My unbalanced 2nd has less inhalation resitance versus my balanced 2nd. Why?

[Leicamshooter]

I've been following the wonderful recent thread on "what is a balanced 2nd." I have a Scubapro unbalanced R108 serving as my Octo, and a R109 adjustable that I just upgraded to a balance type (R156) by my LDS. On a Alum 80 with 2000 PSI, the R108 Octo seems to have less inhalation resistance, despite the fact that my primary 2nd is now balanced & fully open. With the help of a member, whom I feel makes great insightful contributions to scubaboard, I've adjusted the balanced second to be as light as possible without free flowing. It helped, but my Octo still offers a little less resistance. Will the slight difference in inhalation resistance go away when I'm actually at depth, and not in my garage? By inhalation resistance, I’m referring to the initial cracking effort to get air flowing.

Thank you,
G Medina

oops, mispelled resistance in heading!

[NetDoc]

Check it on a magnehelic. Cracking pressure is really hard to discern, ESPECIALLY if the reg is over compensated to shove that air down your throat once it starts. I don't like regs that do this.

[DA Aquamaster]

The Scubapro "High Performance" second stage is numbered as the 080, but seems to be frequently referred to as the "108". I don't know where that comes from other than the next second stage SP introduced was the "109". Of course, the name "High Performance" never made much sense either as it was SP's basic first stage and did not have as high of performance as the 109.

In any event, the 080 is still capable of very good inhalation performance as it is a very smooth breathing regulator (compared to the crappy plastic entry level second stages made today) and is easy to adjust to a very fine level of inhalation performance and that level of performance is useable in the water due to reasonably good case geometry fault issues. It is not uncommon for one to function reliably with no tendnecy to freeflow at cracking efforts as low as 1" of water and it has a moderate venturi effect that requires little or no sustained inhalation effort without forcing air into you.

It's weakness is that it is not very tolerant of intermediate pressure changes. For exmaple if you tune one being used on an unbalanced first stage for the lowest possible inhalation performance on a low pressure (2250 to 2400) psi steel tank and then place it on a full 3000 psi AL 80, it will freeflow due to the higher tank pressure and higher resulting IP.

Similarly, if tuned for minmum inhalation effort at 3000 psi, it will breath significantly harder at 500 psi due to the lower IP that results with that lower supply pressure feeding an unbalanced piston first stage.

When used as an octo on a balanced first stage with very stable IP, the 080 will provide low inhalation effort regardless of tank pressure.

The R156 on the other hand, all things being equal, should be capable of the same 1.0" of water inhalation effort and will do it across a much wider IP range with very little degradation in inhalation effort. So I suspect that not all things are equal.

The problem may be a worn or damaged orifice that cannot be tuned to the same low cracking effort. Or it may be an issue with more mechanical resistance due to a corroded lever or an older lever that is not fully compatible with the new S-wing G250/S600 poppet. If the tabs on the lever are too large they drag on the poppet and cracking effort is increased.

It may just not be properly adjusted. Ideally, the 156 should be adjusted so that it is either just short of or has a slight freeflow when the adjustment knob is all the way out. If however, the tech adjusts the reg for an inhalation effort in the 1.2" to 1.4" of water range with the knob out, it cannot be diver adjusted for the lower performance that it is capable of acheiving.

[H2Andy]

i would say it has to do with adjustment, maintenance, and parts issues

for example, the balanced reg may have been improperly adjusted, or needs maintenance, or has some parts that are worn out

[halocline]

Will the slight difference in inhalation resistance go away when I'm actually at depth, and not in my garage?

Unless your garage is filled with water (which might not be a bad thing for a landlocked diver) then I take it you have not checked this while actually diving, right? I'm pretty sure comparisons on land are not very reliable. You could have the LDS that rebuilt the balanced adjustable have a look at the unbalanced stage; maybe they can make that one breathe harder!

[DA Aquamaster]

Unless your garage is filled with water (which might not be a bad thing for a landlocked diver) then I take it you have not checked this while actually diving, right? I'm pretty sure comparisons on land are not very reliable. You could have the LDS that rebuilt the balanced adjustable have a look at the unbalanced stage; maybe they can make that one breathe harder!

Actually since it is an octo, he is probably lucky they did not set it up to breathe harder. it is not uncommon for some tech to se tthe octo up to breathe with a minimum cracking effort of 2.0 to 2.5" of water. The idea is to prevent them from unintentionally freeflowing, but it makes them breathe like rocks.