Dry chamber (mk17) or not (mk25)?

[Luis H]

Well, I'm confused. Every time I see a reference to the MK17 and the MK25, people, fairly uniformly, agree that the MK17 is better and seem to all but say, "why in the hell would anyone ever buy a MK25 over an MK17?" However, from what I can tell, the MK25 costs more. So what's the deal? Is it really just a case of selling less regulator for more price? There must be something more to it than that. What gives?

FWIW, I have an MK25 and it seems to do just fine in cold water, judging by the 15 or so dives I've put on mine in 5 degree C (or less) water.

Scubapro still considers the Mk-25 there flagship first stage. It probably has a lot to do with Scubapro history, but some with the gas flow performance of the balanced piston regulator (and the potential for advertisement related to this performance).

Scubapro was the first dive company that strongly pushed the piston first stage. When they introduced the flow through piston in the MK-1 and Mk-5 the gas flow performance was taken up several notches. Therefore, IMO, for them to place a diaphragm first stage above there piston design would be going a bit against there roots.

The truth is that the piston regulator 1st stage is a perfectly good regulator for the large majority of divers…that are not interested in cold water.
The piston 1st stage has less parts than a diaphragm. It can also flow more air than a diaphragm 1st stage, but that is academic because well design diaphragm 1st stages provide more air than anyone ever needed.

About reliability, there hasn’t been a regulator to date that has shown to be as reliable as a US Divers Conshelf and that is a diaphragm 1st stage. Part of the reason is that no other 1st stage has been around as long to show its long term reliability…”you can’t build a reputation on what you are planning to do in the future”. Henry Ford?

A well design diaphragm first stage can’t be beat for long term reliability and the ability to work in harsh environments, cold water, contaminated water, etc.

 

[beanojones]

Well, I'm confused. Every time I see a reference to the MK17 and the MK25, people, fairly uniformly, agree that the MK17 is better and seem to all but say, "why in the hell would anyone ever buy a MK25 over an MK17?" However, from what I can tell, the MK25 costs more. So what's the deal? Is it really just a case of selling less regulator for more price? There must be something more to it than that. What gives?

The ScubaPro flow through piston design has been the benchmark against which all regulators have been judged forever. Nothing delivers the quantity of air that a flow through piston design delivers.

Modern diaphragm designs have finally started to come close (which is why S/P finally started carrying them, so they say). But in the end if you want high flow you want a flow through piston.

The guys who started Atomic took the flow through piston design and added the twist of making the first stage of titanium, which elminated the design flaw of a flow through piston. That flaw is that the exposure of the ambient pressure chamber to the environment can make the regs fail due to corrosion. (Don't worry about it failing on a dive. We are talking about years of daily use without ever getting dry.)

So it does not really matter now if Scubapro wants to go all diaphragm: They have some long term customers who would scream if they did. Since there are moving parts of the reg exposed to the environment, the plating and materials of the ambient pressure chamber (which is where the piston and its Oring moves) have to be pretty carefully designed and put together.

ScubaPro will probably never have as its flagship regulator a diaphragm design. And the flagship regulator has to be the most expensive one.


Holy Heck LuisH, we said almost the exact same thing. I was even wondering why my post had someone else's name next to it for a minute.

 

[Thalassamania]

From a design standpoint that is true. But I don't think that actual field experience mirrors the theoretical.

 

[Ken abucs]

Think of it as a car than can go 300 mph instead of 200.

I like this analogy, but it's useful to put it i concrete values. The MK17 supplies ~175cfm where the MK25 supplies ~300 cfm. Most 2nd stages max out at ~70cfm. So either 1st stage will supply more air than a pair of 2nd stages could require.

If you look at the review Scuba Diving Magazine - 17 New Regulators you'll see that both regs perform identically in breathing tests.

 

[Luis H]

The ScubaPro flow through piston design has been the benchmark against which all regulators have been judged forever. Nothing delivers the quantity of air that a flow through piston design delivers.

Modern diaphragm designs have finally started to come close (which is why S/P finally started carrying them, so they say). But in the end if you want high flow you want a flow through piston.


Holy Heck LuisH, we said almost the exact same thing. I was even wondering why my post had someone else's name next to it for a minute.

Well, maybe…except that I don’t agree with a couple of your statements.

Saying that all regulators have been judge against a Scubapro forever is inaccurate…forever is a long time and Scubapro is not that old. Yes, I know it is a lot older than many of the new companies we see today…so this is a minor point.

On the other hand, IMHO, this statement is very inaccurate: “Modern diaphragm designs have finally started to come close”. The Conshelf has not really changed in over 40 years and it has always performed exceptionally (the second stage was decent, but not as good as a Scubapro 109). The Titan use the exact same first stage parts (and the Legend is very similar) and they are all considered excellent performers.

The Conshelf diaphragm first stage is one of the best first stages of all times and I don't consider it a “Modern diaphragm designs”.

The power of advertising is always amazing. Even US Divers (now Aqua Lung) introduced for awhile a flow through piston regulator as the flagship of their line (the 1970’s Calypso).

Oh, I also don’t see much of a need for Titanium on a well design regulator. I have used titanium in Navy sonar equipment due to many other harsh conditions, but a well designed regulator should perform well without the use of materials that are hard to fabricate and are not forgiving.

I like this analogy, but it's useful to put it i concrete values. The MK17 supplies ~175cfm where the MK25 supplies ~300 cfm. Most 2nd stages max out at ~70cfm. So either 1st stage will supply more air than a pair of 2nd stages could require.

If you look at the review Scuba Diving Magazine - 17 New Regulators you'll see that both regs perform identically in breathing tests.

Yeah…the problem doesn’t show up until you attach several second stages (3 or more) to one first stage and all the divers get very…very excited. I let your imagination decide how this could happen…
Note: I have no direct personal experience in any related event.


Just to keep it in perspective at those flow rates (under a steady condition) any Scuba tank would be empty in less than a minute.

 

[beanojones]

Oh, I also don’t see much of a need for Titanium on a well design regulator. I have used titanium in Navy sonar equipment due to many other harsh conditions, but a well designed regulator should perform well without the use of materials that are hard to fabricate and are not forgiving.

It's there for long life. The flaw in ScubaPro flow through pistons has always been the fact they will eventually need replacing because chromed brass pits eventually. Titanium is not there for performance. It is there for non-corrosive-ness.

 

[pescador775]

The piston design will always be less reliable than a diaphragm regulator for reasons already mentioned. Corrosion, salt, sand and ice are contributing factors. This was true in 1970 with the SP MK 5 vs the Conshelf and is true today. The original selling points of the MK 5 were simplicity and flow. Scubapro technicians/NASDS instructors would do a kind of song and dance lauding the size of the flow through piston (featuring a larger bore than the valve orifice to which the reg is attached). A technician would take a MK 5 apart before the goggle eyed student and say something like "SEE, ONE MOVING PART". This implied that it was more reliable and super easy to maintain. The instructor would stand before a class and depress the purge button on a MK5 releasing a blast of air as proof of huge flow. However, not mentioned was the fact that the SP purge would depress the demand lever deeper than some other regulator brands which restricted the travel of the depressor. These theatrics were, and are, meaninless and insulting to one's intelligence.

Back in the day, the SP regulator was simpler. It was also easy to maintain. O ring technology was not as sophisticated then which made the MK5 a notorious leaker. However, the diver could take his regulator to a SP shop and present his plastic card. Problem fixed.

Today, similar claims, or myths, are embedded in some parts of the community. For example, the simplicity issue. A couple years ago, I counted the number of parts in a modern MK 25 and compared the result to the most complex diaphragm regulator in history, the MK18. The finding was that the MK25 had more parts than the diaphragm regulator.

Large flow pistons are dangerous. If this type of regulator is mounted on a large flow valve, the Sherwood 5000, an out-of-air diver will be out of luck. There will be insufficient air for some, inattentive divers to make an emergency, free ascent. However, if a Titan diaphragm reg or similar is mounted to a Thermo valve, and I have tested this in the water, there is time to make a controlled ascent when inhalation resistance becomes apparent.

The "large flow" deal is irrelevant to regulator performance. The "simplicity" claim is false.

The main influence on regulator performance is the second stage and the relatively high scores garnered by SP and some others have been due mainly to efficiency of this component.

The MK25 continues in production for one reason, brand identification, Scubapro will always make a piston reg.

 

[pescador775]

To those who are curious, a MK25 mounted to a Thermo valve will allow as much time to ascend as a diaphragm regulator. The crucial feature is flow restriction. The Thermo valve has a 0.100 orifice, much smaller than the 0.185 bore of the flow through piston and the same or slightly smaller than a typical diaphragm regulator orifice. Bottom line, I urge anyone using a piston reg to consider the Thermo valve or the old style OMS valve. I don't know the size of the HP orifice in the MK 17 but claims of large flow make me nervous. Use the Thermo.

Another PITA common to the piston reg is creep. I've seen diaphragm regs which had been in use off and on for 20 years and had never had the IP adjusted. They were still spot on. The few times that I have personally observed one of these to leak was due to the small O ring in the balance chamber or a bad HP seat. The O ring was the same age, and it was the same Conshelf regulator, 20 years old without any service during that time. The bad seat was in a double hose reg, so long ago, it's history.

 

[oxyhacker]

Another factor is that many of the most enthusiastic SP dealers tend to have been around for a while, and were indoctrinated for life into the BP cult back in the days when the SP BPs really were better than almost everything else. So they still push the BPs over the diaphragms (those that haven't switched to pushing Atomic, that is).

I've run into a few SP dealer who won't even stock any of the diaphragm SPs - they'll special order them if a customer insisted, but reluctantly.

The MK25 continues in production for one reason, brand identification, Scubapro will always make a piston reg.

 

[DA Aquamaster]

It's there for long life. The flaw in ScubaPro flow through pistons has always been the fact they will eventually need replacing because chromed brass pits eventually. Titanium is not there for performance. It is there for non-corrosive-ness.

I do reg repair for a shop that still has some very old 70's era Mk 5's in rental use. They have been heavily used and abused in salt water for decades and are just beginning to reach the point where I refuse to put some of them bac into service as I cannot get them to meet specs.

So yes, what you say is true...but the process takes about 40 years unless you seriously ignore and neglect the regulator.

The fact is chrome plated brass has an impressive record of performance in a salt water environment. Is titanium better - probably, but it will take about 40 years to really know for sure.

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That said I prefer the Mk 17 to the Mk 25. It is bullet proof in cold water, it is easier to rinse, it is not subject to IP creep, it is easy to service and adjust, it has a similar parts count to the Mk 25, I prefer the house routing with doubles with the Mk 17, it is less expensive and it offers excellent performance without the Mk 25's 300 SCFM overkill.

SP does regard the Mk 25 as it's flagship top of the line reg, and as such it costs more than the Mk 25 even though I suspect the Mk 17 is more expensive to produce.

The observation that SP has a bias toward piston regs is exactly right. SP got into the diaphragm reg business with the Mk 14, only because a growing number of customers demanded it. The Mk 14 was someone elses design that SP used to get a diaphragm model on the shelf until they developed one of their own.

That first SP diaphrag reg was the Mk 16, that saw further evolution in sat design and alignment along with a sealed ambient chamber to become the Mk 17. It took a while but you have to give SP credit for doing it right and designing an exceptional diaphragm reg. The Mk 11 is a Mk 17 without the sealed ambient chamber.

I think eventually the finer qualities of the Mk 17 will win out and SP will at some point cave in and admit to the to the reality that the Mk 25 is an over sized, over performing first stage that has gotten away from the fundamental benefits of simplicity that were the hallmark of piston regs. Nothing beats it for performance, but it is a level fo performance that no one needs and that is actually counter productive.

The Mk 17 on the other hand brings to the table everything a diver needs with no faults, shortcomings or extravagent excesses. SP does not market the Mk 19 in the use, most likely because if they did, there would be no real market for the Mk 25 and Mk 25 sales would seriously decline. Under the current price stucture dealers make a bit more profit off the Mk 25, so availability of the Mk 19 woudl be problematic for them. Also, as indicated above, old school SP dealers were steeped in the blanced piston is better tradition and many woudl either revolt, or at least have to eat 20-30 years of crow if they were forced to admit a diaphragm design was better AND was the top of the line SP reg. So I don't see the mk 25 going away soon, but it needs to.

I do see a market for a smaller and lighter Balanced Piston first stage along the Mk 10 / Mk 10 Plus lines but with an updated dry sealed or christo lube filled ambient chamber. A Mk 10V would be a great compliment to a G250V and would kill off potential rivals like the still problematic and unreliable Sherwood SR1.