Which Scubapro piston 1st stage is the overall best?

[Couv]

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[350xfire]

When you say 'how they did it' I'm guessing that you are referring to how SP increased flow rates? I think I can at least partially answer that. First, (you may know this already) the way these flow rates are measured are with an unlimited HP supply and all the port plugs removed. So right off the bat there is a tangential (at best) relationship to real-world diving, because when we dive we are not getting air from an unlimited flow supply, far from it, and we are also not using all four or five LP ports to feed one 2nd stage.

Think about those numbers. 80cft/minute is emptying an AL80 in one minute. 300 cft/minute is emptying one in a little more than 15 seconds. Open the valve on an AL80 and let it breathe down; it takes several minutes, assuming you can keep it from freezing! So the difference between 80 cft/minute and 300 cft/minute would be kind of like the difference between a car that can go 200 mph and one that can go 500 mph. I guess if you live on one side of the Bonneville salt flats and your true love lives on the other side, it might have an active impact on your life, but for the rest of us, not so much. Like the speed limits and road conditions, the tank valve is by far the limiting factor on how much air flows through your regulator.

There is some relevance to flow rates, though; basically, first stages with excellent aerodynamic properties should have less IP drop under inhalation as the air moves more efficiently across the piston edge and re-stocks the IP chamber more quickly. That's the idea behind the rounded piston edge. It allows for smoother and more efficient gas expansion into the IP chamber. That's why the MK10+ flows more than the MK10.

I don't honestly know how they doubled that flow rate with the M25, but I could guess that the bushing system which reduces friction and the lowered weight of the composite piston both contribute to quicker piston response, which should mean that the valve opens more quickly under load. And it seems to work in that IP drop and recovery in the MK25 is very impressive. This means, theoretically, that the 1st stage is supplying the 2nd stage with air at very close to IP even when it's open, which should theoretically lower WOB. Real world? I doubt it matters.

The atomic uses a rounded piston edge and a re-designed seat that I'm sure has very good aerodynamic properties, so I would guess that it's flow rates are very very high.

I am actually taking the tech certification class for the Mk2 and asked what the differences are between the cheap and expensive regs. The Mk25 has a way bigger hole through the piston and it is a flow through design where the cheap Mk2 has a smaller hole(quite a bit smaller) and it is not a flow through it flows off to the side of the piston stem. This is how they get the higher flow rate. Now SP does have their reg CE tested and even the chap Mk2 exceed the CE requirments by 3 times...

 

[halocline]

I am actually taking the tech certification class for the Mk2 and asked what the differences are between the cheap and expensive regs. The Mk25 has a way bigger hole through the piston and it is a flow through design where the cheap Mk2 has a smaller hole(quite a bit smaller) and it is not a flow through it flows off to the side of the piston stem. This is how they get the higher flow rate.

You're talking about something different; the MK2 is a totally different design than any of the balanced piston regs. Our discussion on this thread is about the differences between the various generations of the balanced piston, flow through design. Of course the flow-by (unbalanced) MK2 has lower flow rates, but there are some other benefits, like no dynamic o-rings subjected to HP air, and a very small teflon seat. This makes it an excellent design for high percentage 02.

Now SP does have their reg CE tested and even the chap Mk2 exceed the CE requirments by 3 times..

Nice to see that SP is making sure they put a little free advertising into their tech certification class.

 

[rhwestfall]

One must realize that the true limiting factor in flow is the tank valve. These first stages all can pass a higher flow rate than the tank valve can...

 

[Geo7]

Yes, I think we all agree that all of the 1st stages in question flow more than enough air. Out of curiosity I brought up the question of how flow (ratings) can vary so widely with such a similarly dimensioned piston, which halocline explained. But I actually did not intend to imply that more flow is better. In the spirit of the OP's initial question, my preference is usually for simple designs. I was fine with the Mk2 for a long time, but like the hose routing of the balanced pistons best.

 

[halocline]

The practical value of a high flow rate would be in IP recovery under heavy load at depth. The 1st stage has to flow enough air to re-stock the IP chamber without it dipping too low under demand, otherwise 2nd stage breathing effort, especially with an unbalanced 2nd stage will go up.

That said, I have never noticed any appreciable difference while diving (even air sharing) between any of the SP 1st stages, including the MK2, until tank pressures get down well below 500 PSI.

 

[buddhasummer]

Q: Which Scubapro piston 1st stage is the overall best?
A: The ones that Couv sells. :-D (Too bad I don't have any right now.)

The real answer of course is the MK10 SPEC. Why? Sharp piston, low parts count, easy to service, can be configured with or without grease in the ambient chamber, the grease (if one chooses to pack it) can be silicone or PTFE. The SPEC boot can be OEM or homemade. The HP seat comes in 3 lengths to easily change the IP. The seat can also be OEM, aftermarket, or homemade. Works well on hp tanks...

Most often they have no rubber ring cosmetic or SPEC, either way to me looks like something is missing when it's not there. There's a million plus MK10 over here and I've only ever seen one with the rubber ring it was a MK10 plus that I bought and sold. I want to love them as they are easy for me to get. Given boredom is forever knocking on my door and money is tight perhaps I'll take another run at them.

Scrap that idea, just remembered new pistons are NLA. I like amassing huge stocks of new parts that I'll never use and eventually sell at a massive loss.

Morning madness.

 

[buddhasummer]

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View attachment 665018

Ooh, that one looks "complete".

 

[Couv]

Ooh, that one looks "complete".

It is THE answer to the OP's question.

Simple: No oddball bushing that one will not be able to source years from now. Homemade boot retaining readily available grease in the ambient chamber. The seat is still available from SP, aftermarket, or homemade. The seals are standard, easily sourced o-rings. The piston, if needed, can be dressed with emery cloth.

The MK10 is a popular model found throughout the world so getting spares is not an issue.

Reliably handles the high pressure (3500 psi) of my favorite cylinder. etc, etc...

 

[Kupu]

...Homemade boot retaining readily available...

Does that mean aftermarket MK10 boots can be purchased? I have seen a couple versions made by SB members, but did not think they were an item that was generally available. I considered going ghetto and trying a homemade inner tube boot but never got around to it.

Difficulty finding the boot is one reason I stopped using the MK10 for salt water dives. It took a couple years searching to find an original MK10 boot at acceptable price. The quest to own sealed SP pistons has also resulted in a purchase of MK5 outfitted with environmental kit.