Rebreather Question

[tbone1004]

@joshk

so you keep giving those faces, but won't actually say anything on why you're giving them. I'm hoping to learn something because I must be missing something.

Why do you believe that having a more precise ppO2 reading for your dil flush is more important than at the target operating range of the unit? We are talking within single digits percentage wise, so what I'm saying is that 10% of say .4 has a lot less consequence than 10% of 1.6 if I'm doing dil-flush shallow

What is your normal target dil-ppO2? I use standard gases, so my dil-flush is often going to be real close to the setpoint. If I'm using EAN32 for dil and am in cave country, then my dil ppO2 will be at .8 by the time I hit 50ft, and in many of our caves the dil-ppO2 will be 1.1. I understand this isn't always the case as many use air-dil, but even then, the ppO2 would be .75 which is not going to show significant deviation from the bottom of the calibration point at 1.0. Certainly much more accurate if calibrated at 1.0 and 1.6 than if I calibrated at .21 and 1.0 and much more accurate than the 1.6 would be if I used the standard 2-point calibration with air and O2 at the surface.
Part of the reason that I prefer standard gases is it will give me a dil-flush right around the lower operating range of the unit so I know that the cells are behaving properly in the range that I'm using them vs. a range that I'm not planning on using them.

What are you looking for in the low ppO2 dil-flush that requires high precision that makes it more important than having high precision at the operating ppO2? I dil flush every 45 minutes to lower humidity in the loop and make sure the cells are responding properly and cycling normally, but do understanding that there may be some residual high ppO2 gas in the loop that will throw the dil flush off by a few tenths of a point

 

[joshk]

why do you believe that the dil flush ppO2 accuracy matters more than the target ppO2 accuracy? Clearly I must be missing something

I disagree with the “below 1.0, the accuracy down there doesn't really matter” statement. I think you should expect cells to be accurate within the range you're expecting to use them, and the PPO2 of your diluent on the bottom should be within that range.

Why is it that you think a dead nuts ppO2 on the dil flush is more important than having a dead nuts ppO2 at your set point?

I reacted to the statement that you'd prefer a range of accuracy (inaccurate outside of range) that includes target bottom PPO2 and not diluent PPO2.

Spoiler: Side note

Have you considered writing out a complete thought and then posting? This post first edit later habit you have is nonsense.

 

[joshk]

I'm also talking within our accepted limits of linearity, so if you subscribe to say 90% linear, then a delta of .04 at a ppO2 of .4 during a dil flush is a lot less critical than say a delta of .16 when you are a ppO2 of 1.6 on deco. That's basic y=mx+b since the units create straight lines instead of curves between points, though the Liberty is apparently going to get a curve for it's 3-point calibration

 

[tbone1004]

@joshk sorry for engineer brain turning on.... That's how I look at all of it though. The computer has to turn the cells into y=mx+b and bounds it with our calibration points. It has to assume that outside of those points, the line holds true, which we unfortunately are aware that it does not.

fair enough, so I may have assumed you were looking at a much lower dil-ppO2 vs. target bottom ppO2 than what I run. Since I choose to use standard gases, the odds that the dil-ppO2 is much below 0.8 is pretty small. Most of the time it will be 1.0+0.2.
Right now I 2-point on the meg with .21 and 1.0, and 1-point on the Shearwater which does show discrepancy above 1.0 because the m&b values are different for those two devices. May not be that different, but it is different, and gets more different the higher you go. It's an interesting thing to chart actually.

So why do you prefer to have the higher degree of precision at your dil-ppO2 vs. the target operating ppO2? The consequences of inaccuracy and imprecision at the target operating ppO2 seems to me to be much more severe

 

[joshk]

 

[joshk]

So why do you prefer to have the higher degree of precision at your dil-ppO2 vs. the target operating ppO2?

I never said that. My argument was with the “below 1.0, the accuracy down there doesn't really matter” statement and subsequent follow up that you'd prefer a range of accuracy that does not include your diluent PPO2.

 

[RainPilot]

I never said that. My argument was with the “below 1.0, the accuracy down there doesn't really matter” statement and subsequent follow up that you'd prefer a range of accuracy that does not include your diluent PPO2.

We get it. Could you tell us why you have an issue with the second half. I took the first half to mean that it is less important to him than the top range.

So far you have given several emojis and memes and even a graph of word count but no useful opinion. I for one am a big proponent of choosing correct calibration ranges in my daily life and what Tom says seems valid to me. In my situation it is simpler to do air and 100% calibration but I am running 1.0-1.2 SP on NDL dives with a continuous calibration checking system so my needs are different.

 

[joshk]

Could you tell us why you have an issue with the second half. I took the first half to mean that it is less important to him than the top range.

Your diluent PPO2 needs to be within the range of accuracy of your system and if your target PPO2 is outside that range then you can’t run that target PPO2.

 

[joshk]

So far you have given several emojis and memes and even a graph of word count but no useful opinion.

Honestly I wasn’t aware that it was an opinion. Would you dive a rebreather without an ability to validate cells? It just doesn’t make a lick of sense to me.

 

[RainPilot]

Thanks now I understand where you’re coming from.