Has anyone tried ScubaPro's MK 19?

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george1098

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Just wondering if anyone has tried the new (at least I think it's new) MK 19. It looks like they took the mk17 and added a swivel turret like on their mk 25. I also read a thing or two about how they tried to over balance it hoping to make it a little more popular with the tech divers. I'm thinking about switching from an mk 25 to a mk 17 or even an mk 19 to get a little better cold water performance (my mk 25 free flowed on me last week during an ice dive). Just wondering if anyone knows more about this new 1st stage.
 
The Mk 19 is indeed a Mk 17 with a swivel turret for the LP ports - just like the Mk 18 was a turreted version of the Mk 16.

It would be a good choice if you like the swivel port turret and want that combined with the fully sealed Mk 17 design.

The "overbalanced" thing basically makes me want to scream every time I hear it. It means absolutely nothing in regard to how Apeks uses the term.

The IP in any regulator maintains itself at a constant pressure over ambient pressure. The theory with the Apeks "over balancing" is that the relative difference of the IP over ambient pressure will increase as depth increases so that, for example, if the IP were 140 psi over ambient at the surface (155 psi absolute), it would be slightly more than 140 psi greater than the 65 psi ambient pressure at 132 ft. So rather than 205 psi absolute, you might have 210 or 215 psi.

The purported advantage of this is that the higher pressure will increase flow rate and offset the loss in flow caused by the higher viscosity of the denser air at depth. The problem with this thinking is that viscosity increases are not even a remote factor at anything even close to sane diving depths. At 600 feet or deeper - it might start to become an issue.

The second problem is that if the effect did occur, it would just screw up the tuning on the second stage as the reg would need to be detuned to not freeflow at the higher intermediate pressure found at depth. Alternatively, you could reduce or remove the downstream bias from the second stage poppet/shuttle valve. That however reduces the ability of the second stage to vent excess pressure in the event of a leaking high pressure seat and is unsafe.

More importantly, for the "overbalancing" to have any noteable effect on flow rate, the flow rate would have to be incredibly marginal anyway and it would in short not be a reg you wanted to dive with if it needed overbalancing to get improved performance. So the term overbalancing as Apeks uses it is total and complete marketing BS.

Scubapro defines "overbalancing" differently so the two are apples and oranges terms.

To set the stage, a "balanced" first stage essentially means that the intermediate pressure will not change due to the effects of changes in the downstream force of the air coming from the tank.

In an unbalanced piston reg, this change in tank pressure would cause a progressive decrease in IP as tank pressure fell from 3300 to 300 psi. In the Mk 2 for example a 20 psi decrease in IP is normal over that pressure range. In a diaphragm reg, the opposite occurs as the seat carrier is upstream of the orifice, so the IP in an unbalanced diaphragm first stage would increase as tank pressure falls from 3300 to 300 psi. This change in IP would affect the tuning of an unbalanced second stage resulting in slightly harder breathing at the period of the dive with the lowest IP. With a balanced second stage, the effect would be less pronounced but would still be present as a balanced second stage still has some downstream bias for the purpose of providing overpressure relief in the event of a leaking HP seat.

In a traditionally balanced piston design, the diameter of the piston stem was constant from the place where it passed through the high pressure o-ring sealing it against the reg body to the knife edge on the end of the stem that seals against the orifice. In effect this meant that the reg was not perfectly balanced as the area affected by the downstream force of air from the tank was very slightly smaller than the area of the piston stem due to the small but still present area of the sharp edge of the knife edge. In practical terms this meant the IP dropped about 4 to 5 psi over the 3300 to 300 psi range - not enough to be noticeable, especially with a balanced second stage, but still not perfectly balanced.

In the Mk 20 and Mk 25, the piston stem flares to a slighly larger diameter at the knife edge so that the area of the knife edge is accounted for and the piston is truly balanced with no I change due to tank supply pressure. SP calls this "overbalancing" as "overbalancing is apparently cool. It is also no doubt pandering to the market of ignorantly indoctrinated divers who think that Apeks version of overbalancing is important, but at least it means something.

The same is apparently true with the Mk 17/Mk 19 in that the area of the balance chamber is exactly the same as the area of the orifice creating perfect balancing - but that has never been an issue with the diaphragm design anyway, so it is again just marketing hype to compete with the Apeks marketing hype.

With regard to cold water performance, SP did extensive cold water testing with the Mk 17 at high flow rates at 165' in 35 degee water and the Mk 17 was the only first stage from any manufacturer (including Aqualung/Apeks) to survive the full 200 minutes of the test. It is as close to bullet proof in extremely cold water as you can get with a first stage and it is also an excellent performer.
 
Wow, that's what I call a post. One quick question; if the MK20-25 piston flares out at the edge, how do you get tight tolerances around the HP o-ring area, as you would have to make that area big enough to push the flared end through. Also, I think I read that the MK20-25 did not actually use a knife edge piston, but a rounded edge piston that meant you don't use the bullet tool to install the piston over the HP o-ring. I'll have a look at the diagrams I have for the MK20, but I'm curious. Once the piston edge is in contact with the seat, there is essentially no "horizontal" area for hp air to push on, correct?
 
The HP o-ring is flexible enough to let the slightly flared section of piston pass through.

The area impacted by downstream pressure would be the area inside the knife edge once it is seated on the seat. So with the straight stem it was just that small area of the knife edge that contacted the seat that made the minor imbalance as the area inisde the knife dge was slightly smaller than the cross sectional area of the stem itself where it passes through the HP o-ring.

The Mk 20 used three pistons during it's production life. The first was a stainless steel piston that had a knife edge. The mid production piston was a brass tipped piston that had a rounded surface that sealed against the seat. The rounded edge allegedly improved flow rate in combination with the concave seat, but it caused the reg to have a really mushy lock up with about 5 psi of creep after the initial needle swing. What was worse, the design was not that durable and IP creep tended to develop much sooner than with a conventional knife edged piston and in many cases the IP creep could not be fixed with a new seat and piston replacement was required. The rounded edge also meant that no piston bullet was needed to install the piston.

The final Mk 20 piston was the composite piston currently used in the Mk 25. It again has a knife edge. Scubapro at one point maintained that it is not a true knife edge in that it is apparently not as sharp as the Mk 5, Mk 10, Mk 15 and early Mk 20 piston knife edges, but it it looks like a duck, walks like a duck and quacks, it is probably a duck.

I use a piston bullet when I install composite Mk 20/25 pistons as the edge is sharp enough to potentially damage the HP o-ring (and you can feel the difference when you assemble them with or without a bullet) and there is no downside to using the piston bullet.
 
Just wondering if anyone has tried the new (at least I think it's new) MK 19. It looks like they took the mk17 and added a swivel turret like on their mk 25. I also read a thing or two about how they tried to over balance it hoping to make it a little more popular with the tech divers. I'm thinking about switching from an mk 25 to a mk 17 or even an mk 19 to get a little better cold water performance (my mk 25 free flowed on me last week during an ice dive). Just wondering if anyone knows more about this new 1st stage.

I have a MK11 Which is the same reg as the MK 17 only it doesnt have enviro seal,which the MK 17 is The Same as The MK 19 Only the MK 19 has a swivel turrit for better hose routing.
So having said all this yes i have ,because basiclly the MK 11 ,MK17, and MK19 are the same first stage reg :rofl3:
But to be sure ask DA Aquamaster he is the expert on ScubaPro
 
The HP o-ring is flexible enough to let the slightly flared section of piston pass through.

The area impacted by downstream pressure would be the area inside the knife edge once it is seated on the seat. So with the straight stem it was just that small area of the knife edge that contacted the seat that made the minor imbalance as the area inisde the knife dge was slightly smaller than the cross sectional area of the stem itself where it passes through the HP o-ring.

Thanks, just a couple more questions. The area inside the knife edge, if I'm visualizing it right, would be subject to IP, not HP, correct? Isn't it part of the IP chamber? The other question is, when SP went to the MK15 and later, with bushings holding the HP o-ring rather than just having the slot machined into the body, wasn't that an attempt to tighten the tolerances between the piston and reg body to prevent hp o-ring extrusion and allow for higher supply pressures? If that's so, then I would think those bushings would be pretty snug around the piston shaft, and in fact on my MK15, it's a snug fit. I would think that the flare on the piston shaft would have to be very small to allow for those bushings. Maybe it's just equal to the "width" of the knife edge that contacts the seat?

I did have a look at the MK20-25 drawings (deep south divers is a great site!) and I'm impressed by how similar it looks to my Mk15; almost exactly the same type of arrangement.

Anyhow, thanks again, you're an amazing source of info about these regs.
 
Exactly right. The area inside the knife edge is at IP - just like the cross sectional area of the piston stem that is inside the HP o-ring - which is backed by IP pressure air above the piston head. With that being the case, piston movement is not affected by downstream tank pressure at all.
 
Ok, I found a great deal on a used SP mk 16/s55o combo. Thinkingabout picking it up for my ice diving reg set. how do you think it will stack up?
 
Ok, I found a great deal on a used SP mk 16/s55o combo. Thinkingabout picking it up for my ice diving reg set. how do you think it will stack up?

The MK16's ambient chamber is not sealed like the MK17, so I would prefer the MK17 or MK19 over the MK16.
 

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