Calibrating Dissolved Oxygen - Xylem EXO Series User Manual

Calibration
5.3
Dissolved Oxygen
First review the basic calibration description on pg 65.
ODO % sat – 1-point
Place the sonde with sensor either (a) into a calibration cup containing about 1/8 inch of water which is
vented by loosening the threads or (b) into a container of water which is being continuously sparged with an
aquarium pump and air stone. Wait approximately 10 minutes before proceeding to allow the temperature
and oxygen pressure to equilibrate.
In the Calibrate menu, select ODO, then select ODO % sat. Calibrating in ODO % sat automatically calibrates
ODO mg/L and vice versa.
Enter the current barometric pressure in mm of Hg (Inches of Hg x 25.4 = mm Hg).
Note: Laboratory barometer readings are usually "true" (uncorrected) values of air pressure and can
be used "as is" for oxygen calibration. Weather service readings are usually not "true", i.e., they are
corrected to sea level, and therefore cannot be used until they are "uncorrected". An approximate
formula for this "uncorrection" (where the BP readings MUST be in mm Hg) is:
True BP = [Corrected BP] – [2.5 * (Local Altitude in ft above sea level/100)]
Click 1 Point for the Calibration Points. Enter the standard value (air saturated).
Click Start Calibration. Observe the readings under Current and Pending data points and when they
are Stable (or data shows no significant change for approximately 40 seconds), click Apply to accept this
calibration point.
Click Complete. View the Calibration Summary screen and QC score. Click Exit to return to the sensor
calibration menu, and then the back arrows to return to main Calibrate menu.
mg/L – 1-point
Place the sonde with sensor in a container which contains a known concentration of dissolved oxygen in
mg/L and that is within ±10% of air saturation as determined by one of the following methods:
- Winkler titration
- Aerating the solution and assuming that it is saturated
- Measurement with another instrument
A
Carrying out DO mg/L calibrations at values outside the range of ±10 % of air saturation is likely to
compromise the accuracy specification of the EXO sensor.
In the Calibrate menu, select ODO, then select ODO mg/L. Calibrating in ODO mg/L automatically
calibrates ODO % sat and vice versa.
Click 1 Point for Calibration Points. Enter the known mg/L concentration for the standard value. Click Start
Calibration. Observe the readings under Current and Pending data points and when they are Stable (or data
shows no significant change for approximately 40 seconds), click Apply to accept this calibration point. Click
Complete.
Rinse the sonde and sensor(s) in tap or purified water and dry.
|
68
pg