MK25's and G250v's or MK17's and A700's

[CPH]

Those little doubles are adorable. How old are they? 13months?

Those little doubles are holding 4200 litres of air and my other set holds 6000 litres.
The 300bar double 10s are a good first dive set and the 300bar double 7's are a great second dive set.

 

[silver02ca]

MK17 and G250V all the way. Speaking from a technicians point of view the environmental seal on the MK17 protects the regs insides from wear and tear due to the all the crap that can get in. The G250V just because of its reliability and because it has less parts that can break. Don't get me wrong all 4 are great but if I had to choose.

 

[James R]

Single tank diving I'd go with Mk17/A700 (and that is what I currently dive).

Doubles/sidemount (tech/cave/wreck/etc) I would go with MK25/G250v for better hose routing and other benefits already listed in the thread.

 

[henrik242]

I use MK17 / S600 + MK19 / R295 on my doubles. I guess I'll replace the latter second stage some day. The hose routing is quite good, though. I've used this setup in -1 C water temps without problems.

 

[Karl_H]

Unless it's very cold water or really polluted with all kinds of nasty corrosive fluids I'd always go piston over diaphragm, so Mk25 unless it's essential for the environmental seal.

The G250 and S700 are pretty similar, except the G250 is bigger and more sturdy. If you're planning on going deeper, use the 250, it has a bigger diaphragm so works better at depth - the inside is also easier to get to once the pin to hold the logo the right way is removed too Its the G250 all the way on this one, for all circumstances.

 

[halocline]

The G250 and S700 are pretty similar, except the G250 is bigger and more sturdy. If you're planning on going deeper, use the 250, it has a bigger diaphragm so works better at depth

Would you explain this please?

The current G250 is the G250V and is quite a bit less similar to the S600 than was the previous G250HP. Not that it matters much, but I am interested in your explanation of how diaphragm size translates to better performance at depth.

 

[Karl_H]

Would you explain this please?

The current G250 is the G250V and is quite a bit less similar to the S600 than was the previous G250HP. Not that it matters much, but I am interested in your explanation of how diaphragm size translates to better performance at depth.

Oops, should be bigger volume reg, not diaphragm, I was still thinking diaphragm from the first stage issue.

The bigger the reg volume the easier it is to breath as it gives a larger buffer between the first stage opening and closing so allows for a smoother transmission. I'm more familiar with the 250HP but can't see the design being that different? I'll look into that one.

The bigger diaphragm should also help in theory, but that wouldn't be a main driving factor, nor would it be something anyone could sense without some kind of measurement.

 

[halocline]

Oops, should be bigger volume reg, not diaphragm, I was still thinking diaphragm from the first stage issue.

The bigger the reg volume the easier it is to breath as it gives a larger buffer between the first stage opening and closing so allows for a smoother transmission. I'm more familiar with the 250HP but can't see the design being that different? I'll look into that one.

The bigger diaphragm should also help in theory, but that wouldn't be a main driving factor, nor would it be something anyone could sense without some kind of measurement.

I don't believe any of this is true. The larger size of the G250 means that there is more air at ambient pressure (although not much) and if anything, that would cause a theoretical increase in WOB due to less efficient venturi assist and greater position sensitivity. What I think you are referring to is an increase in the size of the IP chamber, which includes LP hoses, so that when the diver inhales, demanding air from the IP chamber, the IP drop is lower because there is a relatively larger volume of air at IP from which to draw. This is a theoretically true statement. One way to at least get an idea of the effect would be to take one 2nd stage, first on a very short hose, then on a very long hose. Maybe try a 22" (do they make shorter?) hose first, and link together two 7ft hoses for the second try. Then try both hose set ups on the same 1st stage. You can't hook them up at the same time with two identical 2nd stages because you'd then be breathing off the same high-volume IP chamber. Anyhow, you could hook up an IP gauge and see if there is any difference in drop during inhalations with a very low volume IP chamber (short hose) and a high volume one (long hose). Measuring accurate IP drop during air flow has its own challenges, though!

The size of the diaphragm in the 2nd stage will have essentially zero influence on how the regulator responds to changes in depth. A much larger diaphragm does theoretically offer a greater mechanical advantage on levers of identical size, but that has nothing to do with depth, and there are other issues, in particular the geometry of the lever, that could play a large role in how easily the reg breathes at any depth. 2nd stages pretty much function the same at 5 ft or 200 ft. It's the 1st stage that is taxed more at greater depths.

The size of the 1st stage diaphragm on its own likewise has zero influence on WOB or depth compensation, as long as you have a balanced valve. (I.E. the relationship between diaphragm area and orifice area is irrelevant) There is something, though, about mainspring tension and overall 1st stage diaphragm size that's nagging at my brain, so maybe Luis can chime in. He knows everything!

 

[Karl_H]

I take your point on the theory but having a larger reg volume does help significantly in breathing at depth, it slows down the rate the second stage needs to open and makes the initial breath much smoother - there's a noticable difference in the first part of the inhale. The ambient pressure air is the bugger for the second stage regulator mechanism while the LP hose is the buffer for the first stage. That's also backed up in practice by a lot of time underwater using both G250 size regs and smaller the S700 style regs.

The larger diaphragm gives a bigger surface area vs reg volume ratio, in theory this should make the WOB easier? Albeit a tiny difference, it's effectively just a thin piece of rubber after all!

With regard to the OP I'd still say G250 either way

 

[halocline]

I take your point on the theory but having a larger reg volume does help significantly in breathing at depth, it slows down the rate the second stage needs to open.... That's also backed up in practice by a lot of time underwater using both G250 size regs and smaller the S700 style regs.

The larger diaphragm gives a bigger surface area vs reg volume ratio, in theory this should make the WOB easier? Albeit a tiny difference, it's effectively just a thin piece of rubber after all!

Sorry, but this simply isn't true. I'm not denying that your G250 has felt better to you at depth than your S600, but your reasoning is false. 2nd stages operate the same at any depth. The added volume of ambient air in the somewhat larger case does nothing to alleviate the opening of the 2nd stage valve; and in fact adds some complications with venturi assist and position sensitivity as I mentioned before. You might try diving with a double hose sometime; those have a relatively huge volume of ambient air and you can immediately see how it affects WOB. Good dh regs have a SERIOUS venturi assist to overcome the added effort of moving ambient air through the hoses. I certainly agree that the larger volume of ambient air changes the feel of the reg; it allows the air to warm and slows it's flow. But that's not 'easier' breathing, it's just a different feel.

Assuming the 2nd stage was a sphere (which it's not, but it's easier to visualize) a larger size would have a smaller surface area to volume ratio, not a larger. In practice, I suspect that the ratio of diaphragm surface to volume is about the same in the G250 and S600, but again, this alone has zero affect on the 2nd stage's depth compensation or even overall flow capacity.

There was a thread a while back about 2nd stage function at increasing depth, if you search for it you might find it interesting.