Xylem WQ pH SENSORS - Manual (Page 13)

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e. The dissolved oxygen sensor must be stored where the sensing
element remains wet or the sensing element will be damaged. It can
be stored in a container of clean water or a wet sponge can be placed
inside the protective cap shipped with the sensor. Place the cap over
the sensing element to keep it wet. The sensor can then be stored on
a shelf or hung on a wall.
f. Dissolved oxygen (DO) refers to the volume of oxygen that the water
contains. The quantity of oxygen that the water can hold depends on
the temperature, salinity, and pressure of the water. Gas solubility
increases with decreasing temperature (colder water holds more
oxygen). Gas solubility increases with decreasing salinity (freshwater
holds more oxygen than does saltwater). Finally, gas solubility
decreases as pressure decreases. Thus, the amount of oxygen
absorbed in water decreases as altitude increases because of the
decrease in relative pressure.
g. Calibration should be checked monthly using the following procedure.
You will need the following equipment:
2 small containers
1 one-gallon container
1 package of zero oxygen solution
1 power supply
1 current meter
Connecting wires as necessary
Connect the sensor to the power supply and current meter in
the following way. Attach the black wire to the ground terminal of the
power supply. Attach the white wire to the positive input of the current
meter. Connect the ground terminal of the power supply to the ground
of the current meter. Attach the red wire to the positive terminal of the
power supply. See Appendix C. Warning: Always connect the
sensor with the power turned off.
See Appendix A for the dissolved oxygen calibration worksheet.