Which Scubapro piston 1st stage is the overall best?

[agilis]

One thing that surprises me is that the Mk20/25 has such a higher flow rate (300 cuft) than the Mk10 (80 cuft). Not sure about the Mk10+, but I heard an estimated 150 cuft. (Also interesting that the Mk5 had 87 cuft at 2000 psi, which dropped to 48 pis at 300 psi tank pressure).

Has the Mk25 piston the same bore dimension and all its other flow paths have been increased? Or is it the opening travel of the piston different?

Finally, does the Atomic first stage have a flow closer to the Mk10+, or closer to Mk25? I know all of the above is sufficient, but inquiring minds want to know...

I've seen two divers easily breathing off the same Mk10 at about 80fsw with no problem whatever. I don't think these higher first stage flow rates make much difference in actual performance. Even a Mk 5 at low tank pressure seems to provide all the air needed, at least under normal circumstances.

 

[rx7diver]

One thing that surprises me is that the Mk20/25 has such a higher flow rate (300 cuft) than the Mk10 (80 cuft). Not sure about the Mk10+, but I heard an estimated 150 cuft.

Flow rate depends on feed pressure.

1. My 1996 Scubapro Catalog (p. 11) reports flow rate for the Mk 20 is 150 scfm @ 3,000 psi (and 102 scfm @ 500 psi).

2. Same Catalog (p.12) reports flow rate for the Mk 10 Plus is 135 scfm @ 3,000 psi (and 101 scfm @ 500 psi).

3. My 1994 Scubapro Catalog (p. 20) reports flow rate for the Mk 15 is 87 scfm @ 3,000 psi (and 60 scfm @ 500 psi).

4. Same Catalog (p. 21) reports flow rate for the Mk 10 (no Plus) is 73 scfm @ 3,000 psi (and 59 scfm @ 500 psi).

rx7diver

 

[Geo7]

I've seen two divers easily breathing off the same Mk10 at about 80fsw with no problem whatever. I don't think these higher first stage flow rates make much difference in actual performance. Even a Mk 5 at low tank pressure seems to provide all the air needed, at least under normal circumstances.

Agreed. I am just curious how they did it, and to see if I can trace the same changes in the Atomic. I dove for 20 years with a Mk2 down to 200 feet, under ice, freshwater lakes in winter, and it was fine.

This calculator is cool to see how flow rate varies with pressure:

 

[Geo7]

Flow rate depends on feed pressure.

1. My 1996 Scubapro Catalog (p. 11) reports flow rate for the Mk 20 is 150 scfm @ 3,000 psi (and 102 scfm @ 500 psi).

2. Same Catalog (p.12) reports flow rate for the Mk 10 Plus is 135 scfm @ 3,000 psi (and 101 scfm @ 500 psi).

3. My 1994 Scubapro Catalog (p. 20) reports flow rate for the Mk 15 is 87 scfm @ 3,000 psi (and 60 scfm @ 500 psi).

4. Same Catalog (p. 21) reports flow rate for the Mk 10 (no Plus) is 73 scfm @ 3,000 psi (and 59 scfm @ 500 psi).

rx7diver

Thanks for compiling the data!

 

[Pressurehead]

I have Mk10, Mk10+, Mk25 and Mk25Evo first stages [and a mess of 2nds, 190,2xG250,2x S600s, 195 etc]
Just love the look and design of the Mk10, that said , the Mk25Evo and S600s are used more often, 10,10+ and 25 ,they all breath great, logic does not come into it , it's my "go to" at the moment.
Overall best? All of the above.

 

[rx7diver]

Thanks for compiling the data!

Well, the Catalogs are consumer marketing literature (rather than technical spec lit), so I wouldn't put complete faith in these numbers.

rx7diver

 

[halocline]

Agreed. I am just curious how they did it, and to see if I can trace the same changes in the Atomic.

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.

 

[Geo7]

Thanks, that's the most thorough explanation I have seen so far! I did not know they used all ports to achieve the flow, and assumed they used the straight (fifth) port. My interest was particularly in the remaining flow at low tank pressures, but noticed that reduction is negligible in the regs after the Mk5. Again a mystery why the others have almost the same flow across supply pressures, when the flow formula through an orifice dicates a drop (see online caluclator above). I wonder if the Mk17 is also immune to that drop but can't find numbers for 300 psi.

And yes, it takes me much longer to bleed tanks empty. I'd be happy with any of these piston 1sts. Just wish I could find a Mk20 SPEC boot like @lexvil showed...

 

[lexvil]

Thanks, that's the most thorough explanation I have seen so far! I did not know they used all ports to achieve the flow, and assumed the used the straight (fifth) port. My interest was particular in the remaining flow at low tank pressures, but it since that reduction is negligible in the regs after the Mk5. Again a mystery whey the others have almost the same flow across supply pressures, when the flow formula through an orifice dicates a drop (see online caluclator above). I wonder if the Mk17 is also immune to that drop but can't find numbers for 300 psi.

And yes, it takes me much longer to bleed tanks empty. I'd be happy with any of these piston 1sts. Just wish I could find a Mk20 SPEC boot like @lexvil showed...

I have two extra boots George and may have some new silicone ones if James79 is successful with his experiments.

 

[Geo7]

Great news, PM sent!