Zero current

Each polarographic oxygen sensor has a certain zero current, i.e a current that flows even at an oxygen concetration of zero. This current is taken into acount by the zero calibration. The zero current is recorded as a gain dependent voltage and visible during zero claibration or at the end of a sensor test if the raw signal is observed. This voltage at zero oxygen can be expressed as % of the voltage at air saturation. While new sensors have a zero voltage of below 1% of the voltage at air saturation (for the same gain), used senors may have higher zero currents, up to ca 10% of volatge at ars saturation without negative consequences, provided the zero current is stable. The stability of the zero current can be assesed by keeping track of calibration values using the "O2k-Calibration-list.xls" template. This template and the calibration procedure can be found at [MiPNet12.08]. A high or unstable zero current is usualy caused either by the POS, by the POS connector or by a bad connection between those two parts. Cleaning and applying contact oil to the gold pin and thread conecting the POS head and the POS conector will frequentyl solve the problem. Otherwise, the problem has to be localized by switching components.

If the problem is located on the POS head an intensive sensor service, should be applied at least two times. If this does not help the sensor proable has to be replaced.

If the problem is located at the POS connector cleaning the connection as desribed above should be tried again. If the problem was descreased but not solved by clenaing and applying contact oil, the cleaning process may have to be repeated several times. If cleaning the conector does not help the POS conector may have to be replaced.