SP MK-20 - IP Creep

[fmerkel]

My early dive history is having it serviced by the LDS for several years like most divers. I trusted them, didn't check the IP.I don't know what it was doing.

Now I service my own and I check. The MK20 creeps....slowly. If you never check carefully when when it's like that you will never know and will work fine on a dive. The next breath bleeds the creep off before it's ever an issue.

You can take an "oh well it works fine" attitude and get on with the dive. I don't think it's actually unsafe, a slow creep that is. I am letting my own wife dive this reg. Mostly it probably just bothers us. It does me. So I try to get it at least to the level I can rationalize it's working within acceptable, and safe, parameters. That spread is a bit larger than it was a few years ago.

If it creeps aggressively and leaks out the second, then you really do have to deal with. I don't see you having that problem.

 

[buddhasummer]

Understand cheers. For me I get a little obsessed trying to figure out why something is not functioning as it apparently should. I'd guess either of my MK10 Plus rebuilds would have been safe to dive, as you say the creep bleeds between breathes and never has the opportunity to get high. But the fact (?) is, it shouldn't be behaving this way and I want to know why. This reg is virtually new, as in unused, prior to my dissasembly and cleaning. I've meticulously put it together, certainly the last time anyway, I guess I'm missing something though...I'll keep at it, it's a mechanical device so a cause should be able to be identified and corrected.

I do like playing but only to a point, eventually my hope is the thing I'm playing with actually works properly.

After the next rebuild if it doesn't work properly I will reassemble using same shim set up as it originally came with, no centering washer and two washers above spring. I only have one more seat to try. The piston I've taken great care to ensure no nicks etc.

This is perhaps indicative of my lack of experience and knowledge with piston regs. I recently re built two Apeks first stages, a conshelf, a MK11 and a MK17 and all went together without any issues at all, lucky? I don't see too many threads on issues people are having servicing diaphragm regs, or perhaps it's just that SP piston models are more prolific with the DIY crowd. I'm starting to understand how the Gurus became so.

For shits and giggles I'd almost like to get it serviced by the SP service center to see how it comes back but the cost would buy me several regs on auction and I'm not confident it would come back any differently.

 

[halocline]

This is an interesting thread. I think that the MK10+ is probably destined to creep a little regardless of what you do. There are 2 reasons I think this:
1. The rounded pistons, except maybe the newer composite piston, seem to generally not lock up as consistently as the older knife edge pistons.

2. The MK10 has a smaller piston head diameter, which means that the total force on the piston head pushing the piston shaft into the seat is less. I.E, if the MK5/15/20 piston head area was 2 sq inches and the MK10 was 1 sq inch, then at an IP of 150 PSI, there would be 300 lbs of pressure pushing on the MK5/15/20, and only 150 lbs on the MK10. This doesn't necessarily mean that the actual pressure against the seat is higher, because it should be compensated for with different spring strengths. But I can't help thinking that something is different. One thing for sure is that friction on the piston shaft will play a disproportionate role with the smaller piston head. That might have an impact on lockup.

I believe SP introduced the rounded pistons for their airflow characteristics; they supposedly flow a lot more air by providing an aerodynamically better path for the air to flow into the piston shaft. That's the only true advantage I can think of. I guess theoretically they should last longer between rebuilds because the piston is not cutting into the seat. But I have a MK15 going on five years and my MK5s routinely go that long. I really think the motivation was airflow.

Why the rounded piston on the MK10+ results in apparently more IP drop during demand than the standard MK10 is a mystery. When I get my MK10+ parts (thanks buddhasummer!) and convert one of my MK10s maybe I'll have an idea about it. A long time ago I did an experiment because I noticed that my MK15 was apparently dropping more under demand than my MK2. Clearly the MK15 is a much higher flowing reg than the MK2. I even talked with Peter Wolfinger about it at length. The only answer either of us could come up with is that the way we measure IP results in venturi forces within the ports. Basically, you have air moving through the hoses, and regardless of where you put the IP gauge, when air is flowing, it's moving across the opening to the IP gauge, which results in a pressure drop at that spot. The only way to accurately measure flow through a hose is with a flow meter, not using IP drop as an inference to flow.

The experiment I did was to take a full tank, put two balanced/adjustables (those have monster purges) and measure IP drop under full purge on both 2nd stages at the same time. Sure enough, the MK2 dropped quite a bit and stayed there. The MK15 dropped quite a bit initially then recovered to just a few PSI under lock up during the purge.

To me this experiment suggested that measuring IP drop in the way that we do using a gauge on a LP inflator hose is not really a reliable indication of air flow. It also suggested that I had a bit too much time on my hands....

 

[fmerkel]

To me this experiment suggested that measuring IP drop in the way that we do using a gauge on a LP inflator hose is not really a reliable indication of air flow. It also suggested that I had a bit too much time on my hands....

I think you had just the right amount of time. I like your theories.

 

[buddhasummer]

Worthy of his "Guru" status.

Interesting, thank you Halocline. I'm hanging out for your results and findings once you get the parts and time to convert, hoping they are consistent with mine and can give a green light, or red.

 

[buddhasummer]

Sorry to repeat a post, just wondering if anyone can advise on this. Cheers.


Update: took a jewelers loupe to the piston, looked "ok", nothing stood out. Used some 3200 micromesh and Brasso. Reassembled, starts at around 142psi and over a few mins creeps to just shy of 150psi and seems to be holding. So looks like a creep of about 8psi.

Next question, I'm using no shims on seat carrier, I've assembled hp side as per schematic with the spring centering washer and above that a washer/shim.

To reduce the ip I'm going to need to remove something. I'm guessing if I remove the washer and leave the spring centering washer in place it'll drop around 5psi?

Given many MK10s, even though latest schematic shows both MK10 and Plus using spring centering washer, do not use this centering washer can I remove it?

Am wondering what to remove internally as I can make up ip if I need to using seat carrier shims.

Any advice is appreciated. Cheers.

 

[Couv]

To reduce the ip I'm going to need to remove something. I'm guessing if I remove the washer and leave the spring centering washer in place it'll drop around 5psi?

That is "washer" is actually a shim and can be left out to lower IP.

Additionally, see SP Engineering Bulletin 242 for information on the allowable IP drift (a bit more liberal than a real MK10) and recommendations on the lubricant.

 

[buddhasummer]

Thanks couv. Sorry, I'm unclear, the spring centering washer 10-500-118 must be left in place? Or can it be removed along with the additional shim that sits above it against the body 01-060-101? Removing both would leave the spring "floating" and sitting against the body.

The shim 01-060-101 is only going reduce the ip by around 5psi which still leaves ip over 145 and other than the SCW 10-500-118 there is nothing else I can remove. The SCW is thicker than the shim that is currently sitting above it so would guess that it's removal would result in a greater ip drop but am unsure if it's advisable to leave the spring "floating". I'm guessing it's ok as normal MK10s often (?) don't have a SCW just normal shim/s. sorry if this is elementary. Cheers.

 

[buddhasummer]

And yes. I've seen that bulliten courtesy of a post you made in another thread. I've been using silicone grease only, I'll try christolube. Cheers.

 

[buddhasummer]

Delved further into that bulliten and it would appear my problem may well be, at least in part if not all, that I'm using silicone grease. I'll order the recommended grease and then re do. Does this grease get used on everything or just the piston and piston stem O ring? Cheers.