Thanks for all the very informative first-hand replies - I appreciate your time.
XiSKiGuy, I will try the configuration this sunday in the Channel Islands. Very pointed experience.
DA, I've just spent about 2 hours reading all your posts on the X650 since 2003. This one from 2003 stands out:
The advantages of the X650 have a lot to do with the Darth Vader wierd case design and are not immediately obvious unless you understand the concept of case geometry fault.
The X650 appears to use the same basic air barrel and poppet assemblies as Scubapro's more conventional balanced reg designs and on the surface would not offer any real improvement - except for the case geometry and that can make a great deal of difference in terms of how much of that really low cracking effort you can really use in the water.
For example, even a very old Balanced Adjustable second stage can be updated using the same poppet a G250, G500 or S600. Equipped with this poppet, all four regs can be adjusted to give a very low cracking effort under 1.0 inch in the in the .7 range out of the water. However, the regulators adjusted to that level of effort are not fully usable in the water without some diver adjustment to increase cracking effort as the cracking effort is not the limiting factor in the system.
In a conventional case design, the position of the regulator is critical. When the diver is vertical in the water, the top of the exhaust valve is about .2 inches lower than the center of the diaphragm and consequently the exhaust valve is securely seated by water pressure. When the diver rotates in the water to a normal swimming position with the reg angled down about 45 degrees, the top of the exhaust valve is now about .8 inches above the center of the diaphragm and this pressure differential bleeds air out the exhaust valve. And if the diver rotates further to a face down position, the difference in depth increases this pressure difference to about 1.25 inches.
What this means is that in a normal swimming position, a conventional second stage that is adjusted to a cracking effort of .7 to 1.2 on the surface will work ok when the diver is vertical but will freeflow slightly in a normal swimming position or when face down as the air leaking out the elevated exhaust valve will create enough suction to open the poppet and continue to supply air to feed the leak out the exhaust valve. So in the real world the diver ends up having to detune the reg by turning in the adjustment knob to increase the spring pressure and the cracking effort to the 1.2 to 1.3 range anyway to stop the slight freeflow of air out the exhaust valve.
With my own regs of this design, I adjust them on the bench for minimum cracking effort just because.... but I also recognize that on most dives I will end up with the adjustment knob turned in 2-3 turns if I want to set the reg up so that I do not have to mess with it during the dive.
So in short with a conventional case design and a high perfromance poppet you have a lot of potential low cracking effort performance that you cannot use in the water without a lot of adjustment every time you change position in the water. To make the second stage diveable in an adjustment free manner throughout the dive (or in the case of a non adjustable second stage where it cannot be adjusted) you are limited to a minimum cracking effort of around 1.2 to 1.3 inches if you are going to avoid a freeflow in the worst case face down position. A lot of people talk about how low their reg's cracking effort is on the bench, but it is kind of irrellevent if it cannot be used in the water without position related freeflow issues.
With a more unconventional case design like the D400, the exhaust valve is located in the center of the diaphragm and the worst case pressure differential is only .6. And as an extra benefit, this worst case positon occurs when you are rotated 135 degrees from the vertical and rarely occurs in the water. So a D400 adjusted to provide a cracking effort of .6 could be used throughout the dive with no adjustment required and with about half the real world cracking effort of a G250, G500 or S600. It's a very real advantage that never really shows to it's full subjective effect on a work of breathing chart nor was it an advantage SP every really marketed but one that is obvious is you dive a G250 or S600 and a D400 side by side. The D400 just "feels" better.
My first thought when I first saw a picture of the X650 was "oh goody, an angled diaphragm with a co-axial exhaust valve just like the D400". My second thought after looking closer was "oh darn, it has a separate exhaust valve, gimme back my D400."
I am now firmly middle of the road about it until I get to dive with one. I have still not seen one in the flesh but it does appears to have its diaphragm angled 30-45 degrees and the case is relatively flat in terms of diaphragm to exhaust valve distance. So while it does not appear to offer the same extreme degree of CGF advantage as the D400 in terms of case/diaphragm/exhaust valve design, it should still allow a lower usable cracking effort of under 1.0 inch without being subject to freeflow in the face down position.
With the same poppet assembly and potentially low cracking effort as the G250, S600, etc combined with a case design that appears to reduce CGF issues, it should be a very good performer in the water. The X650 like the D400 also appears to have a large exhaust valve and should have lower exhalation resistance than SP's other conventional cased second stages. It also has a real exhaust T, unlike the D400, so it should have less bubble interference than a D400.
Your technical reviews and especially how you write in simple terms and objectively is quite simply fantastic. I am an engineer myself and have worked in a few countries, but I feel like an idiot and am embarassed when I read your posts. The board is lucky to have you around so long and always posting objectively.
Question: Is the MK17/S600 equal to the MK17/X650 in cold water? The free flow in the S600 is caused by the MK25? Therefore, by getting a backup S600, I lose no advantage over the X650 except for smoother breathing which I have experienced?
Kuna
