I think I have a pretty good handle on how constant mass flow using an orifice works, which is why I'm posing the question, but if I am wrong about any of this please correct me. I have taken several classes related to fluid flow, I started off studying Chemical Engineering and switched to Biosystems Engineering, both of which require quite a few classes regarding fluids, so I am basing my understanding off of this.
From what I understand, the volumetric flow rate through the orifice is limited by a Mach number of 1, because the orifice has a fixed cross sectional area and the flow velocity cannot be greater than the speed of sound of the gas in relation to the orifice itself. In the case of O2, this velocity (and therefore volumetric flow rate) limit is reached approximately when the pressure at the inlet of the orifice is equal to or greater than twice the pressure at the outlet. To ensure constant mass flow, all we have to do is "fix" the pressure at the inlet to the orifice to some value that is at least twice the pressure of the outlet, and now we have a constant velocity (and therefore constant volumetric flow rate) and a constant pressure at the inlet, so we can determine the mass flow rate entering the orifice (which obviously equals the mass flow rate at the exit of the orifice, therefore the mass flow rate entering the loop).
The issue with the orifice is that the pressure at the inlet must be constant to ensure constant mass flow. this translates to fixing the IP (using a non depth compensating or "blanked" first stage regulator). This has the consequence that as depth increases, the pressure at the orifice outlet increases while the pressure at the inlet remains constant. At some point the inlet pressure to the orifice will no longer be greater than or twice the pressure at the outlet, and the mass flow is no longer constant because the volumetric flow rate becomes variable, or worse, no flow occurs if P1 = P2.
Back to the topic of rebreathers...
From my understanding there are primarily two types of fully closed-circuit rebreathers, which seems to be heavily based on two idealogies primarily held among rebreather operators. Again, I do not claim to be an expert on anything I am talking about here, every sentence here essentially has a question mark behind it.
Manual rebreathers and those who prefer them keep an eye on the PO2 in their loop and manually add O2 (or flush) the loop to make adjustments. I suppose this is attractive to manual rebreather operators as you become "one with the machine" much like operating a vehicle with a manual transmission. Requiring constant user interaction to operate a vehicle (or rebreather) keeps the operator "in tune" and aware of the function of the machine itself, which accomplishes two things. First, it establishes a strong sense of "trust" for the machine as small variances in performance are readily noticed by the operator, and second, the machine remains mechanically simpler and therefore its function is easily understood during operation and problems can therefore be understood during operation.
Electronic rebreathers, on the other hand, operate much more like an automatic transmission. Users should keep an eye on their PO2 to ensure that the machine is functioning, but this is not enforced through operation of the machine itself. The machine will adjust the partial pressures of the gasses in the loop using a combination of sensors, microprocessors, and automated controls. Essentially, the microprocessor not only reports the PO2 readings from the sensors to the operator, but also uses these readings to operate solenoids. This is very convenient and allows for more precise control over the PO2 in the loop than a manual rebreather. However, it bypasses the operator's decision making process, the machine makes the decision and the user is supposed to double check to see if the machine made the right decision.
I think of it as if there is a professional relationship between the operator and the rebreather. The operator is the boss and the rebreather is the employee. The question then becomes "how much do you trust your employee to make responsible decisions and do their job?" With a manual rebreather, the "boss" doesn't trust the employee very much, so the "boss" opts to do the job him/herself. "If you want something done right, do it yourself." if you will. This works, and statistics appear to indicate that it is a bit safer, but as with any professional relationship the "boss" can only do so much and sometimes he or she needs to hand off responsibility to the "employee" so that the "boss" can divert attention to other things. With an electronic rebreather, the "boss" trusts the employee to do their job, so the boss simply "supervises" the "employee" but lets the "employee" work autonomously. I believe this can be risky, because the longer the "employee" does its job correctly, the less likely the "boss" is to supervise the "employee" with a great amount of scrutiny, and lets face it, eventually every "employee" will make a mistake. mCCRs ask the operator "do you trust your oxygen cells?" while eCCRs go ahead and assume you do trust your cells, and it is up to the user to "double check" and decide if they trust the cells.
A hybrid CCR appears to be the best of both worlds and doesn't fit this analogy as well as the other types. A hybrid CCR is designed to be operated manually, which is arguably safer (note that "arguably" doesn't mean that it actually is), but also has the option to operate autonomously when needed. Some liken this to having a "parachute."
I am posing this question about rebreather design because the "parachute" analogy seems to indicate that the autonomous operation of the hCCR is a backup safety measure. Since autonomous operation of the CCR is not the primary mode of operation, and arguably not the safest mode of operation for the machine, it seems that this functionality should be likened more to "cruise control" rather than a "parachute." Instead of relying on it to save you when you forget to add O2 to the loop, why not delegate this function to being a convenience feature instead and utilize other methods more common among manual rebreathers as safety precautions. This makes sense to me because hybrid CCRs are supposed to be operated manually "most of the time," so why not include systems that are common among manual rebreathers to assist in manual operation of the eCCR or hCCR that is being operated manually? And indeed, this is true of the rEvo hybrid rebreather which has a constant mass flow orifice in addition to the computer controlled solenoid.
So, back to the question. A constant mass flow orifice, which is common among manual rebreathers (all of them it appears, aside from the UTD MX rebreather), appears to "lengthen" the time between required manual injections of O2 into the loop, or give the user a bit more time to "remember" if they "forget" to inject O2 into the loop. This is accomplished by providing constant mass flow into the loop equal to the minimum expected metabolic rate of the diver. However, these devices often impose a hard depth limitation on the rebreather itself, as they require a non-depth-compensating first stage to achieve constant mass flow and if the manual addition valve (and/or the solenoid) is plumbed off of the same non-depth-compensating first stage then there are NO means of addition oxygen to the loop below this "maximum operating depth."
So my question is: Why do hybrid (or manual) CCRs with a constant mass flow not utilize some sort of secondary non-depth-compensating regulator with a higher IP than normal SCUBA regulators, plumbed off of the high pressure line from the O2 tank, so that the primary first stage that the manual addition valve (and/or solenoid) is plumbed off of may be a depth compensating regulator?
EDIT: on a side note, this question can be broadened. Why don't dive gear manufacturers "do more" with high pressure lines? It seems as if there are many applications for plumbing things off of the high pressure side of a regulator, yet rebreathers and other specialty dive gear never seems to touch the high pressure side of a regulator except to monitor gas pressure in the tank. UTD, for example, opts to use the intermediate pressure lines in their "sidemount manifold" which appears to me to not actually solve the percieved "problem" of having isolated gas supplies when using sidemount configuration. A manifold that instead runs off of the high pressure lines, connected by the same hosing used for "fill whips" at filling stations, seems as if it would be mechanically simpler and offer better functionality and design flexibility.
