Which is better? Mk25 or an Overbalance first stage?

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DatSRBoi

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I am still new to scuba diving so sorry if these two can not be compared. I am still learning. I understand that Scubapro does not make any over balance first stage but instead they make one to bench flow so much are that maybe even elephants can breath off it (inside joke) while other company comes out with overbalance first stages to cope with flow as you go deeper. My question I guess is which is better? I guess I am still trying to understand them. I brought a hollis overbalance first stage which I am sure will always supply plenty of air for me 100feet plus under water but would a MK25 still out perform it?
 

cbrich

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I have both kinds of regulators, and I prefer my MK25/A700 regulator over the Hollis DC3/212. I was not even aware of an over-balanced regulator, just balanced and un-balanced regulator. The MK25/A700 or S600 breathes excellent down to 124'-126'. I was turned onto Scubapro when I rented a MK25/S600 from my LDS, and took it down to 124', and I did not mess the inhalation knob, once I got it dialed into the way I like. I found myself constantly adjusting the inhalation knob on my Hollis; this constant adjustment got to me and I had it rebuilt, reason why I rented the Scubapro. I dove my Hollis a few times after that, and descided to get a Scubapro.

I know of a guy that has constantly taken his MK25/S600 down to 200'+ several times, and he still prefers the MK25/S600 over regulator out there.
 

herman

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Without getting into a discussion of the internals of the regs, the bottom line is, you as a user will not notice the difference in first stages assuming you are using comparable second stages. Make your decision of a reg based on the second stage, not the first.
 

ajduplessis

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MK25/s600 rocks!!! You can dive nearly any condition at any depth. This combination works even better the deeper you go.

I will differ to the comment made by herman. I base my second stage decision on the first stage. What is the use in having a crappy first stage and a great second stage? If the fisrt stage is not delivering good reliable perfomance, what is the benefit is a good second stage? It is the combination of the 2.

If you have the ca$h upgrade to the MK25/A700.
 

rcs9250

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Overbalancing is achieved on diaphragm regulators that have an environmental diaphragm in addition to the primary diaphragm. The environmental diaphragm , the one that is in contact with the water serves a couple purposes.
1. keeps dirty water out of the main spring area.
2. Helps insulate the main diaphragm from very cold water and helps
minimize the effects of adiabatic cooling inside the reg from air pressure
differentials.
3. And in the case of overbalanced regs it does the overbalancing.

The actual overbalancing is caused by the outer diaphragm being slightly larger than the main diaphragm. It is a matter of simple mathematics with respect to air and hydrostatic pressure. The result. Easier breathing the deeper you go. Especially helpful because the air gets denser the deeper you go. In normal rec diving to 130 feet, you'll never notice the diference.
Just breathe trimix. It will make it easier. :)
 

DA Aquamaster

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Well...I'd argue that "over balancing" as described above is just trying to make a silk purse out of a sow's ear. In other words, the dry sealed chamber is less than perfect at maintaining a constant IP relative to ambient pressure and they are trying to call that engineering error a positive attribute.

There is no upside to this type of "over balancing". If the reg really has enough performace in terms of flow rate, there is no benefit to increasing the IP at depth by a few percent as if you are within a few percent of not having enough flow rate - its a crappy reg that has no business being at that depth in the first place.

And that dubious "advantage" comes at significant cost. Since even a balanced downstream second stage has a downstream bias (to ensure it can act as a safety valve in the event of a high pressure leak and resulting creep in the IP), you'd have to detune the reg to ensure it does not freeflow at depth. On a non adjustable reg, that means you get higher inhalation effort at shallow depths and on an adjustable reg it increases work load as you have to adjust it with significant changes in depth if you want peak performance.

The "denser" air argument has some truth to it, but that truth is that increased density due to increased IP does not become a factor in terms of significant flow reduction until you are down around 600' and by then you have long since added He to the mix and he flows through the reg much easier. In short, there is no density problem to solve.

In other words, this concept of "overbalancing" is pretty much a load of crap.

There are however other definitions of "over balanced"

Scubapro defined "over balanced" in an entirely different manner. The MK 1, 5, 6, 7, 8, 9, 10, 15 and early Mk 20 were all "balanced" flow through piston first stages, but because the piston stem had a constant diameter (the area of the end of the piston stem was the same as the area in the middle of the stem where it passed through the HP o-ring) that actual area of the knife edge that rested on the seat was not balanced. This resulted in a 4-6 psi difference in IP as supply presures dropped from 3300 to 300 psi. Not a big difference but a difference.

With the Mk 25 (and later Mk 20s) the end of the piston stem is slightly larger so that the area inside the knife edge is equal to the area of the stem that passes through the HP o-ring. This results in near perfect balancing with no change in IP. Scubapro called this oversized diameter on the seating end of the piston, "over balancing". In this case it actually meant something more than marketing BS as it was an engineering solution to less than perfect balancing of a flow through piston design.
 

herman

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MK25/s600 rocks!!! You can dive nearly any condition at any depth. This combination works even better the deeper you go.

I will differ to the comment made by herman. I base my second stage decision on the first stage. What is the use in having a crappy first stage and a great second stage? If the fisrt stage is not delivering good reliable perfomance, what is the benefit is a good second stage? It is the combination of the 2.

If you have the ca$h upgrade to the MK25/A700.

The reason is 99% of the performance of a reg assembly comes from the second stage, not the first. Assuming you are using a quality balanced second stage, the first stage has pretty much no effect on the performance of the second stage as long as the first keeps the IP within a reasonable range and can provide the reguired flow rate, something pretty much any first stage on the market today can do. Hook your S600 to a properly working MK-2 and you will not be able to tell the difference, reguardless of depth unless you get far deeper than most of us dive.

DA, I did some snooping around a while back on the whole overbalanced thing as it relates to AL/Apeks regs and from what I could find, if it is working per design the "overbalance" worked out to about an additional 15 psi at 100ft, not enough to make any real difference to pretty much any diver, esp when hooked to a quality, balanced second stage.
 
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halocline

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The really dumb thing about the supposed "overbalanced" bit is that the higher IP just means more pressure in the 2nd stage balance chamber pushing the seat closed, so it pretty much washes out; it's adding pressure in both directions. There is the highly dubious "higher IP means higher flow" idea, but since the 2nd stage is the limiting factor in flow for any regulator, increasing the IP will not result in any more air flowing in normal use.

But, people still fall for it as demonstrated above.

I tend to run my MK5s and MK10s at 125PSI for IP, which is the low end of the spec. As the seat wears, IP is only going up.
 
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DatSRBoi

DatSRBoi

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My head hurts. I have more learning to do. So would that mean my DC2 212 hollis reg is not that great? I know there regular price was 600+ and they dropped it to 279 due to that swivel copyright issue. I guess I fell for the overbalance thing. Wonder if its a good buy now.
 

halocline

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Look on the bright side, maybe it's not really "overbalanced" and they were just making it up. :D Overbalanced isn't the right term anyway for what they're talking about, it should be something like "over depth-compensating."

I'm sure your reg is fine. I would not worry about the MK25 outperforming it. Even entry level regs from major manufacturers work really well when they're tuned correctly, and yours is (I imagine) a high performance model from a respected company. Sounds like you got it at a reasonable price, too.

The dive gear companies love to throw regs with gimmicks around; that's because A) regs have not really improved over the last couple of decades, and B) properly cared for, regs last a lifetime. Getting divers to buy new ones takes some salesmanship.
 

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