Mk10 creep

[Open Ocean Diver]

I rebuilt my regulator, and noticed with a tank filled to 3200 psi the IP will creep to 165 in a about 5-10 min, at 2000 psi it will lockup at 142 with maybe 2 psi creep in less than a minute. Don’t understand why it would creep 25 psi with higher tank pressures. Was thinking seat but why would it lock up nicely at lower tank pressures. At 300 psi it locks up at 126.

Any ideas before I open it up again?

 

[rsingler]

Yes. It's knife edge and seat.
Think of the area of the impression of the "knife" edge in the seat. These are very rough numbers but let's call it 0.26" diameter x .06" wide. Using the formula pi x D for circumference, that means that the area contacted by the knife edge is
3.14159 x 0.26" x 0.06", or 0.049 sq in.

With an IP of 145, 145/.049 = 2960psi
So with a tank of 3000psi, the seal will leak.
With a tank of 2000 psi, the seal will hold.

With a leaking seal, the IP creeps until the pressure on that tiny area of contact with the knife edge equals or exceeds the tank pressure.

At 160psi IP, 160/.049 = 3265psi, and the seal holds.

Rough numbers.

 

[buddhasummer]

No idea, but looks scientific so must be right.

 

[lexvil]

I thought this was a thread about couv.

 

[buddhasummer]

I thought this was a thread about couv.

That's some funny sh.it.

We love you @couv

 

[Couv]

Is that because you think I'm an "Expert" or because I fail under pressure?
An Ex is a has-been.
A Spurt is a drip under pressure.

Me thinks my friend @rsingler did one hell of a good job at EX-plaining the issue. It would be helpful if he would provide a link to his equally well explained knife edge dressing post.

Edit: He's on the left coast and still asleep right now-here's the link:
How to Restore a Knife Edge Piston

BUT before the op does all of that, try this:
Remove the seat retainer (leave the the seat in the retainer) and thoroughly clean the sealing surface of the seat with a lint free cloth and alcohol. Do the same to the piston knife edge.

Let it dry; put the retainer back in place and try again. If no joy it remove the piston and give it the Dr. Rob treatment.

 

[Open Ocean Diver]

Yes. It's knife edge and seat.
Think of the area of the impression of the "knife" edge in the seat. These are very rough numbers but let's call it 0.26" diameter x .06" wide. Using the formula pi x D for circumference, that means that the area contacted by the knife edge is
3.14159 x 0.26" x 0.06", or 0.049 sq in.

With an IP of 145, 145/.049 = 2960psi
So with a tank of 3000psi, the seal will leak.
With a tank of 2000 psi, the seal will hold.

With a leaking seal, the IP creeps until the pressure on that tiny area of contact with the knife edge equals or exceeds the tank pressure.

At 160psi IP, 160/.049 = 3265psi, and the seal holds.

Rough numbers.

OK don’t kill me here, but I made a mistake it’s not my MK10 it’s actually the NOS MK5 just recently bought.

The math makes sense but I knew it would be the opposite of common sense.

So I opened it up and guess what I have a Nick in the knife edge.

I bought it off eBay, and the guy interestingly enough said he replace the O-rings and wanted me to be sure that he used a bullet when replacing the piston internal O-ring.

Its a beauty with a crappy piston. Ugh

 

[Couv]

...well it's off to Singler's School of Sharpening for your Mr.

 

[halocline]

Rob that's a great explanation but I imagine the knife edge width as it cuts into the seat is way less than .06". It might be .006 (one tenth as wide) but probably less than that. A guitar string has a diameter of .010 (the thinnest one) and it's far bigger than the groove cut by the piston.

Anyhow, since the OP found the nick, which is causing a leak, there's your answer to the existence of the IP creep. Why it's much worse at high tank pressure is probably just because the pressure gradient on either side of the piston is more severe when the tank is filled, so there's more force pushing the air through the leak. air leaks into the piston shaft, it causes IP to rise, which then pushes the piston further into the seat. Eventually it pushes it in far enough so that it seals despite the nick. My guess is that if you did nothing, the piston might form enough of a groove in the seat to seal better after a few dives. That's just a guess, though. I have cleaned up the knife edge on many pistons using Rob's method and it works great.

 

[Couv]

(I remember the days when we called it Matt Magic.)