MK10 IP Creep

[halocline]

The weird thing is, there was NO creep when I rebuilt it, maybe a little initial drift as the seat was breaking in. Then it worked great on a dive, then I put it aside for a few weeks, then took it out to dive today, and cycled it with the IP gauge a few times to check it out, and on the third or fourth cycle, off it went steadily up through the range, past 150, then I shut off the tank. Obviously something went wrong, maybe some crud got caught between the piston and seat, who knows. Anyhow, the spare piston did the trick, dove with the reg this morning, and I bought a new kit today so I have a new seat.

Gary, I thought you were heading off to the FG?

 

[redacted]

FGB was weathered out - trying again on the 22nd. I'd suspect a piece of crud. The used seat is not a good test as the new groove overlapping the old groove will produce creep. Give the knife edge a good inspection - I expect the old piston is still good. I've got a few of the older style flat seats - I'll bring you one next time we meet. I think they have a bit more of a tendency to develop a whistle but they work fine.

 

[Couv]

Matt,

A nick or cut in the piston head or stem o-ring will not cause an IP drift; however, it is still possible that the o-ring is the culprit. An o-ring that is too soft could extrude between the piston stem and body causing drag on the piston. This will be more pronounced at higher tank pressure. The MK 10 bodies that do not have the bushing system are probably more prone to this ailment and high supply pressure will exacerbate it. Remember, on one side of the stem o-ring is supply pressure and on the other is ambient.

So, if you have eliminated the soft and hard seats as the problem, try a new, well lubricated, duro 90 o-ring. While you have the piston out lube the stem too.

c

 

[halocline]

I borrowed a loupe to get a 16X look at the knife edge and the seat. There were lots of nicks and imperfections in the seat, so I used the loupe to find one in my stash that was in good shape, and it works fine now. I thought I'd try that before installing a whole new kit. The knife edge on the older piston looks good, except a VERY slight flare that seems uniform. I'd almost say it looked like the tiny bit of flare that the later pistons are supposed to have for the "overbalanced" bit to prevent the 2-3PSI drop in IP at low tank pressures. Since to my knowledge (which is mostly limited to reading DA's posts) SP only did that on later models, like MK25, I assume the flare is either an optical illusion (it's really slight) or maybe the pistons wear that way. Given the shape of the conical seat, I would assume that any distortion of the knife edge over time would be a flare inward, not outward. So, like the higher IP drop in the MK15 vs the MK5, this one is a slight mystery to me, but since everything's working fine, I'm happy. Speaking of IP drop, the MK10 acts just like the MK15; an initial 20PSI drop under purge, then really quick lock up. AWAP says that the reason the MK5 has less of an IP drop than the MK10 is due to the greater "leverage" afforded by the larger piston head. That makes sense, but since the MK15 went back to the larger piston, it does not explain that one. I'm thinking there might be something in the turret and/or LP port design that changed between the MK5 and later models that results in a slightly longer lag time (a few milliseconds) of the LP air exiting the reg. That would explain the greater initial drop in the late model regs.

 

[Couv]

Assuming you are using the same tank at the same pressure and the same 2nd stage, then my guess (and that is exactly what it is) would be that the higher flow rate of the MK10 and MK15 is showing the limitation of the tank valve's ability to supply air.......
......Time to call Luis, DA, Pesky and AWAP.

Now change that o-ring like I told you so you don't have any more surprises….and don't jam the bullet into the piston so hard that you flare the orifice again.

c

 

[redacted]

While we are talking about it, does the Mk10 put less force on the HP seat via the piston knife edge when it locks up. Smaller piston head diameter with the same IP so, unless I slept thru all my physics classes, it should. And should that extend the life of the seat?

I'm just trying to find some advantage in the Mk10 design over the Mk5.

 

[DA Aquamaster]

Per the flow rate data SP used to publih the Mk 5 and Mk 10 had very similar flow rates at high tank pressures, but at low tank pressures the MK 5 flow rate fell off quite a bit (into the 60-65 scfm range if I remember correctly).

The Mk 10 had a smaller piston head, but the passage inside the piston stem was shorter, 1.42" versus 1.66" (and in the Mk 20/25 is it shorter still at 1.34") so I suspect that may make some difference in flow rate. Otherwise the difference in flow rate at low pressure would have to be due to better porting in the Mk 10.

The downside of the smaller piston head is that it aggravates any change in IP due to less than perfect balancing in the straight stem piston in the Mk 10 and the smaller area makes it potentially a bit less reponsive - although that may be offset by the lower piston weight.

The nice thing about the piston in the Mk 20/25 is that it has the large diameter head of the Mk 5 with the short lenght of the Mk 10 combined with very light weight due to it's composite design and it is optimized in all three areas.

The Mk 10 was a little smaller and lighter, but pesonally, I think it was a move in the wrong direction with the smaller piston head and with the bearing surface for the piston head o-ring in the reg body rather than in the swivel cap. SP apparently agreed as it dropped both those features in the Mk 15 and never went back.

The larger piston head with its larger area would at any given IP have more force acting to press the orifice against the seat, but I am not sure it would have any effect on seat life.

 

[superstar]

Lets say that when you are checking the IP, you cycle the purge a few times and the IP slowly creaps up from 135 lbs to 140 lbs and it takes 10 seconds to do so. Is this really a problem since you actually breath every 3 or 4 seconds preventing the IP from actually reaching 140 lbs. Or is what I just attempted to explain a case of IP float and what wouild be the acceptible range of float before doing a complete service.

 

[redacted]

No, it is not a problem and is fairly common with a used seat. Even when the creep continues higher toward the 150 upper limit, it is manageable but a clearer indication that time for service is approaching. For local diving day trips, I don't mind 5 to 10 psi creep. But those regs get watched closely and serviced before they go on a longer dive trip.

With a new seat, it would indicate a problem. As a DIYer. I'd give it a little time (use) to see if it just needs a bit more break in. If it did not resolve with a day of local diving, I'd probably try another seat. If that did not correct the problem, it's time to look real closely at the piston. I probably would not replace a piston over that little creep but would just moniter it closely. I have had some success touching up the knife edge with a real light abrasive but that is not a recommended solution.

 

[DA Aquamaster]

It depends a bit on the piston and the seat. The Mk 10 Plus for example uses a rounded seating edge on the piston that improves flow but gives a slightly mushier lockup. For a rounded seating edge piston liek the Mk 10 Plus or the mid production brass tipped Mk 20 piston, a fast initial swing followed by a soft lockup and a slow creep of another 5-7 psi over 5 or 6 seconds before the IP finally stabilizes is "normal".

With a regular Mk 10 and its knife edged seat, you should get a rapid initial swing and then a very solid lockup with no creep. Sometimes with a new seat, it needs to break in a bit and some minor creep may occur until this happens. This does not happen with all pistons and when it does, I first swap the seat to see if it may be a flaw in the seat. If the creep still occurs, I will leave it pressurized overnight and see if the creep is eliminated. If not, I replace the piston.

With any reg, any degree of continuous creep is bad. You may breath 10-12 times per minute implying a creep above 150 psi in 30 seconds is not an issue, but that is not the case. The creep is present because there is a leak. It will also be an issue on the boat when gearing up, on the surface or after the dive when you have the reg pressurized but are not breathing off it. More importantly, 99.9% of the time the leak gets worse which causes the rise in pressure to occur faster. This means it will fairly soon progress from leaking out the second stage in 30 seconds to leaking in 3 seconds to just leaking continuously, to leaking a LOT continuosly. I have seen this progression occur over as many as 10 dives and I have seen it occur over during one dive, depending on what was wrong and what was causing the damage to the piston or the seat.

It is often false economy to delay service as well. For example, if the leak is occurring due to a piece of metal or rust that got pinched between the seat and knofe edge and driven into the seat, the leak may start out very slight but as the metal eats into the kife edge the leak gets worse, and more importantly from a cost stand point, the knife edge gets progressively more damaged. If caught early the nick in the knife edge may be minimal and it may work fine with a new seat. However once the damage accumlates over time, the knife edge may not seal at all on a new seat and/or may chew it up quickly and cause a new leak with the result that a new piston will be required. So worst case you may pay for your delayed service, and then discover IP creep after a few months and have to go back for a new service and probable piston replacement - for a total cost about 3 times what you would have paid just getting it serviced when you first discovered the IP creep.

If you have an old (as in can no longer get a new piston for it) first stage, the knife edge may be able to be resurfaced, but it would need to be ground or honed with a hone worked to create just the right angle, and exactly square with the piston stem. And you want to take off the absolute minimum of material as by shortening the piston, you are also increasing the IP as the spring will have to compress more to close the valve. So you pretty much have only 1 shot to get it right.