Scubapro backup reg.

[davebe]

I plan on using a bungeed backup (necklace) for a Scubapro mk17/s600 setup, and am trying to decide which Scubapro reg. to use. I’d like to go with the smaller profile, partly because it’s hanging on my neck, but the r295 has replaced the r390, and it doesn’t have the venturi switch, which I’d also like. So I’m thinking of using the s555 for the backup, but then got to thinking that maybe it makes sense to use a s600 for backup since I could then use the cracking pressure knob to manually detune it. Is that a reasonable approach, or am I spending extra money needlessly?

Seems like I read somewhere that it’s not good to leave the cracking pressure knob in the closed (detuned) position because it can create a seating groove, which will then cause the reg. to leak when I turn it to the open position. I’m also concerned that perhaps the s555 or s600 might be a bit sensitive to free-flow. If I detune the s600 using the knob, or have my LDS detune the s555, will they be just as free-flow resistant and reliable as an r390 or r190?

For the foreseeable future, I’ll be doing recreational diving. Also, I’ve decided against using a combined inflator/regulator, at least for now. I've read the numerous related threads on backup regs and octos, but am left with the above questions.

Thanks,
Dave

 

[DA Aquamaster]

I'd go with a backup second stage that performs as well as your primary as in an emergency you will be the one using the backup and it needs to preform in any condition where you would have your primary.

You are correct that you can detune the S600 using the inhalation adjustment knob and this makes it preferrable to the S555. Both are preferrable to the lesser performing unbalanced R190/R380/R390/R290/R295 second stages.

As for a seating groove forming, that is a possibility with any second stage where the spring pressure is not fully released between dives. The best approach is to turn the inhalation adjustment knob all the way out to minimize this pressure when the reg is stored between dives.

Scubapro incorporated a system that fully removed spring pressure anytime the reg was not pressurized, but that introduced more complexity and more problems than it resolved so the system was short lived. In practice, the formation of a seating groove does not cause a problem as long as you have the reg serviced every year or so.

 

[davebe]

DAA -- I really appreciate your informed responses.

What do you recommend for the adjustment range of the inhalation adjustment knob? I saw elsewhere you suggested a cracking effort adjustment range of about .6 to 2.5 inches of water for the primary reg. Do you suggest the same for the backup?

 

[DA Aquamaster]

.6 is a little light for most purposes as it will free flow in a face down position due to the design of the case and the location of the exhaust valve and diaphragm. But 1.0 is pretty bullet proof in almost any position.

I'd set your back up to breathe at 1.0" to 1.2" of water with the knob all the way out and it should then be fine with the knob in a couple turns.
For the most part once the case of the reg is flooded, it does not matter too much where the inhalation effort is set.