MK2 overhaul question

[redacted]

Any explanation for this claim?

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 use..

I do understand the idea of setting IP on the high side for a main kit with a Mk2 and an unbalanced 2nd, but I use my Mk3/R190 on a pony and like the drop in performance when tank pressure falls below 500 psi. The difference is noticable but not that great. Probably less than a quarter turn on the orifice. I'm not sure that the reduced stresses from the lower IP are not well worth the very slight loss of performance whe the tank pressure gets low enough that the diver should be headed for the surface anyway.

 

[bl6394]

Any explanation for this claim?

Yes. That is the manufacturer's direction to technicians. They designed the reg - and have more insight into it's performance characteristics as a function of the initial IP than me. I won't try to speak for them. If you'd like further clarification - you'd have to get that from them.

 

[redacted]

Yes. That is the manufacturer's direction to technicians. They designed the reg - and have more insight into it's performance characteristics as a function of the initial IP than me. I won't try to speak for them. If you'd like further clarification - you'd have to get that from them.

So the answer is "Rene said..." That is disappointing. Most good engineers realize that "why" is often more important that "what". It is the difference between knowledge and understanding.

But I'm pretty sure I know their rational - it brings the gear back to the shop more often.

 

[bl6394]

So the answer is "Rene said..." That is disappointing. Most good engineers realize that "why" is often more important that "what". It is the difference between knowledge and understanding.

But I'm pretty sure I know their rational - it brings the gear back to the shop more often.

Awap - you make a fair point.

Frankly, I'm a software engineer; but only a regulator technician. I can describe in terms of first principles why in object orientated development the Builder pattern is frequently a superior implementation to simply using a telescoping contructor when constructing objects. However, with respect to the regulators I'm servicing - I was not involved in their development or design. I've been given directions from the manufacturer to follow - and without extensive research on my own - which would be both time consuming and of limited value - I have no basis to contradict the manufacturers recommendations. A bit like an automotive technician, he may not know why the GM 5.3 V8 flex fuel engine uses an oxygen sensor with a voltage range of 2.8 - 3.3 Volts. As opposed to the sensor on the GM 4.3 V6 - which has an different O2 sensor with a range of 2.5 - 3.0. He simply knows enough to verify function - and properly service the vehicle.

Perhaps you're right about Scubapro in your assertion above. Or, it could be that they choose an initial value of 145 because they anticipate an IP drop in the MK2 of 20 psi as it goes down to 500psi supply and they may have found that their unbalanced second stages perform poorly when the IP drops below 125. They didn't explain their reasoning to be in the class - and this is solely conjecture on my part.

 

[axxel57]

Halo is right, why should exact 145psi IP be so important, if while diving we are anyway only very short in that range.

So why is the chief technician of SP making himself a fool as it looks?

The only idea I came up now:

Over the last years I got used to the fact that brand new MK2 had an IP difference from full to near empty tank between 15-17psi and used ones around 20psi.

I think that in the nineties we were not talking about a difference of 15-23psi, but more about 8-15psi if I remember right.

I read now a brochure about the new MK2 EVO which supposedly flows now 15% more air than the older model. It says the new Evo has a bigger piston and this is why, which is not really clear to me.

To my understanding is, one has to make the orifice in the 1st stage a little bigger to reach a higher flow.

I haven’t had up to now one of those new MK2 EVOs in my hand, so I don’t know if the new replaceable orifice is really bigger than the ones I’m working on right now.

But if I should be right then the downstream power of the tank pressure on the soft seat should be bigger than with the smaller orifices which means the tank pressure would apply a bigger part of its force on the soft seat (for example 18psi instead of the usual 15psi).

This higher part of the downstream force would be missing in the end of a dive with a near empty tank, increasing the difference in IP between full and near empty tank, means the balancing is now a bit worse than before with the older models.

That SP doesn’t give a lot on the balancing qualities of some of their products they have shown with their diaphragm 1sts as long they can boast with flow rates.

The flow rate of the current (before EVO) MK2 models was 92 SCFM and I think in the nineties the flow rate of the MK2s had been lower, but I’m not sure and could not verify it this morning, maybe somebody can clarify.

So if SP has over the time increased their flow rate by the means I described, then they are to my understanding at the limit of their non balancing if they want to keep their 125-145psi range for their MK2s.

This would be for the moment for me the only reasonable explanation why teachers of Tech Courses could insist on the 145psi adjustment at full. They would simply not know how Techs could otherwise stay within the current specs.

So my idea is simply that SP was accepting a higher and sustained IP difference for their MK2s in change for a (now) 15% higher flow rate which nobody needs and that they got away with the same thing before.

I’m not sure if everything is technically right as I described it, but I’m sure, if not, some of the ‘usual suspects’ will enlighten me.

 

[halocline]

My assertion that worn a worn IP seat is causing Awap's IP to increase over time is just conjecture....

I suspect that if it was just worn orings - and HP gas was going around the seat somehow - the reg would have an IP creep - until the second stage burped the excess pressure - and then the whole cycle would repeat.

Worn HP seats do cause IP creep. IP is determined not only by the amount of pressure required to push the piston into contact with the HP seat, but also the amount of force required to get a positive seal between piston and seat. As the seat wears and inevitably deforms, the sealing fore will increase. This is typically seen as IP creep.

In SP piston 1st stages, both the MK2 and MK5/10/25 etc designs, there is no way any worn o-rings can cause IP creep. The only thing separating HP air from IP is the HP seat/piston connection. The "burping" you're talking is usually a problem in the 2nd stage balance chamber; IP air leaks out, IP drops, the 1st stage opens up, and IP is restored.

If there's enough IP creep to cause the 2nd stage to start flowing, it keeps flowing.

 

[halocline]

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.

Balanced 1st stages are designed to maintain constant IP throughout the full range of tank pressure; that's what 'balanced' means. Unbalanced piston 1sts drop IP as tank pressure drops, unbalanced diaphragm 1sts (of which there are probably none currently made) raise IP as tank pressure drops.

Using a balanced 2nd stage with an unbalanced 1st does have a real benefit; balanced 2nds compensate for changes in IP do a degree, so the characteristic of cracking effort increasing as the tank empties will be largely mitigated.

 

[halocline]

I read now a brochure about the new MK2 EVO which supposedly flows now 15% more air than the older model. It says the new Evo has a bigger piston and this is why, which is not really clear to me.

To my understanding is, one has to make the orifice in the 1st stage a little bigger to reach a higher flow.

You're right, the orifice would have to be bigger for increased flow. But if you make the orifice bigger without increasing the size of the piston head proportionally, the IP drop over the supply range will increase, because downstream force on the seat will be a larger percentage of the overall force determining IP. So they likely increased the piston size specifically to allow an increase in the orifice size. There's probably a slightly larger hole in the side of the piston shaft and a slightly larger diameter piston shaft.

Getting back to the "IP at 145 for best performance" bit, it is true that the higher the IP, the higher the flow rate. So on paper, in a lab, with a test machine, the numbers would be different.

But in the real world, the flow-through balanced piston regs all supply way more air than even a high performance 2nd stage can possibly flow. So lowering the IP (and subsequently the flow rate to the 2nd stage) on a MK10 or MK25 will have little to no effect on actual dive performance at anywhere near reasonable depths (like less than 200 ft) as long as the 2nd stages are tuned to the specific IP of the 1st stage.

I set my MK5s and 10s on the low end, between 125 and 135. IP is a little like blood pressure, it's only going up with age. So I set 'em low and get 4-5 years out of them between rebuilds.

 

[halocline]

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.

This is not true. The term "overbalanced" in diaphragm regs has nothing to do with how IP relates to tank pressure. It is used to describe sealed 2nd stages in which the seal diaphragm is larger than the internal diaphragm, and consequently IP rises more than the increase in ambient pressure as depth increases.

The MK11 and 17 experience a rise in IP throughout the supply range because they are not fully balanced; the area of the balance chamber must be slightly less than the area of the orifice. The old unbalanced diaphragm regs like the DA aquamaster had a pretty sizable IP rise throughout the supply range.

I realize you are trying to helpful and participating in a DIY thread voluntarily. However, if you flaunt credentials as a professional, and then provide inaccurate or simply wrong info, you can expect we 'amateurs' to hand it back to you.

Sorry for all the consecutive posts. I have not yet figured out this new SB software very well.

 

[axxel57]

You're right, the orifice would have to be bigger for increased flow. But if you make the orifice bigger without increasing the size of the piston head proportionally, the IP drop over the supply range will increase, because downstream force on the seat will be a larger percentage of the overall force determining IP. So they likely increased the piston size specifically to allow an increase in the orifice size. There's probably a slightly larger hole in the side of the piston shaft and a slightly larger diameter piston shaft.

Getting back to the "IP at 145 for best performance" bit, it is true that the higher the IP, the higher the flow rate. So on paper, in a lab, with a test machine, the numbers would be different.

But in the real world, the flow-through balanced piston regs all supply way more air than even a high performance 2nd stage can possibly flow. So lowering the IP (and subsequently the flow rate to the 2nd stage) on a MK10 or MK25 will have little to no effect on actual dive performance at anywhere near reasonable depths (like less than 200 ft) as long as the 2nd stages are tuned to the specific IP of the 1st stage.

I set my MK5s and 10s on the low end, between 125 and 135. IP is a little like blood pressure, it's only going up with age. So I set 'em low and get 4-5 years out of them between rebuilds.

You are right, that’s probably what the brochure is referring to.

I was so much concentrated on the downstream force of the tank air and the main spring force that I forgot against which forces those two forces are working and which factors are responsible for that ( like the size of the piston head).

So for the moment I have no other idea why a supposed well educated tech teacher could insist on such a blunder.

In my remark that I would not believe that awap would lower his IP on an unbalanced MK2 when heading for strenuous dives to save some money, I was referring to the missing downstream force of the lower IP on the mechanism of a typical unbalanced 2nd ( which I just supposed that was meant, because those simple Mk2s normally don’t come with a balanced 2nd, that would be again a different thing), which would not allow a low adjustment of the cracking effort (CE), which would not allow the best performance (WOB).

I know that awap probably would not adjust the IP of his flow by piston 1st that low, but just for fun I adjusted this morning a MK2 to an IP of 130psi at 3000psi tank pressure and adjusted on a R190 the cracking effort as low as I could, it was in the end 1,2inch/h2o.

I lowered the supply pressure to 500psi and ended up with an IP of 108psi.

For me surprisingly the CE increased only to fully acceptable 1,6inch/h2o

With the same 2nd stage adjustment and a 1st stage at a ‘crisp’ 145psi IP I started with a CE of 0,9 inch/h2o, which increased to 1,2 inch/h2o at 500psi supply pressure and an IP of 128psi.

At flow at 5 SCFM the dynamic IP of the to 130psi IP adjusted 1st stage lowered to 101psi and a CE of 2,3inch/h2o until shortly after the venturi kicked in the CE lowered to acceptable 1,5inch/h2o at 7,5 SCFM and 1,4 & 1,3inch/h2o at 10 and 12 SCFM

So far you guys seem to be right that the lower adjusted IP doesn’t mean automatically a significant worse performance (WOB). I had expected a bigger differences.

Taking a MK2 with an unbalanced 2nd stage on a strenuous deeper dive with an IP pretty low adjusted to save money still seems to me a bit pushing the envelope and I personally wouldn’t take chances.

I wouldn’t expect any problems in performance lowering the IP of balanced 1sts or if you are diving balanced 2nds.

I'm not sure how you use the term flow rate. Of course are the flow rates of modern 1sts and 2nds, as I it understand, more than enough for any dive profile near the recreational level.

But if lowering the IP has no or extremely little effect on the performance of a MK10 or MK25,then that has to do with the fact that they are balanced, so the downstream force has hardly any effect on the IP at full or near empty tank.

For me flow rates could have an importance if you are planning to hang 15 divers on one MK25 at 100 feet and let some of them panic.

But anyway, maybe we are talking about the same thing........