joshk
Contributor
He also designed a rebreather that only uses two cells. I get the idea behind it, I just don't agree with it, hence added a third cell to my unit.
Is your disagreement solely for eCCR units?
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He also designed a rebreather that only uses two cells. I get the idea behind it, I just don't agree with it, hence added a third cell to my unit.
If you turn the unit on and it's reading the same thing as it was last time (air/O2 surface/O2 at depth before and after dive) what is clicking calibrate on the handset helping? Honest questions here
Shearwater felt that the pressure sensor in the NERD 2 drifting up to 50 mbar was unacceptable and redesigned the computer to use a new pressure sensor. 50 mbar is a difference of 20 inches of water. If your two computers don't read the same at the same depth I would contact Shearwater.
If you turn the unit on and it's reading the same thing as it was last time (air/O2 surface/O2 at depth before and after dive) what is clicking calibrate on the handset helping? Honest questions here
Is your disagreement solely for eCCR units?
Well, again this might be unit specific, but the question is if the surface O2 flush is as accurate if you do it manually compared with what the unit does for the calibration sequence. In that case, clicking calibrate might just be the easiest way to get your loop to 100% O2 at 1 ATA. Although I guess if you really lean on the MAV for a while, it would eventually do the same thing, just using more O2.
So to turn the question around, if you are checking your surface O2 before each dive anyway, why not do it with the calibration sequence?
Because the mV air point is literally the last thing which is going to change on an O2 sensor. [...] They will read 0.21 in ambient air for a long long time after they are useless at higher ppO2.
To confirm that assumed line is "close enough" to true linearity you need to use your cell checker.
Shearwater's statement that you should dive with potentially poorly calibrated cells to "see how they've changed" really contradicts every basic principle of scientific instrumentation.
Normally when using any kind of meter to measure some environmental condition you calibrate on either side of the target values. So in a perfect world we would calibrate CCR O2 sensors at 0.21 and 2.0 and then the line between those points would be more likely to reflect the true value in the 1.0 to 1.3 range. Since we can't do that, we have to make assumptions about where the origin (0,0) or air (1, 0.21) and 100% (1, 1.00) are and draw a linear relationship which extends past the 100% to 160%+ (ppO2 1.6). Slight variations in the 100% mV point can affect the slope of the line and misalign it from a true 1:1 relationship.
I cant get a cell over ~1.5ish at 20ft/6m or ~0.90ish on the surface post dive.
Bottom line is that:
1) Our surface calibration procedures are a kludge but they are the best kludge we have until someone integrates a pressure pot into a CCR head to calibrate above the expected measurement range (>1.6)
2) Not calibrating is not a valid scientific practice for any instrument.
3) Not calibrating is not a reliable way to detect cell drift.
4) If you are concerned about deviations from linearity above ppO2 1.0 you need to use a cell checker.
I don't know if the JJ differs here, but I do an O2 flush and then click calibrate on the controller. It's not an automatic sequence. You can also use a cap with a BC connector on it but I rarely use that since it's a bit of a hassle.
Because I'm most often checking my surface O2 in the water unless it's been a while between dives.
And if it's been a while between dives and I do an O2 flush and it reads the same as the last time I did it (when accounting for surface pressure) why should I click over to the calibrate screen?
I think that we are getting into inter-unit variation. Clicking calibrate on the JJ controller causes an automatic O2 flush that is presumably O2 efficient because it is terminated when the mV become stable. On the SF2 you manually flush the loop with O2 and then calibrate?
I don't think I understand. Why are you checking your mV surface O2 when you are in the water? Why wouldn't you check it before you get in the water if you are going to bother checking it at all before a dive?