Are rebreathers getting safer over time?

[tbone1004]

Pete, is the added complexity worth a slightly higher degree of resolution or is what we have "good enough" since everything is measured with a micrometer, marked with chalk, and cut with an axe?

For a CCR it would have to have a way of measuring inhalation frequency *TPS*, track the rate of ascent and descent and try to anticipate it, but unlike an automatic transmission where it can use the torque converter or clutch packs as a buffer if it overshoots, the CCR has no way of automatically dumping gas if it misses. It would also need to be able to measure flow rate of the air to adapt to everything, similar to the MAP sensor, but even then it is always reacting and if it overshoots it could cause problems.

Better to leave it is a passive system that works "well enough" but is still incredibly simple despite Victor being adamant about them being incredibly complex, it is still just a system that measure PO2, and adjusts based on a set of parameters that it is told to stay within. Any more complexity and then the electronics become suspect, right now it is a single circuit consistent of a controller, a solenoid, and sensors. The controller is told what parameters it must follow and how to choose which sensor it gets it's value from. That is the only real circuit in the system, everything else related to deco etc doesn't weigh into how that solenoid gets fired because it can't. KISS philosophy.

 

[PfcAJ]

As we can see, the person is the dumbest part of the Rebreather equation.

 

[The Chairman]

Pete, is the added complexity worth a slightly higher degree of resolution or is what we have "good enough" since everything is measured with a micrometer, marked with chalk, and cut with an axe?

I think it IS good enough and has been for some time. While I think the micrometer/chalk/axe scenario applies more to deco theory, I think that there are many other reasons not to over complicate. My point was, Oz sensors alone on a vehicle are not enough 'resolution'. They have to be augmented with either user intervention or by some other method.

I was going to learn how to dive rebreathers from the late Rob Davies, many, many years ago. He died on his rebreather about a week before our class was to start. Normally I'm an early adopter, but this gave me significant pause. I'm certified on a couple of units, but none of them has blown my skirt up, so to speak. I'm still looking for 'THE' one.

 

[descent]

[The Therac-25] is the example product they tell you about at the start of a medical device software course. ... It is not at all typical of any modern software in safety critical applications. ...

Yes, modern medical device manufacturers will not build any more 1987-model Therac-25s. Similarly, we could say that Ford and Firestone have learned enough from all the funerals and lawsuits in the nineties to guarantee that they will not try to sell us any more second-generation Explorers.

We have made all the old mistakes already. We have not made any of the new ones yet.

I tell the bright young up-and-comers "No code is bug free."

This is a recipe, not a warning. I say it hoping that it encourages them to go ahead and write no code during those wonderful times when writing code can be avoided. [Edit: I see that TBone already said as much. Yes, KISS.]

 

[wedivebc]

problem there is the ADV automatically compensates by adding dil via mechanical addition to offset the compression. So this is essentially directly correlated to your descent profile. No matter how fast you descent, your PO2 will likely be lower than your setpoint because the ADV is adding dil at a very rapid rate to keep the loop volume full. Essentially no risk of oxtox on a descent as long as the dil add is working because the solenoid can't match the flowrate of the ADV which is basically just a standard regulator in the loop.

So descent is much less of an issue than ascent, your PO2 is going up, due to increased pressure but it is being diluted by the dil at a linear or faster rate, so FO2 is going down either to keep the same PO2 depending on how fast your going, or it is declining faster than your PO2 drop in which case the O2 solenoid will keep firing. On ascent it is a bigger problem, where you can be coming up and because you are gaining volume, the loop is dumping air and you can cut your PO2's pretty rapidly when you're doing that, so the solenoid usually needs some help on ascent, pointing to your example above where the diver wasn't paying attention, went up, and saw his PO2 low because he either didn't fire it manually, the ascent was fast enough that the solenoid couldn't keep up with the dumping of gas, or his PO2 setpoint was set low, 0.8 or whatever, and because he wasn't paying attention it got that low. As long as it is above .15 he stays conscious. Not sure how an ascent would create a PO2 of 1.8, a descent could do it if he didn't have an ADV set properly or was running full manual and didn't add enough dil, but that isn't the units fault in either case.

This post is absolutely false. When descending diluent adds oxygen at at rate proportional to your descent rate (unless you are using a completely inert gas for dil). The depletion of oxygen is based on your metabolic rate. The fact that you make these statements tells me you have never done a fast descent on a rebreather.

 

[tbone1004]

thanks for clarification. Never done a rapid descent on ccr, and after thinking about it you can't metabolize O2 faster than the PO2 would be increasing with pressure. Wasn't thinking in terms of the exponential increase in atmospheres, my apologies

 

[wedivebc]

thanks for clarification. Never done a rapid descent on ccr, and after thinking about it you can't metabolize O2 faster than the PO2 would be increasing with pressure. Wasn't thinking in terms of the exponential increase in atmospheres, my apologies

Depending on your depth. I have spiked PO2 using 8/70 trimix

 

[descent]

... unless you are using a completely inert gas for dil.

Coffee on the monitor, and not the happy kind.

Don't try this at home, kids.

.

 

[Drin]

Coffee on the monitor, and not the happy kind.

Don't try this at home, kids.

.

Neon - it's the new dil.

-Adrian

 

[Dr. Lecter]

Neon - it's the new dil.

-Adrian

I prefer straight argon - it keeps me warmer.