Unexpected IP drift in a newly-serviced first stage
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With his occupation, I'd prefer his obsession on perfection....
Probably some ham fisted tech screwed up the last service. Send it out for some quality repair.
Geo7
Contributor
This sounds plausible, and I am suspecting the contamination from the compressor may not exit the fill line constantly. It is possible that the first tank of a batch filled may receive more contamination than later ones of the same batch or day. The scenario I am thinking of is that the compressed air is constantly cleaning the fill lines, but humidity does it's think over night, and the next morning the first tank gets a blast of particulate rust etc?
OP
I realize that months after taking apart the offending Atomic during our last Seminar, I never reported back on the results!
As I am now coming to expect with piston regs, the U-turn that high-speed gas makes as it reaches the knife edge makes it particularly susceptible to particulate damage compared with the 90-degree turn that diaphragm regs have, or the shallow S-turn that high pressure gas makes in a Poseidon XStream.
Recall my problem:
Newly serviced regs with crisp lockup and no drift/no creep. Go to Indonesia for bucket list liveaboard trip and come back 39 dives later with 8 psi of drift from what had been a perfect valve. WTH?
Tore down the reg and the seat was fine:
But the piston was a different matter.
The side of the shaft facing the tank opening was clearly different from the other side.
And in one particular spot, some piece of grit clearly made it past the sintered metal filter and gouged the knife edge as high speed gas made its U-turn. This was NOT there before the trip!
As is my habit with virtually every piston at service, I spent about 10 minutes with Micromesh in grits from 4,000 to 12,000 dressing the shaft. Here's how it looked after 10 min work:
Back on the gauges, and lockup was once again crisp.
As for the cause, I think others have suggested likely possibilities. Boat compressor with uncertain filter status; first fill of the day? Trash in the line that my tank got first?
My wife's identical set was unfazed by the trip, and still locks up fine.
But I think there's alot more to "300 dive/3 year service interval" than manufacturing quality.
I never expected this after 39 dives, and it reinforces my standard practice of checking IP before the first dive of the day.
Was this unsafe? No. It wasn't IP creep, that continued past specification if the pressurized regs were left untouched. It was IP drift that sealed up 8 psi higher, with a little more pressure on the seat. Breath to breath, I was still breathing my regs at the same IP the seconds were tuned to. But the first breath after the tanks were pressurized for the morning pre-check on the way to the dive site was at a higher IP. If my drift had been worse, and if my second stage were an unbalanced reg, I might have bled off half my gas before I splashed.
I wish I could say I don't see IP drift (or frank creep) very often when I service pistons. But I do. And I think it's stuff like this. Tank particulates/oxides flaking off from the tank wall and getting into the air stream and past the sintered filter has a consequence. It's just one more reason why my Mk19EVO or my Deep6 Signature are my go-to first stages these days. I was a piston guy for 20 years, but have gradually come to see the value of the other side. And as @Luis H pointed out to me recently, it's not the huge bore of the piston shaft that makes a piston better than a diaphragm with a tiny opening. It's the valve lift multiplied by the circumference of that bore. That is the area that matters in terms of maximum gas delivery.
That's one reason why the Mk19EVO can have a 250 SCFM max flow (if it had a tank valve capable of providing it), compared to the Mk25EVO's 300 SCFM, despite a much smaller opening (AND a pin in the middle of it). Valve lift is probably higher.
Anyway, all's well that ends well. But among folks that say "all my regs lock up just fine, and I haven't serviced them for years", there are probably not a few folks that aren't looking closely enough.
As I am now coming to expect with piston regs, the U-turn that high-speed gas makes as it reaches the knife edge makes it particularly susceptible to particulate damage compared with the 90-degree turn that diaphragm regs have, or the shallow S-turn that high pressure gas makes in a Poseidon XStream.
Recall my problem:
Newly serviced regs with crisp lockup and no drift/no creep. Go to Indonesia for bucket list liveaboard trip and come back 39 dives later with 8 psi of drift from what had been a perfect valve. WTH?
Tore down the reg and the seat was fine:
But the piston was a different matter.
The side of the shaft facing the tank opening was clearly different from the other side.
And in one particular spot, some piece of grit clearly made it past the sintered metal filter and gouged the knife edge as high speed gas made its U-turn. This was NOT there before the trip!
As is my habit with virtually every piston at service, I spent about 10 minutes with Micromesh in grits from 4,000 to 12,000 dressing the shaft. Here's how it looked after 10 min work:
Back on the gauges, and lockup was once again crisp.
As for the cause, I think others have suggested likely possibilities. Boat compressor with uncertain filter status; first fill of the day? Trash in the line that my tank got first?
My wife's identical set was unfazed by the trip, and still locks up fine.
But I think there's alot more to "300 dive/3 year service interval" than manufacturing quality.
I never expected this after 39 dives, and it reinforces my standard practice of checking IP before the first dive of the day.
Was this unsafe? No. It wasn't IP creep, that continued past specification if the pressurized regs were left untouched. It was IP drift that sealed up 8 psi higher, with a little more pressure on the seat. Breath to breath, I was still breathing my regs at the same IP the seconds were tuned to. But the first breath after the tanks were pressurized for the morning pre-check on the way to the dive site was at a higher IP. If my drift had been worse, and if my second stage were an unbalanced reg, I might have bled off half my gas before I splashed.
I wish I could say I don't see IP drift (or frank creep) very often when I service pistons. But I do. And I think it's stuff like this. Tank particulates/oxides flaking off from the tank wall and getting into the air stream and past the sintered filter has a consequence. It's just one more reason why my Mk19EVO or my Deep6 Signature are my go-to first stages these days. I was a piston guy for 20 years, but have gradually come to see the value of the other side. And as @Luis H pointed out to me recently, it's not the huge bore of the piston shaft that makes a piston better than a diaphragm with a tiny opening. It's the valve lift multiplied by the circumference of that bore. That is the area that matters in terms of maximum gas delivery.
That's one reason why the Mk19EVO can have a 250 SCFM max flow (if it had a tank valve capable of providing it), compared to the Mk25EVO's 300 SCFM, despite a much smaller opening (AND a pin in the middle of it). Valve lift is probably higher.
Anyway, all's well that ends well. But among folks that say "all my regs lock up just fine, and I haven't serviced them for years", there are probably not a few folks that aren't looking closely enough.
Geo7
Contributor
Excellent analysis!
There is something to be said about a titanium flow through piston 1st stage that is subjected to contamination and yet won't need servicing until returning home. Not even humidity inside would be a concern for ruining that kind of reg.
There is something to be said about a titanium flow through piston 1st stage that is subjected to contamination and yet won't need servicing until returning home. Not even humidity inside would be a concern for ruining that kind of reg.
Wow,....just when I thought I was beginning to get a grasp on diving principles and equipment! 
The Equipment Specialist class is very basic, compared to your description of your findings! I still have a whole lot of learning to do!
The Equipment Specialist class is very basic, compared to your description of your findings! I still have a whole lot of learning to do!
AKPDB
Registered
OP
I have mixed feelings about AA's Monel piston. While it is a "submarine" metal, supposedly much more resistant to moving salt water corrosion than other stainless steels, it is definitely susceptible to corrosion under trapped salt water. There's a metallurgical term for that, that I can't recall, but hopefully an expert can enlighten us.
Here's an example of a Monel piston from a guy in the tropics who regularly got wet fills or was sloppy at tank changes. This is very disappointing.
The knife edge holds up well, and can be dressed easily, but I am replacing the piston with SS in a couple of my Atomics to see how it compares after a few years. Since I'm pretty meticulous about water in the intake during tank changes, I've never seen this in my regs, so I may never get a good comparison. But I'd be interested in a SS substitution for someone in the Caribbean or Hawaii, where corrosion like this seems to appear in my shop regularly.
You can check it yourself more easily. Just pop off the rubber bumper under your HP end and look at the Monel seat carrier. I commonly see green corrosion there from trapped salt water.
Maybe I'll switch in a SS seat carrier and see how it looks after a year.
To answer your specific question, I don't see enough SS pistons compared with Monel to give you an abrasion resistance answer. But if you own an Atomic with a SS piston, just watch for IP drift. If it doesn't start to appear, you either have really clean gas and tanks, or maybe SS is better in this regard. Time will tell.
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