Which ScubaPro MK First Stage is This?

[ams511]

The MK2 is an unbalanced regulator. Using little words: that means that it doesn't compensate for changes in depth, which means the deeper you go, the harder it will be to breathe from it.

Don't you mean that an unbalanced first stage does not compensate for changes in tank pressure?

 

[Zung]

... Heck, I even own a MK2, along with MK5 and a couple of heavily modified MK20's...

I'd like to hear about the mods you've done to the MK20's.

 

[SDAnderson]

I'd like to hear about the mods you've done to the MK20's.

Nothing custom, all the parts are off the shelf...

• The original inlet bolt and saddle were replaced per the recall.
• The original piston was swapped for the composite version.
• The original piston cap was swapped for the finned version.
• The original spring was swapped for the TIS spring and bushing.
• The original seat retainer was swapped for the adjustable version.

I haven't swapped out the original decal. Yet.

 

[halocline]

There is no alternative universe where a MK2 is suitable for cold-water diving....Even if the ice doesn't lock everything up, it tends to cut away at the working soft bits, which means the two piston o-rings have a tendency to fail, sometimes dramatically.... Or you can compare a MK2 with a MK20/25 and see the changes SP has made when they get serious in their attempts to address the problem.

The MK2 is an unbalanced regulator. Using little words: that means that it doesn't compensate for changes in depth, which means the deeper you go, the harder it will be to breathe from it.

I'm a working repair technician and have been for many years. I've been through the ScubaPro training more than once, including their Master level class....I'm very comfortable that my training, knowledge and experience with regulators measures up well against yours. Even if your post count has mine beat.

Okay, lets recap:

1. You're a professional working repair technician. You've taken (and presumably passed) the SP "Master Technician" course.
2. You believe that the "two piston o-rings" on a MK2 will tend to fail in cold water, or that water will "freeze solid in the piston chamber".
3. You believe that the MK20 and/or 25 is better in cold water, meaning less likely to freeze. Did I read that bit correctly?
4. You believe that unbalanced means that the reg doesn't compensate for changes in depth.

Am I accurate with those statements? And, I do apologize for the tone of my previous post; no reason for me to get snitty here. It's just that what you're saying about regulators reveals that you don't understand some basic concepts about how they work, and yet SP declared you a "master level" technician and presumably people pay you to work on their regs.

If you want to continue to explain how regulators depth compensate, what "balanced" means, or how they react in cold water, be my guest.

 

[halocline]

They're very reliable, which is why I think of these things as being disposable. Unless you have access to parts and are servicing them yourself, most folks will realize the lowest TCO by purchasing a new one, using it until it kludges up and then dumping the carcass....

While I agree that annual service regardless of use is usually a waste of money, explain how this would be cheaper than simply servicing the reg when it needs it. Also, what do you mean by "kludges up?" Are you saying that the reg no longer works well? Maybe a leak somewhere? Not something you want to discover on a dive trip, is it? Then you start missing dives or using rentals while you frantically search for a repairman, hopefully one with a clue.....

I'm also quite surprised that a professional regulator technician such as yourself would describe a regulator that needs service as a "carcass." So what does that make you, an undertaker?

 

[redacted]

Lets take these point by point.

There is no alternative universe where a MK2 is suitable for cold-water diving. It's issue that even a knob divemaster should be aware of and understand; as the gas flows through the 1st stage, adiabatic cooling causes a temperature drop than can easily amount to 50F, Do the math; if you're diving in 50F water, this cooling sucks the heat out of the regulator body and can cause the water inside the piston chamber to freeze solid, locking the piston in place. Even if the ice doesn't lock everything up, it tends to cut away at the working soft bits, which means the two piston o-rings have a tendency to fail, sometimes dramatically. If you don't believe me, check with ScubaPro about how effective their T.I.S. experiment has been and what kind of temperatures they recommend for the MK2 - they'll tell you much the same thing. Or you can compare a MK2 with a MK20/25 and see the changes SP has made when they get serious in their attempts to address the problem.

Yes, let's compare. The Mk20 is a "flow through" piston which is why it is balanced. That means that all the gas you breath through it flows through that 3/16th inch or so channel in the piston stem. That is where the cooling occurs that may result in ice buildup in the ambient chamber and around the spring. Hence the plastic an rubber bits that "insulate the head of the piston and the coated spring to resist ice adhesion. Nothing really needed down in the body where ther is a lot more metal to distribute the heat differential and a lot more warm water (relatively speaking) to absorb it. Now the Mk2. It is an unbalanced piston, also called a "flow by" piston. That means that the gas you breath flow by the piston and not through it. The IP chamber in the cap does see an IP drop of perhaps 10 to 25 psi which pulls the piston away from the orifice causing flow but the breathing gas does not flow through that piston so there is little cooling effect at the piston head and spring in the ambient chamber. The cooling effect is mostly contained in the body where, like the Mk20, there is lots of metal surrounded by warm (relative again) so there is little cause for icing. Now, neither the Mk20 or the Mk2 are sealed so, if you try hard enough, you can get a Mk20 to freeze up and you may even be able to do the same with a Mk2.

For much the same reasons, you don't want to have a MK2 in dirty water. It's far too easy for crud to get inside the piston chamber and interfere with the operation of the spring/piston. Over the years I've cleaned sand, sticks, snails and worse from the insides of regulators that aren't environmentally sealed. There are multiple evolutions of environmental sealing systems out there, some work better than others. Yeah, there was an optional environmental kit for some of the later MK2's, I don't think that's available anymore.

Yes, many Mk2s have fairly large ambient chamber holes and some stuff is liable to enter. Surprisingly, the ambient chamber holes on the current Mk25 are quites similar in size. The original Mk20 and older Mk2s have the smaller holes which should have reduce the problem of dirt encroachment bat that design seems to have been abandoned by Scubapro, so I guess they must see this as not a big problem. If the water is dirty enough, a sealed regulator may be a better choice.

The MK2 is an unbalanced regulator. Using little words: that means that it doesn't compensate for changes in depth, which means the deeper you go, the harder it will be to breathe from it. I'll plead guilty to a bit of hyperbole: they make great pool regulators and I know several divers using them on 20 foot decompression bottles without issue. Where, exactly, a minor inconvenience transitions to a nuisance and then to a hazard is largely dependent upon the diver but ultimately it's all math and work of breathing. Most divers will notice a significant degradation in performance by the time they get to 3 ata, by the time they get to 5 ata they will be working uncomfortably hard.

Now you are getting scary. Are you sure you are an experienced tech??? ALL WORKING SCUBA FIRST STAGES COMPENSATE FOR CHANGES IN DEPTH. (If the dry bleed system on a Sherwood fails, it might not compensate for depth but that is a failed regulator) They have to in order to maintain a somewhat constant IP for the 2nd to work with. Otherwise, IP could drop 75 psi over the range of recreational diving depths (90 psi if you consider 160ft a recreational depth. No 2nd stage would perform worth a damn with a supply pressure of 40 to 60 psi.

A balanced first stage compensates for the change in tank pressure as the gas is consumed. An unbalanced 1st does not compensate for that change so IP will tend to fall about 12 psi as the suppl pressure falls from 3000 psi to 300 psi. That IP drop would probably be unnoticeable with a balanced 2nd stage.

I have to hope your statement of no depth compensation was just a brain fart and not the kind of nonsense any professional scuba tech actually believes. See Below

I'm a working repair technician and have been for many years. I've been through the ScubaPro training more than once, including their Master level class. For what it's worth, I've been through many of the manufacturer technician training programs over the years, including Atomic, Aqualung/Apeks, Dive Rite, Edge/Hog, Guardian/Aga, Mares, Sherwood/Genesis, TUSA, Ocenanic/Aeris/Hollis, Zeagle, yadda, yadda. In a quiet year I overhaul several hundred regulators of various makes, including a MK2 within the past few days. Heck, I even own a MK2, along with MK5 and a couple of heavily modified MK20's. I'm not a stranger to the MK2, to ScubaPro or to regulators in general. I don't know much about you but I'm very comfortable that my training, knowledge and experience with regulators measures up well against yours. Even if your post count has mine beat.

Once again SCARY.

To be completely fair, I am not a certified scuba tech. Never took a single class. But I work on my own regs because I have run into too many tech who did not know what they were doing.

 

[halocline]

Some added clarification of Awap's comments: In the MK2, the metallic surface closest to the air expanding from supply to IP is the orifice, machined into the body. In the MK20 it's the piston edge itself. So, the cooling is conducted into the piston much more rapidly and efficiently in the MK20. It's this same piston shaft that then comes into contact with water in the ambient chamber. In the MK2, there's a plastic seat imbedded in the piston, and the piston itself never is in contact with air expanding from supply to IP. This is a much more inefficient means of heat (well, cold in this case) transfer, which is one reason that the MK2 design is so much more freeze-resistant. Further, there's a lot less flow in the MK2, although still more than any single 2nd stage can handle, and certainly plenty of flow for anything close to recreational diving. However, the lower flow also means less adiabatic cooling.

But, maybe we'll get a little more education from the SP master technician.

 

[Six2Life]

I don't know much about you but I'm very comfortable that my training, knowledge and experience with regulators measures up well against yours. Even if your post count has mine beat.

Hey check out this interesting quote I just ran across...

"Ignorance more frequently begets confidence than does knowledge." Charles Darwin

Seems to apply in this situation.

I've been through the ScubaPro training more than once, including their Master level class. For what it's worth, I've been through many of the manufacturer technician training programs over the years, including Atomic, Aqualung/Apeks, Dive Rite, Edge/Hog, Guardian/Aga, Mares, Sherwood/Genesis, TUSA, Ocenanic/Aeris/Hollis, Zeagle, yadda, yadda.

You might want to consider asking for a refund.

 

[DA Aquamaster]

Some added clarification of Awap's comments: In the MK2, the metallic surface closest to the air expanding from supply to IP is the orifice, machined into the body. In the MK20 it's the piston edge itself. So, the cooling is conducted into the piston much more rapidly and efficiently in the MK20. It's this same piston shaft that then comes into contact with water in the ambient chamber. In the MK2, there's a plastic seat imbedded in the piston, and the piston itself never is in contact with air expanding from supply to IP. This is a much more inefficient means of heat (well, cold in this case) transfer, which is one reason that the MK2 design is so much more freeze-resistant. Further, there's a lot less flow in the MK2, although still more than any single 2nd stage can handle, and certainly plenty of flow for anything close to recreational diving. However, the lower flow also means less adiabatic cooling.

But, maybe we'll get a little more education from the SP master technician.

This is pretty much it in a nutshell.

The disadvantage of a flow thru piston like the Mk 5, 6, 7, 8, 9, 10, 15, 20 , and 25 in very cold water is that the piston stem itself is in contact with expanding gas and gets very cold - creating a cold surface on which ice can form. In constrast, as halocline states, the flow by design of the Mk 2, 3, 200 and 2+ puts a seat between the gas and the piston and rather than being in the middle of things, the piston in a flow by piston design is out at the far end of things so the piston stem itself stays much warmer.

Prior to the Mk 17, etc, we sold a ton of Mk 2's to PSDs in the frozen north who needed a reliable cold water and ice diving first stage. The Mk 2 performs well even under ice diving conditions where the Mk 25 won't offer any degree of reliability.

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FWIW, I've attended the pro tech and expert tech courses from SP and like their basic course, anyone who show up is going to get a certificate unless they actually manage to blow something up (and I did see an amazingly loud and spectacular failure with a totally misassembled Mk 15 about 10-12 years ago in a tech seminar). The model Scubapro uses is to have studnets be referred by a shop, and then after completion of the course for those students to continue to be mentored by a much more experienced tech. The course is not designed or intended to be "everything" a tech needs to know.

I commented at the last seminar I attended that SP got rid of their academic materials (written by Pete Wolfinger) that were very good and were basically a short version of his "Reg Savvy" book. The comment the instructor made was not impressive, He commented I was proibbaly the only person who ever read it, so it was not of much value. I think that reflects a lack of rigor in the course itself, and seems to communicate that SP is ok with giving out certs to peopel who may lack technical knowledge.

To be fair, it's in line with many other companies' training programs, but it puts a firm qualifier on what a certificate means by itself - which is not a whole lot.