Scubapro MK20 Maintenance Issue/Question

[halocline]

Fair enough. I'd say that there were several contributing factors:
1. Have never heard of this condition before
2. Have never experienced it before (have 1200 dives on my own gear)

I will certainly buy one now and use it appropriately. How often or, after what events, should one test IP? After each service, before each trip, before each dive?

Best,
Tracy

Don't worry about Dumpster's post. An IP gauge is a good thing to own, actually what's a bit troubling to me is that the so-called tech on the liveaboard apparently took several dives worth of obvious IP problems before he checked it.

Since you are such an experienced diver, you might want to invest some time into learning a bit more about how regulators work. Here's a bumper sticker version of how your 1st stage works.

Air from the tank enters the regulator, and travels up the shaft of a hollow piston that is suspended above a seat by a big spring. As air exits the piston shaft on the other end it fills a chamber, which pushes on the head of the piston. When the pressure in this chamber reaches a preset amount, it forces the end of the piston shaft against the seat, which cuts off further air flow. It stays that way until you take a breath on the 2nd stage, which draws air from that chamber, lowering the pressure enough to allow the spring to push the piston a bit off the seat, allowing air to flow until the pressure once again builds up and pushes the piston back against the seat, closing the flow of air. It does this with every breath. That pressure in the chamber is called IP.

Any leak between the piston edge and the seat will cause IP to rise beyond what it is supposed to be. This causes freeflows in your 2nd stages and if unchecked, could easily burst a LP hose. I check IP on my regs before each trip. It's just a good thing to understand what's actually happening and what inconsistent IP indicates.

As you're finding out, dive shop technicians don't always do a good job; and since you are the person actually diving with the gear, it's good to have enough familiarity so that you can effectively inspect your gear, and communicate clearly with the tech, and understand if whatever he is telling you is nonsense or something reasonable.

One strange thing is that you say this has never happened before with 1200 dives on this regulator. That certainly indicates that something changed at the last service. Maybe it is a bad seat, or the guy damaged your piston during service or just did a bad job checking his work. Or maybe you've had some IP creep all along, just not enough to cause a freeflow. Regardless, that brass piston is notorious for creeping and you're going to get better performance from the new piston.

Now if it turns out that you don't have the brass piston, you can ignore all the comments about that, but for sure, it's either the piston or the seat.

 

[halocline]

Was the stainless piston knife edge?

I think these are the three MK20 pistons. Although the brass one does look like it's more rounded, you can see that none of them have the sharp knife edge from the earlier regs. It just wouldn't work with the new seat. And I'm pretty sure this whole rounded edge design was developed to increase the air flow around the piston edge, not so much to increase IP stability. The MK15 has pretty great IP stability, just not the gaudy flow rate numbers of the MK20/25. Who knows what they were trying to accomplish...

 

[dumpsterDiver]

Fair enough. I'd say that there were several contributing factors:
1. Have never heard of this condition before
2. Have never experienced it before (have 1200 dives on my own gear)

I will certainly buy one now and use it appropriately. How often or, after what events, should one test IP? After each service, before each trip, before each dive?

Best,
Tracy

Compact IP Gauge (PSI and BAR)

Sorry they are $9.00

You plug it into the BC hose and get a reading.. takes 5 seconds... do it before a big trip, once every few months and anytime you have a bubbling or leaking second stage.. This tells you if you should be messing with the second stage or not.

and.. if you are really disciplined in using it, you can often spot a regulator that is starting to show some creep before it has any problems.. then you can get it serviced before it causes a problem.

 

[aquaursa]

I have no problem with Dumpster's post. I come to sites like this for unvarnished feedback.

I have no doubt that I am overly lax in my reliance on my LDS for service and oversight. (Can likely be said, as well, for my accountant, tax guy, lawyer, doctors and auto mechanic.) But I do recognize that regulators are rather unique in the list of items upon which I rely on others; I know several people who service their own regs. I know even more who will point out the incidence rate of failures that immediately follow servicing. I've avoided taking the class for one primary reason: I trust myself even less. However, as you point out, being able to better diagnose, have an intelligent conversation with the boat or resort tech and then with my own tech would be a smart idea.

Yes, "so-called tech" is a valid term. One of many multi-taskers on the boat. He was a DM (probably an engineer as well). The DM who first saw the problem was a different person. There likely was no chain of discussion between the two.

Thanks for the refresher on regs and IP. I probably knew that early on but my laxity has led me back to a state of ignorance. I appreciate the lesson and will get an IP gauge and use it at the beginning of each trip.

And, yes, I already had experienced poor work from dive shop techs. Twice.

Just to clarify. I have 1200+ dives but the reg in question is my girlfriend's and she has 800 dives on that unit. I'm compiling all the feedback from this thread, and discussions with several friends, so that I can address the issue with our tech (and likely with a separate shop for further validation).

Is it best to come back to this thread to report the final outcome? (Or will you all be ready to move on before then?)

 

[aquaursa]

View attachment 389032

I think these are the three MK20 pistons. Although the brass one does look like it's more rounded, you can see that none of them have the sharp knife edge from the earlier regs. It just wouldn't work with the new seat. And I'm pretty sure this whole rounded edge design was developed to increase the air flow around the piston edge, not so much to increase IP stability. The MK15 has pretty great IP stability, just not the gaudy flow rate numbers of the MK20/25. Who knows what they were trying to accomplish...

Great, thanks Halocline, much better than the diagrams I've been viewing.

 

[aquaursa]

Compact IP Gauge (PSI and BAR)

Sorry they are $9.00

You plug it into the BC hose and get a reading.. takes 5 seconds... do it before a big trip, once every few months and anytime you have a bubbling or leaking second stage.. This tells you if you should be messing with the second stage or not.

and.. if you are really disciplined in using it, you can often spot a regulator that is starting to show some creep before it has any problems.. then you can get it serviced before it causes a problem.

Thanks Dumpster Diver, I appreciate all the guidance.

 

[Couv]

...Is it best to come back to this thread to report the final outcome? (Or will you all be ready to move on before then?)

I would be interested in hearing the findings and the response from the LDS.

In the meantime you may find this Regulator Inspection and Checklist (Rev-7) helpful.

 

[redacted]

A reasonably competent tech should be able to get a good idea of what is going on by removing the seat retainer and inspecting what that reveals. I would not be surprised to find a defective HP seat or a bit of foreign matter in the seating area. That may avoid unnecessary disassembly and fix the problem rather easily.

 

[Centrals]

Every diver should have an IP gauge.
Anyone who know how to attach the inflator hose to the power inflator........same procedure.

 

[Couv]

A reasonably competent tech should be able to get a good idea of what is going on by removing the seat retainer and inspecting what that reveals. I would not be surprised to find a defective HP seat or a bit of foreign matter in the seating area. That may avoid unnecessary disassembly and fix the problem rather easily.

Speaking of seats. The MK10 plus/MK20/25 apparently take the same seat. I've inspected a few of them and see a number written on the "piston" side surface. I don't remember the numbers, but I do recall that they were not all the same. What is the significance of these numbers? Some older white MK 10 (not plus) seats also have numbers on them that I assumed were the precursors to the + = - (or is it X -l ll.....Ben Casey might know) markings we see on the gray seat kits; but I'm not certain. If that is the case, why would SP go back to a numbering system?

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