Main spring sag...

[emoreira]

Been overhauling a couple of Atomic Z2 first stages... one had some rust pits in the main spring I didn't like the look of so got a new one.

Comparing new to old it was immediately apparent how much the old one had sagged over a decade or so!
New vs Old
View attachment 444475
New 28.50mm
Good used 27.75mm
Old 25.80mm

Well, in fact the length is rest position means little. Is the elastic constant the same for both materials and wire diameter ? One would expect the same size.
The spring is not located inside the first stage in rest position, so the IP check is the definitive measurement of the spring performance.

 

[Fibonacci]

Wire diameter was identical, but the new spring appeared to have a protective clear lacquer coating 0.1mm thick.

Typically auto valve springs have a series of specifications that can be checked with a Rimac style machine:
http://www.mwamachine.com/productline/brochures/RIMAC Brochure Spring Testers.pdf

• Free height
• Installed height
• Pressure at installed height
• Pressure at maximum travel (ie fully compressed within design travel envelope)

For some reason I've always struggled a bit with, SCUBA equipment manufacturers don't seem to list these specifications. Perhaps it doesn't matter...

It would be easy to determine the installed height (+2 shims) by measuring the first stage body and cap recesses. Then each used spring pressure could be compared against a new reference spring at installed height. Maximum travel could be measured by removing the HP seat and measuring from the end of the piston then deducting the height of the HP seat and carrier.

I don't know the root cause of the Atomic main spring failure recall in 2001 but I have just had a G250 second stage regulator main spring break 10mm from the end... a helical crack propagating from a rust pit.

Having a critical spring break in either first or second stages underwater is not going improve your day... so now I am just a bit paranoid about checking such things in every overhaul

 

[lexvil]

So, this was just an exercise for curiosity’s sake, I bought this Z2 just for the second stage but having a spare first laying around never hurts, or 10.
I picked this up today and tested it when I got home, it popped to 135 on my tank with 500 psi and did a long slow creep to 160. I’m out of service kits so I wanted to see what I could do with it sort of like it were a field repair, pulled both stages apart and cleaned them in simple green and hot water, scrubbed with a small brush then soaked in hot water with dish soap (I would of just done the soapy water stage but the second stage had a mildew smell) after rinsing I inspected each o-ring and found them all to look good and have good suppleness. I hand dried everything with paper towels, making sure no bits were left behind, lubed it with tribolube 66 lightly and put it back together leaving out the two shims. On the 500 psi tank it went to 125 and slowly creeped up to 140psi ip.

I put it on a 3500psl tank next, very crisp pop to 136 with a lock at 140, I may dive it this weekend just to see what happens.

 

[lexvil]

Pics of the parts

 

[halocline]

The creep is almost certainly caused by an imperfection on either the seat or the piston edge, or both. You could try leaving it pressurized for a couple of hours and tapping the purge a few dozen times. That might help. If you take it apart again, you can inspect the piston edge with a jewelers loupe and if find any tiny scratches, nicks or marks, try polishing the edge with a very mild abrasive. The usual fix for a seat is to just replace it, but you could look at it with the magnifier and see if you can find any imperfections.

 

[lexvil]

I’m pretty sure it’s the seat, I haven’t messed with it since leaving it on a HP tank for two days, locked and stayed at 143 for two days. Will be testing some other stuff tomorrow or I would give it a real world test.

 

[rsingler]

I’m pretty sure it’s the seat, I haven’t messed with it since leaving it on a HP tank for two days, locked and stayed at 143 for two days.

Respectfully, I would beg to differ.

The seat is soft, the knife edge is hard.
Creep is more often the result of sandblasting of the knife edge where it faces the tank intake. Easy to see with loupes.

Where there is a HP leak that locks up, the seat is confirming to some irregularity (its own, or the knife edge). The knife edge acquires one from salt crystals or dust that gets past the sintered filter over a year or two, and sandblasts the knife edge. The seat only acquires one from mishandling at service, which then shows up immediately.

When you get around to it, inspect the knife edge.

My guess.

 

[Fibonacci]

True, but in the revised Atomic J seat assembly the end of the piston is quite blunt vs the usual 'knife edge' and fits into a precision machined chamfered recess in the seat.
Supposedly more resistant to IP creep and one of the reasons for a 2 year service interval.

 

[lexvil]

I’ll open it up this week and see what’s what in there.

 

[guruboy]

Respectfully, I would beg to differ.

The seat is soft, the knife edge is hard.
Creep is more often the result of sandblasting of the knife edge where it faces the tank intake. Easy to see with loupes.

Where there is a HP leak that locks up, the seat is confirming to some irregularity (its own, or the knife edge). The knife edge acquires one from salt crystals or dust that gets past the sintered filter over a year or two, and sandblasts the knife edge. The seat only acquires one from mishandling at service, which then shows up immediately.

When you get around to it, inspect the knife edge.

My guess.

He is re-using the used HP seat though.