CCR First Stage Regs - IP Stability

[scubaphidoux]

I have a puzzle that I'm trying to solve. I have a JJCR with the standard Apeks US4 first stages. Although, I have no reason to change them at this stage, they are specified as >10°C. What if I wanted a sealed firsts stage? My investigations lead me to understand that the sealed Apeks (DS4 and alike) are apparently overbalanced and gain 0.2 IP per 10m of depth which will get to a point that will impact the O2 solenoid functioning properly. I noticed that some of the GUE JJ rigs are using Halcyon H-50D (ScubaPro MK17) sealed first stages. Apparently the H-50D sealed first stages are not overbalanced and hold a constant IP over the depth range. From a cursory look at the internals of the sealed Apeks and Halcyon/ScubaPro regs, they look similar and seem to function along similar principles.

My question is this please: could someone please help me understand what is technically different about the Halcyon H-50D (ScubaPro MK17) that enables it to be sealed and maintain a consistent IP (as opposed to the Apeks sealed first stage). I have attached a couple of charts that I have found on other forums (I am not sure of the source of the information).

Many thanks!

 

[KenGordon]

Are you concerned that the factory fitted, CE tested, first stages will freeze?

The >10C thing is for OC use where vastly more gas is flowing. Say you use 100bar out of a 3 on a 1 hour dive that would have used 100 bar out of twin 12s, 300l vs 2400l. Eight times as much gas. Worst case is likely to be filling a wing on descent. But that is still much less gas than filling a wing and breathing off the same reg, or worse a drysuit too in a single setup.

 

[Valyngar]

Haven't had any freezing problems when diving 2-3 degrees with JJ. It was taught to me that apeks are non sealed exactly why you wrote. Sorry can't help with why scubapro is better.

 

[scubaphidoux]

Thanks @KenGordon and @Valyngar. At this stage I'm not really concerned about the firsts stages freezing over (and I'm unlikely to be diving water that cold any time soon). Your explanations make good sense, thanks. I'm more interested to understand the technical details behind maintaining a stable IP with a sealed diaphragm reg. The puzzle I'm trying to solve is why (technically) the two regs I mentioned as examples are so different despite their function appearing so similar.

 

[Dsix36]

The Apeks has a plastic bit under the sealing rubber that pushes on the diaphram when external pressure is applies (more depth = more pressure). I know of many people that have removed that bit to keep the IP stable.

 

[scubaphidoux]

The Apeks has a plastic bit under the sealing rubber that pushes on the diaphram when external pressure is applies (more depth = more pressure). I know of many people that have removed that bit to keep the IP stable.

Yeah, thanks. The Apeks and Scubapro sealed regs both have the plastic Hydrostatic Transmitter / Piston. I’m trying to work out what‘s different between them that the Scubapro can hold IP but not the Apeks

 

[KenGordon]

Yeah, thanks. The Apeks and Scubapro sealed regs both have the plastic Hydrostatic Transmitter / Piston. I’m trying to work out what‘s different between them that the Scubapro can hold IP but not the Apeks

You understand this is a ‘feature’. From the XTX200 page.

• Unique over-balanced, diaphragm design - as the diver descends, the over-balancing feature allows the medium pressure gas in the hose to increase at a rate faster than ambient. This results in superior performance at depth.

 

[cathal]

Yeah, thanks. The Apeks and Scubapro sealed regs both have the plastic Hydrostatic Transmitter / Piston. I’m trying to work out what‘s different between them that the Scubapro can hold IP but not the Apeks

Just my 2C, but if you check back on the posts you got those diagrams from, I believe the cause of the drop in I.P at the highest pressure was that the plastic Hydrostatic Transmitter became deformed under these high pressures. They actually had accompanying photos of the remains of the transmitters. So what would happen to these regs at these extreme depths were that you would get less and less gas out of them.
From what we can see it happened to both regs in your diagrams but at a much later stage for the scubapro. This would lead me to believe that the scubapro Hydrostatic Transmitter is made with a material that has a much greater resistance to crushing/I.P than the Apeks plastic one, hence the delivery of a more stable I.P.

 

[scubaphidoux]

Just my 2C, but if you check back on the posts you got those diagrams from, I believe the cause of the drop in I.P at the highest pressure was that the plastic Hydrostatic Transmitter became deformed under these high pressures. They actually had accompanying photos of the remains of the transmitters. So what would happen to these regs at these extreme depths were that you would get less and less gas out of them.
From what we can see it happened to both regs in your diagrams but at a much later stage for the scubapro. This would lead me to believe that the scubapro Hydrostatic Transmitter is made with a material that has a much greater resistance to crushing/I.P than the Apeks plastic one, hence the delivery of a more stable I.P.

Thanks @cathal - your reasoning for the drop makes sense to me (do you have a link to the info you refer to?). My question is about the rise of the IP though, not the fall at higher pressures / depths. The Scubapro and Apeks designs are similar yet the Scubapro first stage holds a stable IP through a very large depth range. My question is: what's different in the scubapro design that enables it to hold a stable IP (and hence be more desirable for rebreather use) compared to the Apeks sealed first stage?

 

[rsingler]

My question is about the rise of the IP

I'd defer to @herman on this, but I believe the rise is due to a subtle difference in the difference in diameters between the main diaphragm and the environmental seal. In the graphs above, if you think about what causes the rise in absolute IP due to increasing ambient pressure, it is the pounds per square inch placed on the sea water side of the main diaphragm in an unsealed regulator. Emphasis being on the "square inch".
To have a perfect match between a regulator in its sealed and unsealed state, it is necessary that the transmitter piston put exactly the same amount of force on the pin hat as the main diaphragm would have if exposed directly to seawater. As noted above, the deformation of the piston shaft by excess pressure resulted in it being unable to transmit that full force, which is why the Apeks reg failed at the deepest depths.
If by design, manufacturing limitation or some other unknown factor, the area of the environmental seal exposed to ambient pressure is larger than the effective area similarly pressurized in an unsealed diaphragm, then the small piston transmitting force to the pin hat will press harder then it would have with a smaller environmental diaphragm. That increased pressure results in a greater differential between ambient pressure and the set IP, giving you a rising net IP.

The Apeks has a plastic bit under the sealing rubber that pushes on the diaphram when external pressure is applies (more depth = more pressure). I know of many people that have removed that bit to keep the IP stable.

For a similar reason, but the opposite effect, it is absolutely incorrect, and in fact dangerous to remove the transmitter piston.
Removing the transmitter piston will cause a result rather like the falloff in one of the graphs above, which I have reproduced below:

With the plastic transmitter piston removed, the only way the main diaphragm is exposed to increasing ambient pressure is as a result of the increase in pressure inside the small air pocket between the environmental seal and the main diaphragm. Increasing ambient pressure from sea water will deform the environmental seal inward, and it will either rupture (converting the reg to an unsealed diaphragm, which is OK), or it will serve as a buffer between ambient pressure and the main diaphragm. The extent to which the pocket of pressure inside that compartment is less than ambient pressure is the extent to which the main diaphragm will not respond, and relative IP will fall off. As relative IP falls, at some point the breathing resistance will increase radically in the second stage.
The only reason folks haven't died after removing the plastic piston is that our modern balanced 2nd stages are so tolerant of low IP. My new D420 breathes almost exactly the same with an IP of 60 as it does with an IP of 135. So that missing piston can result in a differential between ambient pressure and what the main diaphragm sees of as much as 75psi before the diver will notice a problem. In other words, since 75psi is 5 atmospheres, the diver would have to be deeper then 132' before he felt his breathing get harder. That's the definition of a "near miss".