Intermediate pressure drift or creep?

[Fishyhead]

I bought a couple of Scubapro MK10 and G250s to take a part and learn how these things work then rebuild them and use.

Since I'm still waiting for tools and replacement parts, I decided to hook an IP gauge to one of the MK10s and observed the following:

After purging, the gauge stops at roughly 135 psi - sometimes just below, sometimes just above but most of the time at 135.

If I let it sit, the pressure seems to rise up a couple of psi in 15 minutes but hasn't gone past 138.

Is this considered IP drift or creep? I did the same test on my USD Conshelf and the pressure is a steady 140 (just serviced last year).

 

[tbone1004]

drift stops, creep doesn't. That's IP drift and it's not something to worry about.

 

[GF99/99]

drift stops, creep doesn't. That's IP drift and it's not something to worry about.

As tbone has mentioned this is nothing to worry about. Most manufactures actually even expect this to happen and list an allowable amount of drift in there service manuals (sorry I'm not a MK10 owner/user so I don't have this manual but I'm sure another SP savvy user might be able to point you in the right direction)

Nice article here about IP and difference between drift and creep Measuring intermediate pressure

 

[Fishyhead]

Thank you. That's the article I was reading when I made the post. I just wasn't sure if the drifting was acceptable because I always assumed that the pressure should hold steady and if there was movement that there was something wrong.

PS, prior to that article, I had only heard of "IP creep" so thank you @tbone1004 for clarifying.

 

[halocline]

I have always thought of IP drift as an IP that locks up at a slightly different PSI each time the valve opens and shuts. It's sort of an unstable IP, it's not uncommon on some older regs with worn out main springs. IP creep is the slow increase of IP after the valve shuts. Whether it stops or not is only a matter of how severe the creep is. Most Mk10s in my experience creep a few PSI. MK10+ tend to creep more.

That distinction between IP drift and creep comes from the Vance Harlow book. I know in some SP-produced literature, they use the word 'drift' to describe mild IP creep, but I personally think they're using it a bit euphemistically. They just don't want to admit that their res creep, even a little.

 

[Fishyhead]

The IP on one of my MK10's (the one that also drifts) seems to lockup at different pressures. Most of the time its 135 psi but sometimes its less, sometimes its more. I'll rebuild this one first and swap spring if it the lockup pressure if it continues to vary while cycling.

 

[halocline]

The IP on one of my MK10's (the one that also drifts) seems to lockup at different pressures. Most of the time its 135 psi but sometimes its less, sometimes its more. I'll rebuild this one first and swap spring if it the lockup pressure if it continues to vary while cycling.

I would also have a close look at the knife edge on the piston, using some sort of magnification like a jeweler's loupe. If there are any imperfections, you can carefully clean it up with micromesh. Rsingler posted a good explanation of the process. When I've done it, mostly I just wrap the micromesh around a pencil tip to get a conical surface and then polish the inside of the piston. While you're at it, take any glaze off the piston shaft where it goes through the HP o-ring. This helps it hold some lube and reduce the friction at that o-ring, which can be a problem with MK10s at higher tank pressures.

Another thing to keep in mind is that I have found that if I install the seat and pressurize, then take the reg apart and try to re-use that seat, it never locks up cleanly. The piston cuts a circular groove in the seat and it's very difficult to get it to line up just right when you put it back together. If you're going to change the spring, I would do the whole rebuild at once, new seat, new HP o-ring (90 duro is best), cleaned up piston, put it back together and see what happens. I'll bet it's much more stable.

 

[Fishyhead]

I was going to do the rebuild with the existing spring to see if the IP is stable then replace the spring if it isn't. Do you guys replace the springs as a part of a rebuild or just as needed? What about shims? I'm assuming that if I change the spring, the shim configuration may yield different results than with the original spring, correct?

I'll definitely take a look at the knife edge of the piston before putting it all back together.

 

[halocline]

I was going to do the rebuild with the existing spring to see if the IP is stable then replace the spring if it isn't.

The problem with that is you will need to disassemble the reg to install the new spring, and then you will probably want to use another new seat because of what I said in my prior post about re-installing seats in the MK10. So if you have plenty of seats, fine. But since the IP is jumping around a little, I would just replace the spring when you rebuild and see if that helps. You'll have to find a NOS spring for the MK10. I had some, but I don't any more. Rsingler might have one for sale.

If you don't have a new spring, I wouldn't worry too much about it. My experience is that when the springs wear out, it's tough to get the IP up enough to be in spec. Yours is plenty high at 135. If you can fix the slight creep by polishing the piston knife edge and replacing the seat, and you get a good 90 duro o-ring in the HP spot, and plenty of lube on the piston shaft, maybe the drift (locking up at slightly different numbers with each purge) will disappear as well.

Make sure you check IP at 3000 and 300 PSI supply pressure. You can do that with a full tank. Just pressurize the reg, then turn off the tank valve, and tap the purge until the SPG reads 300-500. Check IP at that pressure, then open the valve again and check at full. It should only rise between 5-8 PSI more at full. That seems to vary a bit from reg to reg, but they all rise. It has to do with the geometry of the piston and the inevitable increase in friction at the HP piston o-ring as tank pressure rises.

 

[rhwestfall]

First thing first, is it a MK-10 or a MK-10+ (10+ has a satin seat carrier). The 10+ is notorious for IP Drift.... It is a MK-20 predecessor, not having the knife edge the MK-10 piston did.