Xylem CellOx 325 Dissolved oxygen sensor - Manual (Page 12)

Maintenance, cleaning, replacement CellOx 325
28
5.5 Checking the sensor for freedom from zero-current
The sensor is zero-current free. Checking the sensor for freedom from zero-
current is only necessary in the case of malfunctions that cannot be remedied
by exchanging the electrolyte solution and membrane head or by cleaning
the electrodes.
There are two possibilities to check the sensor for freedom from zero-current:
l
Measurement in a nitrogen atmosphere (recommended method)
l
Measurement in a sodium sulfite solution according to DIN EN 25814/
ISO 5814.
Caution
If you check the sensor according to DIN EN 25814/ISO 5814, do not
leave the sensor in the sodium sulfite solution for more than 2 minutes.
Danger of sensor poisoning!
Test criterion The sensor is OK if the measuring instrument displays < 1 % DO saturation
after 2 minutes.
Note
For measurements under high pressure the fill must be completely free of air
bubbles.
Readiness to
measure
After approx. 30 to 50 minutes, the sensor is ready for operation. Subse-
quently calibrate the sensor.
Note
If you want to measure very low oxygen concentrations (< 0.5 % saturation),
we recommend to let the sensor rest overnight and then calibrate it.