The impact of lubricant on IP

[Zung]

Scope: Scubapro MK10Plus, MK20 & MK25 only

In 2 "Engineering Bulletins", SP stated:

EB #242: "Both 1st stages should be lubricated with CHRISTO-LUBE * lubricant versus the standard silicone which has been used in the past. If silicone grease is used, you will find in general, that the intermediate pressure range will be higher than what we recommend."

EB #248: "The MK10plus and MK20 1st stages are assembled using Christo-Lube® lubricant, p/n 41-047-000. Your choice of lubricant will affect the intermediate pressure. If standard silicone grease is used, the intermediate pressure could increase by as much as 20 psi. SCUBAPRO recommends using Christo-Lube for these two first stages."

I raised the question in another thread, and the responses were largely negative.

Just now, I've the opportunity to do some experimenting. I've scored yet another MK20 on eBay (I know, I don't need it, but you know how it goes), older generation with the small ambient holes, advertized as "150 dives", which I assume to be 150 dives since last service.

Initial testing gives an IP of 10 bar/145 PSI, rock steady, no creep.

After much head scratching, I decided to take it apart for a service. To my surprise, everything is shiny new inside, so I shoot a "complain" to the seller, who says it's 150 dives in its whole life, serviced a year ago, and very few dives since. Oh well...

Since it's apart anyway, I take a closer look: there's a very slight scratch at the edge of the piston stem, and the lubricant is silicon grease (transparent instead of white). So I clean everything in the ultrasonic, lube with Christo 129 and reassemble.

After settling, the IP is 9 bar/130.5 PSI, creeping to 9.2 bar/133 PSI!

There's absolutely no part substitution, I use the exact same piston o-rings, seat, and the single shim at the seat retainer. Net difference is .8 bar/11.6 PSI, within range of what SP says in EB 248.

I'll wait a while to see if the creeping stops. If it doesn't, maybe I'll give it the mattmagic treatment and report back.

Meanwhile, any comments?

Edit/epilog

The creep is bugging me, so I substitute a spare piston I previously applied the mattmagic on. Initially the IP is a little low, as I thought it would be, so I add a shim at the bottom of the spring. Now the final result is 9.0 bar/130.5 PSI, no creep.

 

[knotical]

OK, I'll bite. What the heck is mattmagic?

 

[redacted]

OK, I'll bite. What the heck is mattmagic?

Polishing the piston mating surfaces with micromesh to remove imperfections that cause friction and leaks reported by Mattboy. Avoids much unnecessary replacement of Mk10+ and older Mk20/25 pistons.

 

[Zung]

Thanks awap.
It's a methodology reported by awap & mattboy; I documented it here.

 

[halocline]

I really doubt the change from silicone to Christolube was responsible for an IP change of more than 10 PSI. This is especially the case since this is a MK20, with the bushing system that lowers tolerances around the HP o-ring, which should result in less friction. Maybe you just lowered the friction by cleaning everything up, and maybe for some reason you got a better piston/seat seal after re-installing the seat. You would think it would be the other way around, but who knows.

But, I certainly could be wrong, it wouldn't be the first time!

 

[redacted]

ABBA would probably be the appropriate experimental design protocol.

It is intriguing.

 

[aquaregia]

Seems like you should do the same thing again but with silicone and see if you get back to 145.

 

[halocline]

What does ABBA have to do with it? "Dancing queen, dancing queen...." wasn't that ABBA?

Second only to air supply in a musician's list of worst pop music ever.

 

[Zung]

ABBA would probably be the appropriate experimental design protocol. It is intriguing.

Seems like you should do the same thing again but with silicone and see if you get back to 145.

Yeah, that would be very scientific, but no thanks, it's working great, and I've learned I shouldn't mess with something that works.
Beside, it's very academic since most of us use Christo anyway.

In case there's a relationship (see below), I wonder if the Tribolube would produce the same effect. It should.

... Maybe you just lowered the friction by cleaning everything up, and maybe for some reason you got a better piston/seat seal after re-installing the seat. You would think it would be the other way around, but who knows.
...

That's possibly the key. In another thread, I wondered whether a superior lubricant would change something because of lowered friction. But then mechanical engineering in general and fluid dynamics in particular used to be my worst nightmare in school. Any mechanical engineers out there?

What does ABBA have to do with it? "Dancing queen, dancing queen...." wasn't that ABBA?

Second only to air supply in a musician's list of worst pop music ever.

With lots of hindsights and as a "pop" group, they ain't as bad as I used to think they were
As a guideline for experimental procedures, it's a must: A -> B then back B -> A. But I'm just an amateur

 

[Couv]

Yeah, that would be very scientific, but no thanks, it's working great, and I've learned I shouldn't mess with something that works.
Beside, it's very academic since most of us use Christo anyway.

In case there's a relationship (see below), I wonder if the Tribolube would produce the same effect. It should.



That's possibly the key. In another thread, I wondered whether a superior lubricant would change something because of lowered friction. But then mechanical engineering in general and fluid dynamics in particular used to be my worst nightmare in school. Any mechanical engineers out there?



With lots of hindsights and as a "pop" group, they ain't as bad as I used to think they were
As a guideline for experimental procedures, it's a must: A -> B then back B -> A. But I'm just an amateur

I ain't no engineer...but here goes my take.

Friction, stiction, and lubrication.

….or should I say, stiction, friction, and lubrication? Anyway, (especially) in a piston first stage, all three of these factors affect the IP. Aside from the spring, the initial force necessary to move the piston must be taken into account. Stiction, aka "static friction" is sort of the breaking point where an object overcomes the adhesive force holding it in place. Likewise we have friction, the dynamic force resisting movement of the parts in contact with o-rings. The amount of force required to overcome stiction and friction will increase or decrease relative to: the material the o-rings are made of, the material the o-rings are in contact with, and the properties of the lubricant used. (Bare with me, I'm getting there.) So how does this affect IP? Well, if the piston is slow to move initially or drags while moving, it will cause the IP to increase by allowing higher pressure to accumulate on the piston head. Taking this into account, that could be the difference in Zung's two lubricants. The PTFE grease could be more compatible with the o-rings you use.

Now having said all that (this is were I say more,) one can have the best damn lubricant in the world, but if it doesn't stay put between uses you'll have seal against metal, which will require more force to move the piston. What can you do about it? Besides selecting a grease that doesn't easily allow the oil to separate from the thickener, you should prep the stem to hold the grease in place. One area where I differ with some of the Usual Suspects is the polishing of the piston stem. I think polishing the stem knife edge that comes into contact with the hp soft seat is a good idea, but IMHO, the area that comes into contact with the stem o-ring should not be polished. In fact, I try to put tiny scratches (preferably a cross-hatched pattern) there. These tiny scratches provide more surface area and nooks and crannies for the lubricant to adhere to. The advantage of this process may not be apparent on a freshly rebuilt regulator as there is sufficient lubricant, but after it has been in service for a while or has been sitting on the shelf, the grease has an opportunity to get wiped away or the oil simply flows away because of the polished surface.

YMMV

c

Edit: link to thread re Christo-lube vs. Tribolube 71
http://www.scubaboard.com/forums/tribolube/263864-tribolube-have-you-tried-yet.html