Collectors list of notably easy breathing second stages?

[Kupu]

There are many high performance second stages available, making it easy to obtain a regulator with very good to excellent breathing attributes. My current collectors fixation is to find models current or past, regardless of availability, that could be considered as above and beyond for excellent breathing performance. In example, second stages that are able to perform better than Scubapro G250 or equivalent.

The current list is as follows. Anyone have comments, corrections, or additions?

• Scubapro Pilot (unconverted). Can be tuned to inhalation effort .4" (or less?), exhalation effort lower than average. Parts not readily available, DIY effort high.

• Sea Hornet Commander & Command Air. Can be tuned to inhalation effort .5". Commander slightly better performance vs Command Air. Exhalation effort average? Parts not readily available, DIY effort medium. Also available under ProSub and Parkway brand.

• Scubapro A.I.R.1 (and converted Pilot). Can be tuned to inhalation effort .6" (or less?), exhalation effort lower than average. Parts not readily available, DIY effort medium-high.

• Scubapro D series (D400 - D350 - D300). Can be tuned to inhalation effort .6" (or less?), exhalation effort lower than average. Generally equal performance across the D series, except later D400 models with plastic orifice slightly less. Parts not readily available but easier to find vs Pilot/AIR1. DIY effort medium-high. Parts and kits no longer produced, but can be found with a bit of patience.

• Kirby Morgan Superflow. Can be tuned to inhalation effort slightly less than .8". Not sure if this one really belongs here, thinking it is on par with several current high performance second stages. Added as space filler. DIY effort medium. Parts and kits available with some effort required to source?

 

[diversteve]

All the Atomic Aquatics line

 

[Brian G]

I'm thinking the OP is referring to regulators that have a design that mitigates the case geometry fault that results from having an exhaust diaphragm that is separate from the inhalation diaphragm. Rsingler explains it nicely in his post Are Atomics worth the cash?. He mentions the Poseidon as also having a coaxial exhaust valve which might put it into the sub-0.8" range as well. I like my Atomic, but I don't think it quite makes it into this more esoteric group.

 

[rockinrebel]

I dove a Scubapro AIR 1 for decades. Nothing else is close to that one.

 

[tbone1004]

All the Atomic Aquatics line

they're not in the same league as what the OP is talking about. Like @Brian G said, these are regulators that have minimized the freeflow at cracking efforts below 1" as part of the case design. The only reg that is currently in production for recreational divers that can come close is the Poseidon Xstream which is stable down to around .7-.8" in most cases

 

[Kupu]

I have considered trying out the Atomics based on reputation, and hopefully will get the chance to do so. I had another glance at the Atomics and noticed the elliptical exhaust valve design, that would potentially qualify as a feature that allows exhalation effort above and beyond. After breathing from a regulator with exceptionally low exhaust effort, it can be very noticeable to switch back to a second stage with "normal" exhalation effort. A great example would be exhaling from the AIR 1, and then going back to G250. Night and day.

It is true that I am after regulator designs that allow performance past the limits of case fault geometry as described in Rsinglers post, and glad that there are coaxial valve designs that offer an alternative. But the search is not limited to coaxial design, also hoping to find other regulator designs that stand out. The Sea Hornet was not on my radar until recently, it appears to be a "standard" case design, but still allows tuning to exceptionally low cracking effort.

 

[Kupu]

I dove a Scubapro AIR 1 for decades. Nothing else is close to that one.

The AIR 1 is a recent addition to my lineup, a wonderfully easy breather that has influenced my interest in finding other exceptional designs.

 

[Kupu]

they're not in the same league as what the OP is talking about. Like @Brian G said, these are regulators that have minimized the freeflow at cracking efforts below 1" as part of the case design. The only reg that is currently in production for recreational divers that can come close is the Poseidon Xstream which is stable down to around .7-.8" in most cases

I have been curious about Poseidon, would the Xstream be as good as it gets from standpoint of breathing performance, or is there another model that would be preferred if it could be found?

 

[tbone1004]

I have been curious about Poseidon, would the Xstream be as good as it gets from standpoint of breathing performance, or is there another model that would be preferred if it could be found?

it's as good as it gets with modern regulators but you have to tune them properly. Since you're in NC, let me know if you get one and I'll point you in the right direction.
The Xstream shouldn't be tuned lighter than 1" under normal circumstances. It technically can be tuned down to near 0" and not freeflow while it is in your mouth in normal diving orientations, but if you tune below about .8" and roll on your back, or generally put the mouthpiece straight down, it will freeflow. Since it's a non-adjustable regulator, it's recommended to leave it at 1" for most people which is more than light enough. The difference with the servo assisted regulators is not necessarily the cracking effort but the area under the WoB curve.

Below is a picture of a standard ANSTI breathing curve. The top curve is the exhalation resistance and the bottom curve is the inhalation resistance. The values are the pressure inside of the case relative to atmosphere so the negative values is suction created by the diver, and the positive values are exhalation created by the diver or a brief blip which is the kick of the venturi.
The two charts are at different depths with air, but the important part is the shape of that curve. Looking at the bottom chart, the bottom curve shows an initial crack, then we go to a positive value as the venturi kicks in, then a steep increase in WoB to maintain the breathing. Exhalation shows a spike to crack the valve, then the valve opening relieving the pressure, then an increase in case pressure as the diver exhales. The value that you "tune" the regulators to is that initial cracking value and then the rest is ignored unless you have a breathing machine because you can't really test it easily but it's also not something the technician can do anything about.

Below is a picture of the Xstream graph. Here you can see the initial pressure required to open the servo, then a pretty good reduction in negative pressure as the main valve opens and "feeds" the diver gas. The exhalation WoB is fairly high compared to many regulators because the exhalation diaphragm is downward facing. In this case, compare this to the top chart of the above since they were at the same work rate. In this case the total WoB was calculated at 0.75j/l which is about as good as it gets. The difference though is the "standard" regulator design has a slightly lower total effort for exhalation because the exhale diaphragm is above the main diaphragm, but it is relatively equal at just under 5mbar for both. With the Poseidon, the total effort for inhale is a third of what it is for exhale. This is important because the machines test case pressure, but your body will be trying to "auto" exhale because your lungs are lower than the regulator in a normal diving position so the exhale WoB doesn't matter quite as much. This is why it is important to look at how the total WoB is created which shows how the regulator is going to "Feel". In the Scubalab tests the Poseidon doesn't do that great at deep depths on air because the exhalation WoB goes up fairly quickly where the inhale doesn't change a whole lot. The machine has the diver truly vertical in the water and the pump is at the same level as the mouthpiece. This means that you don't get any benefit during exhalation from the water pressure differential on your lungs with either design, but it's meant to simulate commercial diving more than recreational diving where the diver is vertical.

 

[Fibonacci]

The Sea Hornet was not on my radar until recently, it appears to be a "standard" case design, but still allows tuning to exceptionally low cracking effort.

It’s not just case design, the diaphragm design also plays an important part in getting consistent cracking efforts to 0.5”.
I’ve had a couple of interesting chats with Peter Katz who was the Chief Engineer at Sea Hornet and designed the Commander/Command Air regs:
“When I designed the Command Air diaphragm we conducted many tests of hardness, thickness, shape and type of silicone for the diaphragm.
...All protocols about those tests and final drawings of the diaphragm remained in the archive of Sea Hornet. I have no idea if this archive still exists.”