Post-servicing breathing resistance?

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Makhno

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Greetings.

I recently had my regulator (Scubapro MK 25) serviced, and now I'm finding that it does not breathe so easily -- it's not truly difficult to draw a breath, but there's a good bit more resistance than there should be with an expensive regulator like this one.

Of course, I'll take it back to the guy who serviced it and ask him to make it right, but I'm curious to know why this should be -- can anyone offer an explanation that's reasonably comprehensible to someone who doesn't know terribly much about the inner workings of my equpment? In my past experience, getting my regs serviced resulted in easier breathing, not more difficult. Thanks for whatever insights you can offer.
 
It's unlikely (but possible) that your difficulty is coming from the MK25 1st stage

Whatever you have for a second stage has 1 or several adjustments that are made as part of the servicing procedure. It is possible that the regulator was set with a little extra resistance to allow for the break-in of the seats. This would be to avoid having the regulator develop a free flow condition shortly after servicing. Some shops have a breathing machine to exercise and break the regulator in while it's at the shop, others just compensate in the adjustment with a best guess of where it will settle in.

Have you made a dive with it yet? I know my regulators do feel much better underwater. If it's a subtle difference that 1 dive may break it in.

It could also be an instrument problem or simple human error. I suggest calling and making an appointment to test the regulator. The tech can show you how they connect the regulator and what they use to make measurements. This way you will have some understanding of what goes on behind the curtain.

Pete
 
If the second stage is adjustable (G250, S600, X650) the tech is pretty safe is setting the orifice to allow minimum inhalation effort with perhpas 1/16 of a turn to anticipate the formation of a seating groove. In practice this results in inhalation efforts in the 1.0 to 1.1" of water range which is the minimum the diver can use anyway due to case geometry fault issues.

With a non adjustable second stage, the tech needs to be sure to fully anticipate the development of the seating groove and ensure the minimum cracking effort is enough to prevent freeflow between inhalations when the diver is in a face down positon. This usually means tuning the reg somewhere in the 1.2 to 1.4" of water range with the orfice turned in perhaps 1/12 to 1/8 of a turn past the point where any leaking stops. This conservative approach is needed as any unwanted freeflow cannot be eliminated with a diver adjustment and means the reg will come back to the shop with a usually annoyed diver. With a non adjustable and unbalanced second stage like the R190, R290, R380, R390, etc, The spring in the second stage is much stiffer and seating grooves tend to be more significant, so the regs tend to be detuned a bit more to perhaps the 1.4 to 1.6" of water range. And some of these regs will not delivert that good of performance anyway. The R190 in particular varies with some delivering excellent inhalation efforts and others doing much worse.

In some cases the orifice can be nicked and if not detected by the tech, the result is that a much higher inhalation effort results due to the excessive pressure needed to seal the damaged orifice against the seat. In other cases, the seat itself may be less than perfect with more pressure required for a good seal (This in my experience happens with about 3 to 5% of the seats in the S-wing poppet and you can usually spot the problem visually in good light.)

If the tech properly bench tests the reg, he or she will note the higher than normal inhalation effort and should then diagnose the issue - particularly on a high performance balanced second stage.

On SP's balanced second stages the design is a "Single Adjustment" design where both the spring pressure and lever height are controlled by the orifice adjustment. Good engineering and close tolerances are needed to ensure that the valve seals with proper lever height. There are some potential problems that can occur:

1. If the spring is too weak, or the seat or orifice is damaged, the orifice must be screwed in too far which sets the poppet back too far which lowers the lever excessively reducing the working range of the valve and reducing the flow rate in addition to increasing inhalation effort.

2. If the spring is to strong, the valve will seal when the lever is still too high meaning it will be depressed when the diaphragm and cover are installed, requiring the orifice to be turned in even farther to set the poppet back farther and lower the lever. This results in a normal working range, but can increase the inhalation effort more than is acceptable.

Ideally, the tech will make the initial air on orifice adjustment before the diaphragm is installed so he or she can see how things are looking without the diaphragm in place and can then know whether the valve is not sealing at a given point due to a seat/orifice issue or an excessive lever height issue. Knowing that will tell the tech whether spring replacement or an IP adjustment may be in order (higher IP helping to accommodate a slightly too strong spring while a lower IP would assist a slightly weak spring.)

A tech who has been indoctrinated to lower the IP on the Mk 25 in all cases to 120-125 psi for improved cold water reliability (a very negligible improvement and in my opinion very bad, or at least incomplete, advice by Scubapro) could potentially create this problem as in effect with a reg with a spring that is already on the edge of being excessive, the spring pressure would now be excessive requiring a deeper set orifice, lowering lever height and reducing working range in addition to increasing inhalation effort.

The non adjustable unbalanced SP regs are a "dual adjustment" design where orifice and lever height are independent of each other. In most cases (no dirt corrosion, salt, etc, present) leaving the spring,seat stem and inner part of the inlet fitting together and ulta sound cleaning it as a unit once the seat and orifice end of the valve has been removed is preferable as in most cases the new seat will be the same thickness as the old seat and will deliver maximum performance with no need to adjust the lever or adjust (and consequently replace) the lock nut. When possible, I think it makes sense to have parts that have been working happily stay together rather than deal with the issues and wear that arise when they have to reorient themselves to each other. However in cases where the inhlation effort is badly off from what it should be or where excessive lever adjustment is required, complete assembly and diagnosis of potential problems is in order.
 
Thanks for the input.

The primary 2d stage is x650 and I can't recall off the top of my head what the alternate is.

I've done 18 dives with it since servicing -- all within just a few days as I went to the Similan Islands for a liveaboard trip. The technician warned me that I might need to bring the reg back to have the 2d stage fine-tuned a bit if it freeflows (which in fact it is doing), but I've noticed the ncreased breathing resistance from both the primary and the alternate 2d stages which leads me to think it's something funky going on in the 1st stage.

Anyway, I'll be finished with my travels and back home in a couple weeks, and hopefully the technician will get everything sorted out to my satisfaction then.

Cheers,
m
 
The existence of the free flow does suggest the second is more than willing to deliver air.

Does the problem get worse at deeper depths? That could suggest that the IP is not compensating.

Pete
 
Free flow and hard to breath suggests that the cracking pressure is too light (orifice not screwed in enough and/or IP is too high) and the 2nd stage spring pressure is too high. A rather ugly combination of adjustment errors.

The start point for correcting this is to get an IP gauge on it to see if the 1st stage is part of the problem.
 
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