2nd Stage regulators for Doubles

[rjack321]

So my smartest options are to get another S600 so that I can have greater control over free flow, or get an unbalanced R295 that I know will free flow much less than say a S555? The reliability factors on the different SP's seem to not be a big deal. Trimix is in my future, and I feel fine breathing my current unbalanced backup at depth. Is the S600 really worth the extra $300 to have for backup if I know that sill in case of an emergency, my 295 does get the job done?

Get the 295. I have a r380 (which I got years ago) and its often considered to be the worst breathing 2nd SP made. Yeah its a little stiff with nitrox at 3-4ATA but its not that big a deal. I have breathed on it extensively in cave1+2 and after the 2nd breath you won't notice its slight increase in resistance. If you were into "deep air" which you won't ever in the GUE curriculum, that minor resistance would be more relevant.

 

[RTodd]

Although this discussion has focused on why a high performance 2nd is so great - something I have not found a need for...



This is why (to me) a non-adjustable primary like the s550 is not really a great idea. You have to detune your primary or risk losing a bunch of backgas in your armpit while on a stage.

Good catch. A high performance non-adjustable second is the worst choice you can make unless you can properly detune it. Then, what is the point.

 

[DA Aquamaster]

... yes I am using Mk25s for everything. Actually two of them started out as Mk20s but have been upgraded.

The adjustable seat carrier on the Mk 25 was originally the defining trait of a Mk 25 and it is nice in terms of exactly matching IP's on different first stages in the event you swap second stages at some point, but it adds a potential failure point.

The adjustable seat carriers have had a habit of leaking in service. The adjustment screw has what looks suspiciously like a non hardening pipe dope on it and a few years ago Sp started adding another o-ring to service kits to seal between the base of the seat, the body of the reg and the seat retainer to create another layer of leak protection.

The adjustable seat carrier adds essentially nothing other than being able to tweak IP in smaller than 5 psi increments (not anything you really need) but it adds 3 more parts and uses a carrier with a hole in a high pressure area rather than using a solid seat carrier, creating one more point where it could leak. Consequently, I'm more inclined to retrograde the seat carrier to bring that portion of the reg back to Mk 20 status more less in the "remove the screen door from the submarine line" of thinking.

Not a major deal in terms oif increased risk as the leaks tend to be very small but there is no reason to "upgrade" a Mk 20 to a Mk 25 seat carrier.

 

[DA Aquamaster]

Good catch. A high performance non-adjustable second is the worst choice you can make unless you can properly detune it. Then, what is the point.

I agree, but I would extend that to any non adjustable reg as well.

With a balanced first stage and a stable IP, you can get pretty decent, almost great performance off most (not all) R190's and the same can be true to only a slightly lesser degree with the smaller diaphragmed versions (R290, R295, 380,R390). Some of those regs that come through for service are capable of exceptional performance and have to be detuned to bring the cracking effort above the minimum spec.

The reg does not really care how the valve operates, it just cares that it takes X amount of force to depress the lever to initiate flow and whether some of the force resisting that is from air in a balance chamber is irrellevent. If the reg has a cracking effort of 1.0" of water, it has a cracking effort of 1.0" of water period - and that level of performance can be achieved with many R whatevers if you tune them that way.

Now the difference with the G200, G200B, and S555 is that they are still single adjustment regs and are easily adjusted for increased cracking effort even without the knob, as the knob is just replaced with a plug that screws in and out and acts on the balance chamber just like the adjustment knob. So they are in effect much easier for a diver to adjust for cracking effort without having to mess with the orifice and the lever height.

So IMHO the choices are from best to worse:

1. An adjustable reg like the S600, G250 etc.
2. A non adjustable single adjustment reg like the G200, G200B or S555 (not readily adjustable in water but easy to adjust out of the water)
3. A non adjustable dual adjustment reg like the entire R series (except the R109, which is also a single adjustment design.)