Calibration question

[joshk]

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?

 

[doctormike]

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

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? The only downside I can see would be the potential for masking a failing cell, but you are checking mV anyway to watch for that. The only difference is that it recalculates the PO2 conversion factor at the same time.

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.

Yeah, I saw that. I have a NERD 1, which isn't supposed to be affected, but maybe I'll take a closer look in different positions next weekend. I just can't stand the thought of being without my NERD during dive season!

 

[rjack321]

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

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. Which is why you can use spent CCR cells in a mixing stick to make 32% OC gas for a long time after they are dubious in a CCR. Once you go squirting O2 in there (on the surface) to bring them up to ~0.99 you might as well hit the calibrate button. If its only been a few days you are mostly compensating for barometric differences (slight). Why would you not calibrate if you have 100% flooding the unit and one sensor is 0.98, one is 0.97, and the third is 1.02? Bring them all to the same (assumed) line based on current performance.

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. And that last dive might have been weeks ago. That is not a reliable check on their linearity anyway. It is good for checking that they go over 1.4ish but not that's a far cry from demonstrating linearity. Most divers assume that if they go over 1.4-1.5 then the lower portions of the range are accurate. Its a reasonable assumption but not always true. I don't set up my calibration stuff for every dive, but I wouldn't go months with an old calibration either.

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.

 

[JohnnyC]

Is your disagreement solely for eCCR units?

Negative. 2 cells doesn't give you any basis for comparison for wonky behavior. A dil flush is not always a positive indicator of cell health. 3 cells allows you your own voting logic. Only difference with an eCCR is that it can start spewing gas into your loop. At least on an mCCR you have to make that choice yourself.

 

[JohnnyC]

I should add, the Pelagian is an mCCR, dude calls is a DCCCR for "Diver Controlled" because it uses a needle valve for O2 addition instead of a fixed flow leaky valve, or no valve in the case of the UTD abortion.

 

[joshk]

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.

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.

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 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?

 

[joshk]

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.

I dunno, you could be right. But all my cells have decayed measurably in air mV. Reading “0.21” in air is all relative, what's the mV output you're assigning to that value?

To confirm that assumed line is "close enough" to true linearity you need to use your cell checker.

I find most of my cells are nonlinear from 0.21 to 1.0 to a certain degree. That knowledge can't be imparted to the computer reading those values. Also I don't own a cell checker, but I might buy the wiring harness for my cells and bug @doctormike

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.

Can you link or quote what you're referencing?

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 think my handle on the math reasonable. Though I wonder, are you using that cell checker with a warm and humid environment containing tons of helium?

I cant get a cell over ~1.5ish at 20ft/6m or ~0.90ish on the surface post dive.

Sounds like bad calibration. Have you tried calibrating the unit post dive and then comparing it on subsequent dives?

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.

1. Sure, current procedures have room for improvement.
2. I don't think anyone said don't calibrate. I said don't calibrate every dive.
3. Who said this?
4. I can draw a line from 100% at 1 ATA to 100% at 2 ATA and check my points along that line. If it's close it works for me. If it's not, it's limiting and time for a new cell.

We're all essentially using a bunch of fabblecobbled bags to contain gas that allows us to have fun underwater.

 

[doctormike]

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.

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?

Because I'm most often checking my surface O2 in the water unless it's been a while between dives.

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?

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?

Again, inter-unit variation. Calibrate screen on the JJ is the easiest, most accurate and most O2 efficient way of filling the loop with O2 on the surface.

 

[joshk]

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?

Yes, you can do this either with a cap for the inhale side and flush O2 over the cells there or you can crack the DSV and use the MAV to flush O2 over the cells.

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?

If it's been awhile (indeterminate period of time I haven't nailed down, but typically 1-2 weeks without diving) I'll check surface O2 before gearing up. I'll sometimes (but not always) calibrate here. I really dislike calibrating with dry rested cells because it typically gives higher output that is not sustained.

Note, I'm not checking surface mV I'm checking surface PPO2. It's really only useful when calibrating. I write down surface mVs in air because it's easier than working backwards from the calibration value to the displayed PPO2.

 

[rjack321]

[QUOTE="joshk,

Post 1 of this thread is where Bruce P is quoted (from RBW) as specifically not calibrating so you can "see" if the cells have drifted. I disagree that this is in anyway a good idea. Not based on O2 sensors in particular, just from the entire premise of what calibrating is trying to accomplish which is to most accurately measure ppO2 on the upcoming dive(s).

I cant get over 1.5 or 1.55 at 20ft at the end of the dive because I have to breathe lol. And the cells are getting soggy. And if I am midwater trying to do multiple complete O2 flushes is mucking with my buoyancy big time. I can usually spike over 1.6 at ~24ft. I don't really do it very often as its only the coarsest of checks.