More question from the great parts box from Skip Cox, why did they ever stop making the MK5?

[lexvil]

Another rhetorical question, I suspect it was to save machining costs of the HP o-ring but here is another zombie from the $35 parts box. This had the yoke and retainer mixed in with all of the other little bits along with the main body and the MK2 in the other post. I sorted the parts out and decided to put this together so I stripped the old o-rings out cleaned it with dish soap and put it together, I used a + seat from a kit I had and no shims.

125 up and no creep, I’ve had it holding pressure for several hours and no change.

I don’t know what I’ll do with it but it seems ready to go I did change out the old key yoke screw for the Mk15 one that was in the box. This is one of the reasons that the people asking if their 15 year old regulator needs to be replaced makes me smile, this pretty much looks to be a circa 1970 but since it was parts I can’t be sure.

 

[txgoose]

It would appear Houston passed a city ordinance barring parts bins at dive shops. That is a neat rebuild.

 

[Couv]

Why did they stop making it? After upgrading to two 7/16" hp ports, 5 l/p ports, a heavy yoke, stepped piston, and a ss turret retainer SP could not improve it anymore......so they made the MK10 which incorporated all those improvements to sell more regulators.

 

[Luis H]

I have a good idea on what I consider is the primary reason why the Mk-5 was replaced, but this is only based on a educated technical guess… and I have not confirmed it with anyone at Scubapro. That said, I doubt that there is anyone left at Scubapro that was around when this decision was made, but I don’t really know that.

IMO, the reason had to do with the increase popularity of higher pressure cylinders.

The Mk-5 design in general does not like the higher operating pressure cylinders. I am not referring about the older yokes. Those were replaced over the years, in two stages.

The problem with the Mk-5 with the higher tank pressures is with the high pressure piston O-ring (the small one that seals on the piston shaft). It will have a tendency to be extruded with the higher operating pressures.

There are two traditional ways to avoid O-ring extrusion on a dynamic gland seal.

The best way is by reducing the clearance or gap between the piston and the body. The problem is that this involves tighter tolerances, which is difficult to do when the shaft of the piston has to seal in the main body and the head of the piston has to seal on a separate part that is threaded to the main body.

In a perfect world when the two parts are screwed together, it should all line up and be concentric. But in the real world, then you have to add up fabrication and assembly tolerances.

So the first solution was the MK-10. The two sealing surfaces for the piston in the Mk-10 are machined into one solid metal body. It is all done in one machining setup and the relative tolerances can be very tight. The concentricity of the two sealing surfaces can be dead on.

IMHO, the only disadvantage of the Mk-10 is that the piston head had to be relatively small to fit into a relatively small package. The bigger the piston head, the more stable and constant is the IP (this applies to both balanced and unbalanced piston first stages).

So Scubapro wanted to go back to a bigger piston head.

Some machining processes with tighter tolerances (at a reasonable cost) have improved a little bit through the last few decades, but not as much as many advertise and tend to believe (at least not in this short time period).

The other solution to reduce the extrusion gap is to use an O-ring with a back-up ring. This is the solution that the diaphragm Conshelf regulator used in the high pressure balancing chamber O-ring since the mid 60’s.

I will admit that I am not super familiar with the newer Scubapro regulators (the Mk-15, Mk-20, and Mk-25), but they all look like they incorporate a high pressure piston O-ring with a bushing/ back-up ring configuration. The purpose of this design is to have a tight O-ring support in a floating orifice surface to back the O-ring and avoid extrusion. This is accomplished without the need high precision (or tight tolerances) between the main body and the piston head closing body.

The disadvantage is the need for more parts and complexity (and associated cost). But that is the tradeoff in order to handle the higher pressures.


Many Mk-5 will handle the 3500 psi cylinders just fine, specially with higher durometer O-ring, but IMO, as a company Scubapro could not push its luck.

 

[Kupu]

Many Mk-5 will handle the 3500 psi cylinders just fine, specially with higher durometer O-ring, but IMO, as a company Scubapro could not push its luck.

Is there any reason to think that a later generation MK5 could be better at handling higher tank pressure vs early generations?

 

[Luis H]

It is possible that Scubapro might have reduced the clearance/ gap between the piston and the body, but I don’t know. It would have required holding tighter tolerances at a higher cost, so I doubt that there would have been much of a difference.

On the other hand, if you are lucky, some individual Mk-5 are going to be better than others.

You can also (potentially) improve the performance with different O-ring material and durometer. I say potentially, because other factors (variables) can play a role.

I know some of the Scubapro DIY (usual suspects) have tried different O-ring material and I have heard of good results.

The condition of the piston surface could also play a part on the results.

YMMV

 

[ams511]

D.A. Aquamaster had a post on this somewhere. From what I remember he attributed the change to the MK-10 because it was easier and less expensive to manufacture than the MK-5.

 

[Luis H]

Cost of manufacturing always plays a factor.

In this case, the cost associated in manufacturing a redesigned Mk-5, with a smaller gap and tighter tolerances (to meet the new pressure requirements) was probably a big consideration, if it was even possible or reasonable to make those design changes.

The interesting thing is that Scubapro then went from the MK-10 to the Mk-15, 20, and 25. All three of those first stages are more complicated with a higher parts count. With very few exceptions, a higher parts count is always associated with higher production cost.

 

[lexvil]

Cost of manufacturing always plays a factor.

In this case, the cost associated in manufacturing a redesigned Mk-5, with a smaller gap and tighter tolerances (to meet the new pressure requirements) was probably a big consideration, if it was even possible or reasonable to make those design changes.

The interesting thing is that Scubapro then went from the MK-10 to the Mk-15, 20, and 25. All three of those first stages are more complicated with a higher parts count. With very few exceptions, a higher parts count is always associated with higher production cost.

Just my opinion but the 15, 20 and 25 were simpler to machine in progression, having to machine the o-ring grove in the mk’s 5 and 10 are a more costly proposition than the straight bore job for the HP seal, the 15 was a bad design with its deep set clip retainer, the reverse bore of the 20 makes more sense with the pressure holding the seal in place, one less step in machining and quicker to service.

 

[Sam Miller III]

Interesting !
Now that all the principals of SCUBA Pro are no longer with us questions arise ...
Gone and probably forgotten are
Gus dela Valle
Dick Bonin
Jim Christianson
Sam Ichikawa
Mike Burke

Only one who was at SCUBA Pro at the time of the MK Five (Mk V ?) was Jim Decker who suddenly and unexpected departed SCUBA Pro and never reentered the diving world as an engineer.

A huge Thank You to Luis H ! As always your engineering expertise is appreciated

SDM