[quote=dave t;164305]
Quote: (Originally Posted by
Gilles)
sorry dont understand that, as I said before the controller interprets the MV output as a po2, all you do in calibration is to correct the reading to a known factor (stored?) dont you? but you cant change the MV output so if the cell puts out say 10MV in air and air is assumed to be 0.21 thats the reading you will see??
The controller is a machine. Completely unable to do any interpretation. It converts a mV into a PO2 depending on your calibration. Calibration is not a correction, it is a setting that determines the correctness of your controllers "interpretation" (please tolerate my sarcasm
). The difference between 0.21(air) and 1.0 (O2 @ 1 AtA) is a factor of about 4.8. Your O2 (@1 ATA) mV setting needs to be 4.8x that of your air mV in order for your controller to "interpret" correctly.
Quote:
<<<<In practise, if you calibrate your PO2=1 point at the correct mV reading, and then you in water test to 1.6 @6m with oxygen, you have certainty in both your calibration, and in cell health.>>>>>
only as long as you are accurate with cal gas and depth measurement!
That is true, but it (the correct depth and purity) is largely implied already if your mV at PO2 =1 jives, and you get 1.6 @6m with your cal gas.
Quote:
whats the difference in watching a po2 decay or a MV decay?
The main difference is that in the former, you do not know if you have a calibration issue, or a cell that is changing its nominal mV, or if it's a dying cell.
Quote:
<<<Contrary to your opinion on the matter, you can catch a faulty cell at least some of the time. This is quite easy with handset mV readings.>>>
if you have a test chamber, yes but still the po2 readout or MV readout from a raw data will decay the same
It is best to have a cell checker (test chamber), and yes PO2 and mV readouts will respond equally, it is just that the former is more ambiguous because healthy cells do need periodic re-calibration. When mV readings start to behave excessively non-linear (by this I mean to consistently not reach its theoretical value, something like erectile disfunction), this is a good indication to suspect your cell.
Quote:
dont agree, with a 2 point cal you are correcting the error at two points (maybe) introducing an error that wasnt there due to inacurate cal gas/depth sensor/ atmos pressure. There are some that say calibrating in air is better than calibrating in (O2 or both) due to the fact that an air sample is more consistantly reliable than an o2 sample (at least in our operational field) and most cells stay linear beyween 0.21 and 1.00bar anyway. A current limited cell will still calibrate within its range but unless you can calibrate higher than your setpoint how do you know where the current limit is if its outside your one or two cal points?
For reasons thoroughly articulated in the above-mentioned essay, air-cals are the worst possible types of cals.
Granted your argument about the higher risk of making errors in a 2-point cal.
Oxygen cals are ok because then (at least) you can observe how the cell drops back to air. If the value starts staying above 0.21, this is an indication of cell erectile disfunction
(for reasons detailed in above attachment). A correct 2-point cal however will result in a PO2 readout that is slightly less in error than a O2 single point cal. (again detailed in above essay).
I have observed current limited cells that don't/can't reach 1.5 or so, start not reaching their respective ideal PO2=1 mV by some 2-3 mV. The non-linearity starts to show already at PO2=1.
The above works at least some of the time, and is thus worthwhile if you have the convenience of handset mV's.