Rebuilding a sealed diaphragm first stage, how many purge cycles?
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Pretty simple. Folks sitting around during Surface Interval. One diver heard a little hiss/freeflow from his primary second stage. A quick purge and it stopped. IP checked at 140. Yes - tank had been left on. Changed tanks and did the next dive. But later on, a prolonged check showed IP creep to >155 over about an hour, and second stage breakthru. First stage serviced.
Sorry, I'm being dense or you're assuming I understand what happens to a first stage as a result of being left on. Does it damage the seat? How, exactly?
Whatever it is, I believe it happened to a HOG D1 of mine a few weeks ago. The tank had inadvertently been left on while I drove it around in the car all week. Then at the quarry I turned it on and a few seconds later the second stage began hissing. At home, I watched the IP rise until the second stage released it. So I serviced it. I took a look at the soft seat, but I really didn't know what to look for. I thought I might see two overlapping circular impressions, but I couldn't discern that.
Ah, my apologies.
No, there's nothing wrong with leaving a first stage pressurized for days on end. I've done it many times. It's not really more stressful on the seat than the percussion of thousands of breaths over many dives.
What is significant about a long period with a closed valve under pressure is the microscopic leak of air past the seat. Poseidon even has a specification for it! If a valve seal isn't perfect, as air seeps past the seal, the Intermediate Pressure downstream of the seat and upstream of the second stage valve begins to rise.
At some point, the pressure exceeds the balance capability of the second stage and it freeflows. But that immediately drops the pressure back to the designed Intermediate Pressure, and the process starts all over again.
What we're discussing is how much of that "IP creep" to allow. I'm a fan of near zero, but only because I have the time and inclination to polish the sealing part so it perfectly matches the seat. Scubapro has an allowable creep from the Mk10+ days of 8psi in the first 45 sec, and zero thereafter. But even here, "zero" further leak is a relative term, because as @Cdncoldwater pointed out, there's no reason you should leave your reg pressurized that long.
In practical terms, if IP is completely stable (after any initial "drift") for 5 min, most shops will call that reg good to go.
In terms of the problem I described above, if IP is stable for an hour after service, there are few situations where enough creep would happen to result in freeflow.
In your case, there was a leak that exceeded limits, though why it developed is not clear. With the reg on and bouncing around for a week, it's possible that the mating parts damaged each other. But that seems less likely than plain seat deterioration since the last service, that was enough to finally result in excess IP creep.
Overlapping seal marks is usually a function of a seat shift when the reg is first assembled. If the reg is assembled HP side first, the valve will be closed until the diaphragm and spring are installed. That can create one mark. When the centering pin, hat and diaphragm go on and the reg is cycled, the seat may shift due to the centering pin and a second mark is created. An IP leak from that "concentricity" problem, as it's called, usually shows up immediately.
In short, your leaving the tank on that long may have only slightly aggravated a seat that was on its last legs in the first place. Without a microscope, it's hard to discern the source of a leak. But as I've discussed in a few other threads, scratches or sandblasting of the orifice due to tank oxides is more common than usually thought. That's the first place I look for IP creep.
No, there's nothing wrong with leaving a first stage pressurized for days on end. I've done it many times. It's not really more stressful on the seat than the percussion of thousands of breaths over many dives.
What is significant about a long period with a closed valve under pressure is the microscopic leak of air past the seat. Poseidon even has a specification for it! If a valve seal isn't perfect, as air seeps past the seal, the Intermediate Pressure downstream of the seat and upstream of the second stage valve begins to rise.
At some point, the pressure exceeds the balance capability of the second stage and it freeflows. But that immediately drops the pressure back to the designed Intermediate Pressure, and the process starts all over again.
What we're discussing is how much of that "IP creep" to allow. I'm a fan of near zero, but only because I have the time and inclination to polish the sealing part so it perfectly matches the seat. Scubapro has an allowable creep from the Mk10+ days of 8psi in the first 45 sec, and zero thereafter. But even here, "zero" further leak is a relative term, because as @Cdncoldwater pointed out, there's no reason you should leave your reg pressurized that long.
In practical terms, if IP is completely stable (after any initial "drift") for 5 min, most shops will call that reg good to go.
In terms of the problem I described above, if IP is stable for an hour after service, there are few situations where enough creep would happen to result in freeflow.
In your case, there was a leak that exceeded limits, though why it developed is not clear. With the reg on and bouncing around for a week, it's possible that the mating parts damaged each other. But that seems less likely than plain seat deterioration since the last service, that was enough to finally result in excess IP creep.
Overlapping seal marks is usually a function of a seat shift when the reg is first assembled. If the reg is assembled HP side first, the valve will be closed until the diaphragm and spring are installed. That can create one mark. When the centering pin, hat and diaphragm go on and the reg is cycled, the seat may shift due to the centering pin and a second mark is created. An IP leak from that "concentricity" problem, as it's called, usually shows up immediately.
In short, your leaving the tank on that long may have only slightly aggravated a seat that was on its last legs in the first place. Without a microscope, it's hard to discern the source of a leak. But as I've discussed in a few other threads, scratches or sandblasting of the orifice due to tank oxides is more common than usually thought. That's the first place I look for IP creep.
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More than you want to do without wearing out your mouth or wasting a tank of air. Several hundred, minimum.
Alternatively, I just pressurize and walk away. If it's the same in an hour, I'm encouraged. If it's the same in 4 hours, I go to bed. If it's the same in the morning, I'm good to go.
Alternatively, if it's crept 4psi in four hours, I may try again in the morning, when I can look at 1, 4 & 8 hours. If it stabilizes after 4 hours, I'm still good. If it's still creeping another couple psi, then I have to decide whether it's worth tearing it down again to polish the orifice (which is not easy in a D1).
But isn't 120psi slightly below spec? Not really a problem for your balanced second stage, but i thought I'd mention it. Just another 1/4 turn on the hex.
(And as you know, reinstall the environmental seal with the reg pressurized.)
This tool, which is factory Apeks, new and coincidentally for sale, push, push, might help with polishing crown:
JBFG
Contributor
So how does one polished the crown on a D1? I think mine may need it when I do my rebuild this winter.
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Use the tool above? It's for Apeks DS4, DB4, DST and FST but see no reason it wouldn't work on your D.
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