Quote: (Originally Posted by
silent running)
This is very interesting Nigel, but I'm a bit confused about what the reference is at any given point during the dive. If it's true that sensors can and do need frequent calibraion, somtimes even during the dive, how then can the PO2 be trusted for depth sensor cal/reference? Thanks, -Andy
First PO2 sensors do not need frequent calibration per se. What they do need is frequent checking as their failure modes can be insidious and catastrophic.
To answer your Question:
PO2 = FO2 * absolute_pressure
absolute_pressure ~= (1 + depth/10) with depth in meters
Thus:
FO2 = PO2 * (1 + depth/10)
If any two of these three variables are known, then the third may be validated. Thus in the normal case, FO2 is known and depth is assumed to be accurate, thus PO2 can be computed and compared to what the PO2 sensors are reading.
Conversely, if the PO2 sensors were known to be extremely reliable (which may be the case one day when there is a breakthrough in PO2 sensor technology), then we may rearrange the equation to give us:
(1 + depth/10) = FO2/PO2
(10 + depth) = 10 * FO2 / PO2
depth = (10 * FO2 / PO2) - 10;
In which case, we could use this equation to validate the depth sensor.
I hope this helps.