Scubapro X650 vs. D400. Which Is Easiest Breather?

[realdiver7]

Wondering if anyone has done a side-by-side comparison of the new Scubapro MK25/X650 and the discontinued D400 using the MK25 to see which ones subjectively breathes better. I did a side-by-side comparison of the MK25/S600 and the MK25/D400 yesterday in the ocean at 64 ft. with a water temp of 56 degrees.

The S600 breathed the easiest I have ever experienced with the adjustment knob only turned out to the half-way position. I thought it couldn't get much better than that until I put the D400 in my mouth. The D400 seemed to have an edge because the breathing seemed to feel much more natural and the breathes were delivered to me with hardly any effort on my part at all. Now I just need to turn the S600 knob all the way out and compare it to the D400, but I still think the D400 is going to feel subjectively easier based on its inherent design and because I have used the S600 enough to know what to expect. I don't think I can wrong with either reg, but I would like to learn more about the X650 now.

One dealer says the X650 has a major design flaw in the angle of the front housing that covers the diaphram (or some kind of degree angle flaw), but another dealer tells me with the latest diaphram, etc. upgrade that the X650 has its problems worked out. At this point, I don't know what to think.

I have heard from one person that the X650, when it works right, actually breathes better than the D400. Can this be possible? Is this really true? Are the problems worked out now? If it breathes better than the D400 AND the problems are resolved, then I don't see how it could get much better!

I will continue to provide updates on any experiences that I feel may benefit this Board. Any input on the X650 vs. D400 would be greatly appreciated. I don't need any further test results on the S600. It's a great performer, and I will use it too.

Thanks!

[DA Aquamaster]

I own both the X650 and D400 and have dove with both regs side by side on the same first stage (a Mk 20). The D400 wins hands down compared to an X650 (or S600) if it is tuned for peak performance as it will deliver cracking efforts around .6 and still offer stable no free flow performance. It still wins hands down even if it is detuned per SP specs to meet CE norms (cracking effort around 1.2) as even at the same cracking effort the D400 will deliver much more normal feeling breathing than an S600.

Plus the D400 is totally adjustment free during the dive as due to the case design and co-axial diaphragm and exhaust valve, it will not be prone to a freeflow in unusual attitudes so no adjustment is need during the dive and the reg can contiune to deliver maximum performance.

With the S600 or other conventional reg design, the poppet's potential perfromance is excellent, but the cracking effort will need to be increased in different positions to prevent a slight freeflow as air slips out the exhaust valve in positions where it is significantly higher than the center of the diaphragm. This means that with the S600 the minimum cracking effort of .6 to .8 in practice normally needs to be increased with the adjustment knob to about 1.0 to 1.2 inches of water to prevent a freeflow in a normal swimming position.

With the X650 the situation is even worse as from a case fault geometry perspective, the reg is in a worst case situation when the diver is in a normal swimming position. So in practice, the diver has to go around with the reg adjusted to a cracking effort of around 2.0 inches of water to prevent a slight freeflow from continuing after the inhaltion ceases. So the X650 poppet is capable of great performance that is nearly as good as the S600 but the case design is problematic as you can only benefit from this reduced cracking effort if you are willing to accept a continual slight freeflow out the exhaust valve.

The X650, like the D400, does deliver much smoother air flow than an S600, but this advantage for the X650 is negated by the generally higher cracking effort required during the dive to prevent a very slight but constant freeflow .

In my case, most of my diving is deep and in cold water and a slight but continual freeflow is totally unacceptable, so I end up having to dive with the X650 adjusted to deliver a measured cracking effort of 2.0. In that situation it breathes like a rock compared to a D400 (or an S600).

So in short, I really wish SP would reintroduce the D400. Anybody want to buy a slightly used X650 (2 dives total) with a not yet activated warranty card included? I'll sell it at my cost of $227 plus shipping. I'd sell it cheaper except I cannot imagine the X650 is going to last long in production and it could be kinda collectable in 10-20 years.

To be completely fair, the X650 is an excellent cold water reg and is in some ways even better than the D400 in this regard as it does not spit the occassional ice chunk at me like my D400's will in 35 degree water.

The X650 also has lots of potential and with some modification could be an excellent design. If I were king of Scubapro, I'd plug the existing exhaust valve and redesign the diaphragm to use a coaxial exhaust valve like the D400 and redesign the purge cover to vent the exhaust bubbles. This would give you a regulator with the case geometry and direct air flow advantages of the D400 with the simple balanced poppet design of the S600. (no more whiney techs complaining about working on D400's) If SP did that boys and girls, we would have a real winner on our hands.

They could update the case on new regs (X700's, X750's ?) to exclude the current exhaust port and exhaust tee. But with a kit including a new purge cover, diaphragm/exhaust valve and a plug for the current exhaust port, exisiting X650's could be ungraded to essentially X whatever standard.

[wjbrandt]

FYI SP has a modification for the X650. They are replacing the diaphragm, with another allowing the cracking effort to be reduced. I would say the old one was like sucking a ping pong ball through a straw. The new is much better. SP will also replace your X650 with an S600 if you like, no charge

[DA Aquamaster]

Unfortunatelty SP just shipped me my new X650 in it's fully updated condition with the new diaphragm and blue X650 sticker so my observations are based on the new one.

But the diaphragm upgrade is not all that relavent as again the problem is not with the performance of the diaphragm/lever/poppet combination (which is actually quite good) but rather with the relative positioning of the diaphragm and exhaust valve.

The X650 shares the same general shape and case design with the D400 but with one important difference - the D400's exhaust valve is in the center of the diaphragm, while the X650's exhasut valve is mounted on the backside of the case.

In the D400 the exhaust valve placement means the highest edge of the exhaust valve will never be more than .5" higher than the center of the diaphragm. In the X650, the upper edge of the ehxuast valve is at least 1.5" higher than the center of the diaphragm in a normal swimming position and is nearly that bad in every other position. The result is that while a low cracking effort can be achieved, it cannot be utilized in the water due to the efrect of air escaping out the exhaust valve reducing the pressure in the case and casuing the valve to open to feed air to the regulator which continues the freeflow cycle.

In my experience, the cracking effort had to be increased to 2.0 inches of water (as measured on the bench post dive with the adjustment knob in the same position) to eliminate the tendency for the X650 to continuously trickle air out the exhaust valve between inhalations.

I was aware that SP was replacing the older recalled X650's with S600's at the buyers discretion. I am not sure they are replacing the new ones, but it's worth checking.