Are rebreathers getting safer over time?

[Akimbo]

The scenario described by descent has been a problem since the first mixed gas rebreathers were on the test bench. Many of these units addressed the problem by replacing orifices and mixing eductors for Oxygen and diluent (HeO2 in the case of semi-closed and pure Helium for totally closed units) depending on the depth. Keep in mind that the design limits were 0-1600' on many of these units.

I have often wondered how many times this mixing problem has resulted in a momentary slug of O2 rich gas causing a convulsion. It doesn’t take much to interrupt the eductor’s operation and for that small particle to clear itself, leaving no evidence. The problem is much more apparent on HeO2 than Trimix due to the dramatic differences in molecular weights. It took a long time before engineers learned how closely they had to monitor in order to observe the problem.

 

[The Chairman]

I'm sorry, Pete, you're twisting my words to win an argument I'm not engaged in. I'm not saying a car has more sensors than a rebreather, I'm saying that the other sensors are for other decisions. Your ABS sensors play no role in whether your car needs to speed up or slow down. Neither does your A/C setting, the song on the radio, or what color your shirt is. What matters is whether or not you're too slow or too fast. Your car's speed is relevant, and your required speed is relevant....not your exhaust temps.

I'm not sure why you're missing the point: the entire system is INTEGRATED. You don't get to slow down or speed up without lots of agreement. Disconnect one wheel sensor and watch your cruise control not work. That's why everything is so 'smooth'. You want to focus only on speed control, but what about simple acceleration? An oxygen sensor by itself just isn't enough to accomplish that. In the beginning of onboard ECMs, most of the work was done by the carburetor and the final mixture was merely tweaked by input from the 02 sensor. With the advent of fuel injection, the ECM has taken over every aspect of fuel and spark control. During acceleration, that millisecond response you refer to is not because of the O2 sensor and it gets ignored as other sensors tell the ECM that more fuel is needed and that the spark needs to be advanced.

The current rebreathers only use input from the oxygen sensors to work their magic. Some are like the Revo, and use an orifice to continuously inject oxygen with the solenoid just clicking in when needed. If they could add in depth and change of depth to the program, they could anticipate the addition of O2 or diluent.

 

[victorzamora]

I'm not sure why you're missing the point: the entire system is INTEGRATED.

I think that's the problem....we're having two different conversations. I'm talking about current flaws in rebreather control logic. You were alluding to potential solutions. Yes, there are other inputs that could be taken to shape O2-injection algorithms more smoothly than what we have now. You could add a lot of functionality...which would be great.

It doesn't change the fact that the analog to a CCR's current control algorithm is a car's cruise control portion of the programming receiving inputs from the speedometer. The smooth acceleration is achieved with other sensors, but knowing that it needs to accelerate is a function of the speedometer. Right now, CCRs have a speedometer that have a several-second lag. Cars have practically instant feedback from the speedometer. It doesn't have to wait 5 seconds to get a measurement. The "millisecond response" you're referring to is not what I'm talking about at all. I'm saying that as a car slows down and speeds up, the speedometer knows about it instantly. In a CCR, when O2 is added it can take an extended period of time for that O2 addition to reach the sensor. It has nothing to do with advancing the spark. I'm not talking response, I'm talking input.....accurate input.

 

[The Chairman]

If you want to dumb things down, you've done a pretty good job here. My hat's off to you for way over-simplifying the issue. I feel your premise about the function being analog to a speed control is not a good one. We were talking about oxygen sensor response from one system to the other and it's obvious that you have no clue how a car uses it's oxygen sensor information nor why they don't rely on just that information.

 

[victorzamora]

If you want to dumb things down, you've done a pretty good job here. My hat's off to you for way over-simplifying the issue. I feel your premise about the function being analog to a speed control is not a good one. We were talking about oxygen sensor response from one system to the other and it's obvious that you have no clue how a car uses it's oxygen sensor information nor why they don't rely on just that information.

What??? I never mentioned the car's oxygen sensor....you did. I was saying that the control algorithms behind the cruise control are much more responsive than a solenoid controller in an eCCR unit. I wasn't talking about oxygen sensors being used in cruise control at all. You're off base here, buddy....it's not what I was talking about in the slightest.

In a control system, you have measured process variables (inputs), desired setpoints, and control elements. I was comparing process variables of two systems, not oxygen sensors.

 

[The Chairman]

What??? I never mentioned the car's oxygen sensor....you did.

I wasn't the first, but I guess you missed that salient part of the whole discussion. As much as you want to put this on me, you're the only one who doesn't get that we were talking about O2 sensors.

 

[hroark2112]

After a year+ diving my mCCR, I have started the crossover course to an eCCR.

I believe that choosing between an mCCR and an eCCR is a personal decision and one that must be made taking into account both the type of diving you'll be doing and the personality and traits of the diver. Knowing that I am someone with a case of ADD and that I am prone to being easily distracted by "shiny" things and squirrels, I chose an mCCR. This forces me to monitor my computer regularly if I want my PO2 to be optimum.

After close to 3 hours on the eCCR, I am finding that buoyancy is more 'touchy' while shallow than on the mCCR, and I think I still prefer how the mCCR performs in the water. It just feels more natural, which may be a result of 100 hours on the unit. Time will tell.

 

[kr2y5]

I think a part of the challenge in this discussion is that it is happening in abstract... Victor, would it be possible for you to describe at least one example with a timeline in seconds, explaining how the diver's depth would be changing, what the delays in all the sensor readings be, what a rebreather would be doing in the meantime, and how this would create a problem? Maybe that could help some of us to get a better grip on it...

So, suppose at time T = 0 seconds, the diver would have been at X feet for a while now, ppO2 would be Y, all sensors would have an accurate reading at the moment, and the diver would begin ascent (or descent) at Z feet per minute...

At time T = 5 seconds, the diver would be at X - Z/12 feet... the rebreather might have done something by now (?), the sensor readings would be still the same ppO2 = Y, since it has not mixed or the mixture has not reached the sensors yet, ...

Maybe if we could plug in some concrete numbers, it would become a little bit clearer...

 

[BRT]

The PPO2 is going to change immediately with depth

 

[muzikbiz22]

This thread has been amazingly informative, educational and entertaining. At this point, I can't help but to ask the OP to revert back to his original question, "do YOU think rebreathers are getting safer" ???