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As the STOX sensors have a built-in zeroing it is only necessary to do a one-point calibration,
but it may be reassuring also to do a zero calibration in an ascorbate solution.
The signal for oxygen concentration is the difference between sensor signal with the front
guard polarized and unpolarized -
the ∆
-signal. Calibration of the STOX sensor should not be
performed using the calibration function in Logger, instead it is recommended to record
uncalibrated data and to do the calibration in a spreadsheet.
If you are measuring low O
2
concentrations with the STOX sensor it is best to calibrate in
that range and not at atmospheric saturation as you would normally do with O
2
sensors.
This may be done by injecting a known small volume of air saturated water into a larger
known volume of water with low O
2
concentration while recording the STOX sensor signal.
The STOX sensor is sensitive to temperature and calibration should be done at the same
temperature as the measurements. If temperature varies during the deployment the
calibration should be done at both the highest and lowest encountered temperature. It may
then be assumed that sensitivity varies linearly between the two temperatures.
The O
2
concentration in air saturated water at a given temperature and salinity may be
calculated with the O
2
-calculator tool in the Logger software:
Tools - O
2
calculator
.
Example:
Procedure:
•
300 ml of N
2
bubbled water is transferred to a conical flask. The STOX sensor is
placed in the water and at least two full polarization - depolarization cycles are
recorded
•
3.0 ml air saturated water is added to the conical flask and gently mixed. At least two
full polarization - depolarization cycles are recorded
•
The amendment of air saturated water may be repeated to obtain several estimates
of the sensitivity
This procedure may at best be performed before deployment and with the FDL Mini
online with the Logger sof
tware to see the response of the ∆
-signal to the O
2