, I stuggle with understanding the benefits of an un-balanced 2nd stage. If the IP creeps, the un-balanced 2nd should freeflow sooner (if the balancing works like on a 1st stage). The balanced 2nd may allow the low pressure hose to blow first (if the balancing works like on a 1st stage).

Maybe the notion of preferring unbalanced 2nd stages came from the unbalanced design needing a higher spring pressure to prevent freeflow in the high-limit IP case. The inherently more conservative spring pressure would make the un-balanced 2nd less sensitive. But that can be rectified in a balanced 2nd stage by jacking the spring pressure up, as suggested above.

The reason I want to use one regulator type for primary/backup and possibly even for stage/deco is logistic simplicity. If possible, I would not want to worry what 'guts' are inside. The other reason for using the same everywhere is that it will be immediately obvious if one goes out of specs before the service. (When I first breathed out of a non-G250 I thought the thing was broken, not immediately realizing that it was a different beast).

From my own experiments and what you guys suggested, I am now pretty confident that a stock G250 can be prevented from free-flowing and from annoying the hell out of a GUE instructor; and GI3 has more important things to do than sending lightning bolts down on me. My whole concern was really more about DIR compliance.

 

[redacted]

The way I understand the balanced system is that the supply pressure acts on both sides of the valve. Therefore, any change in supply pressure will cancel out.

That is a huge advantage for 1st stages as the supply pressure goes from >3500psi to <100psi in the worst case.

However, I stuggle with understanding the benefits of an un-balanced 2nd stage. If the IP creeps, the un-balanced 2nd should freeflow sooner (if the balancing works like on a 1st stage). The balanced 2nd may allow the low pressure hose to blow first (if the balancing works like on a 1st stage).

Maybe the notion of prefering unbalanced 2nd stages came from the unbalanced design needing a higher spring pressure to prevent freeflow in the high-limit IP pressure case. But that can be rectified in a balanced 2nd stage by jacking the spring pressure up (as suggested above).

Like I said, the benefits are small. The balanced 2nd uses 2 additional o-rings (odd size can be hard to find) to seal the balance chamber (actually 1 seals and the other protects that ring from damage by dirt - they are quite small) and a more expensive seat that can also be harder to come by. (I make my own seats for the unbalanced version for pennies each.)

I almost always dive a balanced primary and an unbalanced alternate. With well tuned regs, the performance difference is hard to detect. It is not that I would recommend "downgrading" your G250 but it can be done. I assembled the components necessary to "downgrade" my Balanced Adjustables only because I did not have a reliable source for the LP seats they require.

 

[halocline]

However, I stuggle with understanding the benefits of an un-balanced 2nd stage. If the IP creeps, the un-balanced 2nd should freeflow sooner (if the balancing works like on a 1st stage). The balanced 2nd may allow the low pressure hose to blow first (if the balancing works like on a 1st stage).

Maybe the notion of preferring unbalanced 2nd stages came from the unbalanced design needing a higher spring pressure to prevent freeflow in the high-limit IP case. The inherently more conservative spring pressure would make the un-balanced 2nd less sensitive. But that can be rectified in a balanced 2nd stage by jacking the spring pressure up, as suggested above.

The basic difference between the barrel poppet balanced (G250) and unbalanced is that in the balanced 2nds, there is a small hole in the seat which allows air to travel through the poppet to a chamber, where it pushes back against the end of the poppet, increasing force holding the seat closed. This means that instead of a heavy spring which entirely opposes the air pressure trying to push the seat off the orifice and thus open the valve, you can use a light spring, which works in conjunction with the air pressure in the balance chamber to oppose the IP. Theoretically, both balanced and unbalanced barrel poppet regs can be set to identical cracking effort, but when the reg opens, the IP drops, meaning that the pressure in the balance chamber drops, and therefore the pressure against the poppet drops. So, while the cracking effort is the same, the force needed to keep the valve open is lower. This is what (theoretically) makes balanced 2nd stages have better breathing characteristics, along with the obvious advantage with less-than-perfectly stable IP. As awap said, in the real world, there is a pretty small difference between otherwise identical balanced and unbalanced 2nd stages.

The problem in your situation is that you're not comparing identical 2nd stages. The R190 (0r 380, or 295, etc) is not a barrel poppet design and IME does not have the smoother breathing characteristics of the barrel poppet regs. I have no idea why this is, except the only unbalanced barrel poppet reg I've ever used is the old metal case 109 (same as awap) and those are exceptionally smooth breathers. With one of those you could also get adjustable and unbalanced, which means you can dial out a slight freeflow; a nice feature for an octo that spends most of it's life outside of your mouth.

But the big advantage IMO of the balanced design is the lower rate of wear on the seat. Yes, you can store the R190 with the purge slightly depressed, but I still find that the balanced seat just lasts longer and is less prone to flowing.

Maybe the down and dirty bottom line is that it doesn't really matter; a decent quality unbalanced 2nd stage will work just fine as an alternate as will a G250. I would not go out of my way too much to avoid either choice.

Unlike awap, for whom spending the princely sum of $3 on a balanced LP seat is an unbearable expense, (he even flips his 1 penny seats to make them have an effective price of .5 cents) I am recklessly willing to ruin my financial future and throw away my hard earned money on actual SP seats once every 2-3 years. So, most of my 109s have been converted to balanced.