Balanced piston vs diaphram first stage

[DA Aquamaster]

Personally, I would rather pull maintenance on a US Divers SEA diaphragm type first stage than a Scubapro MK 20 piston stage. Some low end piston regs are fairly simple but a lot has changed since 1971 when a Scubapro MK5 balanced, flow through piston reg could be disassembled in a couple minutes with only one spanner and a wrench. The main selling point was that they were simple. Not so today, with the descendant, the MK20 being a clusterflop of plastic parts requiring several special tools to disassemble. What have they produced other than complexity? Stories persist about these expensive SP piston regs freezing up. In fact, after 30 years, the only improvement that I've seen on SP first stages is the change over to hypalon O rings. These helped transform the once notorious SP piston leakers into a fairly reliable regulator. Nowadays, most problems seem to occur after "service". If you want to blow your mind, try counting the parts in a piston reg. Get out the manual and look. I see no "simplicity" claim to be made for most piston regs.

I agree with you that progress in reg performance has more or less plateaued, and in some areas has actually regressed, but it is an industry wide problem.

I don't agree that the Mk 20/Mk25 is overly complex or tool intensive. You still only need a multi wrench (using one pin spanner to remove the swivel cap and the other to remove the HP seat retainer) and a $12.50 assembly tool to make stacking and seating the bushing and HP o-ring assembly easier (and frankly a similar tool made MK 5 and Mk 10 assembly easier as well). A $5.25 piston bullet (as used on the Mk 5, and Mk 10) is a good idea to use but is not absolutley required on the Mk 20/25.

Personally, I like just pushing the bushings and HP o-ring out of a MK 20 or Mk 25 as opposed to carefully fishing out the HP o-ring on a Mk 5 or Mk 10, hoping you don't nick the bearing surfaces and cause a leak. And I never see problems with HP o-ring pinch in MK 20's and Mk 25's like I did with Mk 5's and Mk 10's. They tended to eat HP o-rings when used at pressures over 3000 psi and/or when they got older with the accumulated wear increasing the tolerances between the reg body and piston stem.

The Mk 20/25 does have a higher parts count, but the HP O-ring retention system is better all the way around than the earlier system used on the Mk 5 and Mk 10. The difficulty in maintaining tight tolerances over time in the earlier system was one of the reasons the newer o-rings were such an improvement, but the harder/more durable o-rings were a quick fix, not a cure and the fact remains that the Mk 20 and Mk 25 are much easier on HP o-rings, particularly at higher service pressures.

Now the Mk 15 is a somewhat different story as the Mk 15 was a little harder to service as the bushings were held in place with a C-clip that required a set of snap ring pliers with very slender arms to remove. Servicing one of those could be a real PITA even with a properly contoured set of snap ring pliers.

 

[pescador775]

DA, I agree with that. However, a parts count of the Scubapro piston MK 20 TIS comes to only 5-6 less than the MK 18 TIS which is arguably the most complex diaphragm first stage ever manufactured. The MK2 with its 28 parts is simpler and fairly easy to maintain. However, maintenance of the MK2 doesn't look advantageous compared to the Cousteau SEA diaphragm regulator, wherein the high pressure poppet and balance assembly can be removed and replaced in 5 minutes with a screwdriver, or with a coin if needs must. The Cousteau SEA regulator has 31 parts, and even including the environmental kit, the count comes to only 35, the same as the Scubapro MK10, which is to some a so called benchmark for simplicity. So, why are some people still saying that piston reducing valves are "simple" compared to diaphragm mechanisms? It has not been a true statement-- in other than a nitpicking, overreaching exaggeration, for at least 20 years.

 

[cerich]

Disclaimer- I have been a long time flow thru balanced piston fan!

The better flow that flow thru pistons have given in the past is now being matched by newer design diaphram regs which also have the advantage of better IP control and cold water performance. Right now I'd give my nod to diaphram regs as being "better".

A good high performance choice of either design will suit most divers (tech or rec) just fine.

 

[pescador775]

Regulators produced in the 1950's produced enough flow to dive to 400 feet. What more do you need? The hoopla surrounding "flow" was a Scubapro trick which related to the second stage, not the first stage. The purge on the Scubapro second was flexible and capable of being depressed all the way whereas, for safety, the competing brands had purge buttons with limited travel. When the shop owner pushed the purge on the SP reg a big "whoosh" would ensue and goggle eyed newbies would automatically genuflect. You see, the entire lives of some divers, decades of opinion formed mostly in their first class, have been based on advertising tricks and tomfoolery like the flexible purge button. In fact, the Scubapro MK5 first stage piston stem had a larger bore than the valve on which the regulator was mounted. So, how could a regulator produce more flow than allowed by the smallish valve orifice of the time? It couldn't and the whole through flow issue was propaganda.

 

[cerich]

Regulators produced in the 1950's produced enough flow to dive to 400 feet. What more do you need? The hoopla surrounding "flow" was a Scubapro trick which related to the second stage, not the first stage. The purge on the Scubapro second was flexible and capable of being depressed all the way whereas, for safety, the competing brands had purge buttons with limited travel. When the shop owner pushed the purge on the SP reg a big "whoosh" would ensue and goggle eyed newbies would automatically genuflect. You see, the entire lives of some divers, decades of opinion formed mostly in their first class, have been based on advertising tricks and tomfoolery like the flexible purge button. In fact, the Scubapro MK5 first stage piston stem had a larger bore than the valve on which the regulator was mounted. So, how could a regulator produce more flow than allowed by the smallish valve orifice of the time? It couldn't and the whole through flow issue was propaganda.

Yeah the first had flow and unacceptable IP drop that made them hard to breath with a standard downstream second, thus balanced seconds which soon became marketing for better performers which is not neccessarly the case.

As far as second stage flow, nothing does it like a servo reg, jetstream, zeta, omega etc.....

 

[DA Aquamaster]

Disclaimer- I have been a long time flow thru balanced piston fan!

The better flow that flow thru pistons have given in the past is now being matched by newer design diaphram regs which also have the advantage of better IP control and cold water performance. Right now I'd give my nod to diaphram regs as being "better".

A good high performance choice of either design will suit most divers (tech or rec) just fine.

I agree completely. As indicated earlier, I have been converted from being a long time Balanced Piston fa MK 10's Mk 15's Mk 20's and Mk 25's to being an ardent Mk 17 fan.

Piston designs have been largely stagnant while diaphragm designs have benefitted from refinements that now make then very attractive options for diving in very demanding conditions.

 

[superstar]

DA Aquamaster,
Refering to the MK-5 you brought up the tolerance between the piston stem and body due to wear. What is the measurement for these parts. It would be nice if Scubapro had a dimension in the tech manual to determine if the part is within tolerance.
Most references are to the MK-5/MK10 or the MK-20/MK-25, why is the MK-15 left out?

 

[redacted]

DA Aquamaster,
Refering to the MK-5 you brought up the tolerance between the piston stem and body due to wear. What is the measurement for these parts. It would be nice if Scubapro had a dimension in the tech manual to determine if the part is within tolerance.
Most references are to the MK-5/MK10 or the MK-20/MK-25, why is the MK-15 left out?

That is probably a trade secret. But if you are having problems with one and have the capability of that fine a measurement, go thru your 010 o-rings and try one that is on the high side of the o-ring specs.