rjack321:
When you do a dilutant flush what do you try to drive the pp02s down to?
When doing a diluent flush to check cells I want to get all three readouts to show the correct partial pressure of diluent's O2 for the depth I'm at. For example for air (about 21% O2) it would be 0.21 ata at the surface. So if I do a dil flush at 20 meters (metric units come in real handy for math involved with RB diving
) were ambient pressure is 3 ata I want them to show 0.63 ata pO2.
As the maximum pO2 that can be reached at the surface is 1.0 ata with 100% O2 (without the use of a pressure pot), and some people dive setpoints above 1.0 ata it has become common practice to flush the loop with O2 at 5m or 6m and verify the cells to 1.5 ata or 1.6 ata pO2, respectively.
rjack321:
This was the surface incident that sticks in my mind as preventable with better engineering
Actually the victim was seen changing his sensors before the dive. That is a major mistake as the sensors are sealed and need some time to generate the full output once exposed to O2. The normal "wake up" period is 24 hours. When you calibrate the rebreather, you flush the loop with O2 and wait for a stable reading. Then you adjust the display(s) to 1.0 ata if necessary, basically telling the electronics that the current mV received corresponds with that. If however this is done when the cells don't generate their full output yet the electronics get a lower value. As the output increases the electronics will keep the actual pO2 lower yet the gauges will read the correct setpoint.
IIRC there was a surface swim involved, too, so the already lower pO2 in the loop was likely not enough to sustain the diver's conciousness. Once unconcious and the mouthpiece dropped, both loop and lungs got flooded. The unit lost bouancy and the diver drowned.
While I agree that this fatality was preventable, I believe it was due to complacency rather than lack of engineering. Rule No. 1: Always know your pO2! Problem is you can't if you don't calibrate the unit properly.
The PRISM's electronics, while simple and not particulary loaded with features, are very well engineered. They are still the only ones available that track the sensor outputs, and their response to changes in ambient pressure (depth) and O2 injection. The unit is able to distinguish between 1 sensor failing (and getting voted out) and 2 sensors failing (and the good one getting voted out - slim chance of that but it has happened on other units). The HUD signals one or more than one cell failing. But as any instrument, the calibration needs to be correct.
In regards to the CO2 sensors, the Infinito has that feature, but less than a handful of them are are owned by private parties. The status of the units is pretty much unknown, at least to me. A batch was ordered by a military, not the US' though. Don't know if they were delivered or are in use, don't even know if the manufacturer still exists.
The electronics complete with deco and other features have been offered, however at the small production rate of all but two or three companies (AP, Dräger, OMG) the price per unit was prohibitive. We're talking about easily doubling the price of Meg or PRISM, a range for which there is not much of a market. Among those features were a second solenoid to inject diluent IIRC, allowing the unit to perform a dil flush if the pO2 went through the roof. As well as diver surface feature that would bring the diver back to the surface for recovery in case the unit noticed it was submerged but no longer used and/or moving for some time. So some of the technolgy is there, it just has to become feasible and available.
Bubbleseekers is one of the companies who announced a CO2 sensor, and the unit I saw earlier this year (without that technology) also had a second solenoid for electronically controlled dil addition.
The same is true for the Open Revolution project that can be found on RBW.
