MK2 overhaul question

[NotSure]

actually, I believe you are both were wrong (not completely, but, since we are splitting hairs here)... both the terms you used (plastic and elastic) are for tensile stress mechanics

for a spring, it would be compressive - fatigue failure, work hardening, etc..... now correctly eluded to in the post above..

link: https://en.wikipedia.org/wiki/Deformation_(engineering)

What I was taught and Wikipedia confirms is that when plastic deformation occurs the spring becomes shorter and harder.

"Work hardening, also known as strain hardening or cold working, is the strengthening of a metal by plastic deformation. "

I just reread my original post and rhwestffall is correct the IP change is a result of the spring becoming shorter not "weaker".

 

[rhwestfall]

Isn't plastic deformation when the object is subject to tensile stress? The failure mode for the compression of a spring is not a tensile stress..... it is metal fatigue, compressive failure, work hardening, etc...

 

[BIGJACK]

Not to change the subject, but is there a difference in the effect of the IP range between balanced and unbalanced 1st stages? I might not have asked the question properly. If I use a balanced 2nd with an unbalanced1st, will there be a real benefit? I didn't think this thread would have gone 2 pages, but then this is Scubaboard. Thanks guys and I really do appreciate the replies and opinions. Time to start a new about why my MK10 (first) rebuild didn't work.

 

[bl6394]

Not to change the subject, but is there a difference in the effect of the IP range between balanced and unbalanced 1st stages? I might not have asked the question properly. If I use a balanced 2nd with an unbalanced1st, will there be a real benefit? I didn't think this thread would have gone 2 pages, but then this is Scubaboard. Thanks guys and I really do appreciate the replies and opinions. Time to start a new about why my MK10 (first) rebuild didn't work.

Any scubapro first stage can be used with any scubapro second stage (at least - all reasonably recent examples.)

Scubapro has a standard optimal IP across it's line of first stages of approximately (125 -145 psi). Balanced / Unbalanced. Maximum IP should be 145 psi.

With that said - the tuning varies depending upon the type:

• With a unbalanced first stage like the MK2 - the regulator is tuned to 145 @3000 - with the understanding that the as the supply pressure drops the IP will also drop up to 20 psi. e.g.145 @ 3000 psi to 125 @ 500 psi

• A balanced / overbalanced diaphragm like the MK11 / MK17 will be tuned to 145 @ 500 psi - and see an IP inversion of 2-12psi as the supply pressure drops. e.g. IP of 128 @ 3000 and IP of 145 @ 500.

• A balanced piston like the MK25 is tuned to 145 psi at 500 - and will see an IP inversion of approximately 5 psi from 3000 to 500. e.g. IP of 140 @ 3000 and IP of 145 @ 500psi.

The best performance for your balanced second stage is going to occur when your IP is nearest the upper limit of 145 psi. That sweet spot will occur at different points in your dive depending upon the first stage used.

What I was taught and Wikipedia confirms is that when plastic deformation occurs the spring becomes shorter and harder.

"Work hardening, also known as strain hardening or cold working, is the strengthening of a metal by plastic deformation. "
.

I'm curious how the springs performance (stiffness) actually changes over time. Does the spring take a set while loaded in the reg? Does dynamic cycling harden the spring and increase the stiffness? Do microscopic fractures weaken the spring? I suppose that's also strongly influenced by the alloy used in it's construction and any heat treatment it received. I'd be curious if anyone had a reference that discussed.

In any event - I'm out of time for Scubaboard tonight. I've got to crate a rebreather and get packed for a trip to Florida tomorrow.

Suffice to say that while there may be some disagreement amongst a few of us - we are all are passionate about providing the best service possible for the tools we use underwater. For some of us - those regs are being used in caves or soft overhead dives where failures are not much fun.

 

[NotSure]

Isn't plastic deformation when the object is subject to tensile stress? The failure mode for the compression of a spring is not a tensile stress..... it is metal fatigue, compressive failure, work hardening, etc...

If I remember correctly the forces on a compression spring would be compression and shear. Plastic deformation is when a material loses it's original properties due to applied forces, not sure if the type of force is relevant. For scuba regulators the reason a shorter spring is a cause for concern is that a stainless steel spring that is shortened by work hardening is also more brittle and likely to break. I started a scubapro spring thread a while back because the best way to determine if you need a new spring is to compare the length to that of a new spring.

 

[NotSure]

Does dynamic cycling harden the spring and increase the stiffness?

Absent corrosion and manufacturing variances I would guess the number of cycles is the most important factor.

 

[redacted]

There is a range within which a 2nd stage mechanical spring is designed to operate most efficiently. With most regs, that range corresponds to an IP range of 125 to 145 psi. What I find hard to believe is the suggestion that, within the designed operating range, a 2nd stage will perform better when it is forced to work harder!!! (The higher the IP, the more work the spring must do to hold it back and the more work the user must do to compress the mechanical spring.)

If that makes sense to anybody, I sure would appreciate an explanation.

 

[halocline]

Considering the fact that IP drops in the MK2 about 20 PSI over the supply range, I wouldn't be too concerned with getting it exactly at 145. Take a few dozen big breaths or purge the 2nd stage a few times, and the IP will drop down to the 143 that b16394 seems to disapprove of. Or start off with a tank that's a bit overfilled, and that 145 will be more like 147 or a bit higher.

The important thing with IP and the MK2 is to get it near the high end of the 125-145 range with a full tank, so that it doesn't drop too low with a near empty tank. That can certainly affect performance. In answer to the question about a few PSI affecting 2nd stage performance, it's true that with an unbalanced 2nd stage, as the IP goes down, the cracking effort will go up, and the MK2 is normally paired with an unbalanced 2nd stage. How noticeable the increase in cracking effort is to a diver depends on the sensitivity of the diver and the amount (and rate) of IP drop over the supply range. Since the MK2/R190 et al is one of the most popular and long-lived regulators in history, it's likely that the performance drop over the supply range is not too noticeable. To me, I start to really notice increased breathing effort well below 500 PSI, at which point a safe diver should be on the surface anyways.

It's unfortunate that arrogance seems to be part of the lesson plans in the weekend-long nobody-fails regulator technician courses that the manufacturers offer. I wouldn't be so proud of a diploma earned over a weekend, even if the rest of us knuckle-draggers are spending that weekend watching football and guzzling beer. There are several pretty knowledgeable folks on this forum.

 

[Wayward Son]

My Mk2 is a Mk2+/R190. It's my daughters set. I've only dove it a couple of times, but from full down to 500psi I can't really tell much difference. have not experienced lower supply pressure than that with it.

 

[bl6394]

OK - Gear packed. I'll take a moment to poke some fun at myself.

Yes - an unreasonable fixation on an optimal IP of 145 is probably not a completely useful thing. I regret that the language in the manual is "best performance at approximately 145 - but no higher" The only other guidance from the reference video is that a deviation of 6 psi (IP of 139 on the initial reading) was worthy of shimming - and it just so happens that they hit 145 on the head. "Demonstration Quality" of course. If Scubapro's specification was tune the IP to "140 to 145" or 143 +/- 2 - I would have much less angst. It's the engineer in me.

I've got goo-gobs of various shims and a fixation on making things as perfect as possible. I probably would fiddle for a minute or two to get the IP right on 145 @3000 - or plus of minus 1psi. If you made a reasonable effort to get pretty close - yeah - I couldn't fault you.

Being OCD about gear maintenance probably serves me well as a technical diver or when I'm on a rebreather. For what it's worth - I also have an unreasonable fixation on buoyancy and trim in my OW classes too. I'm the guy that even tries to get my discover students horizontal in the pool as much as possible. Meh I guess OCD has it's good and bad points.