Xylem WQ-FDO OPTICAL DISSOLVED OXYGEN TRANSMITTER - Manual (Page 7)

Global Water
800-876-1172
• globalw.com
IV. Sensor Calibration
The dissolved oxygen sensor should remain stable for the service life of the
cap and membrane, which is greater than one year. This generally makes
recalibration unnecessary. If the cap is replaced, recalibration should also
not be necessary. The unique design of the FDO sensor allows a memory
chip inside the cap to hold calibration information that is read by the sensor
when a new cap is installed. The sensor then recalibrates itself for the new
cap and adjusts its DO output accordingly. The temperature output should
also not need recalibration.
Recalibrating the WQ-FDO sensor consists of measuring the output currents
at two known oxygen levels. For greatest accuracy, one level should be at
zero oxygen and one should be near full saturation.
There is no easy way to develop a calibration standard that has a known
level of dissolved oxygen. Calibrating an oxygen sensor is generally done
by comparing it to another instrument of known accuracy. The Winkler
titration method can also be used. It provides the highest accuracy but
involves the use of strong chemicals.
The oxygen level of water can be elevated by bubbling air through it with a
conventional aquarium bubbler. Once saturated, the oxygen level can be
measured and compared to the WQ-FDO sensor output. The zero oxygen
level can be checked in one of two ways. The most accurate way is to
expose the sensor to a nitrogen saturated atmosphere. Place the sensor in a
sealable container and fill the container with nitrogen until all of the air has
been displaced. Wait for the temperature to stabilize then measure the
sensor DO output current. A zero oxygen sodium sulfite calibration
standard can also be used.
To have the sensor’s calibration checked, return it to Global Water. Contact
the Technical Support department for more information.