35
DO CALIBRATION
DO CALIBRATION
DO CALIBRATION
DO CALIBRATION
DO CALIBRATION
It is recommended to calibrate the instrument frequently, especially if high accuracy is required.
The DO range should be recalibrated:
• Whenever the DO probe is replaced.
• At least once a week.
• Before BOD, OUR, SOUR measurements.
• After testing aggressive chemicals.
• When calibration reminder is activated (“DO Cal Expired”).
• If the readings are far from the calibration point.
Note:
BOD, OUR and SOUR readings are automatically derived from the DO readings and no specific
calibration is needed.
The following options are available for the Dissolved Oxygen calibration:
•
one point automatic zero calibration at 0% saturation or 0 mg/L
•
one point automatic slope calibration at 100% saturation or 8.26 mg/L
•
1 point manual calibration using a standard value set by the user in % saturation or mg/L
When automatic calibrations are performed it is assumed that the standard value is the saturated DO value at 25 °C,
0 g/L salinity and 760 mmHg.
When manual calibrations are performed it is assumed that the standard value is the DO value at the current
pressure, temperature and salinity.
Initial preparation
Make sure the probe is ready for measurements, i.e. the membrane is filled with electrolyte and the probe is
connected to the meter.
For an accurate calibration, it is recommended to wait for at least 15 minutes to ensure precise conditioning of
the probe.
Remove the protective cap from the DO probe.
Make sure the salinity value has been set to the salinity of the standard.
Probe conditioning
At
startup, the probe is under polarization with a fixed voltage of approximately 800 mV for 1 minute.
Probe polarization is essential for stable measurements with the same recurring degree of accuracy.
With the probe properly polarized, oxygen is continually consumed when it passes through the sensitive
diaphragm and dissolves in the electrolyte solution contained in the probe.
Whenever measurements are taken with a non-polarized probe, the oxygen level revealed is both that of the
tested solution, as well as that present in the electrolyte solution. This measurement is
incorrect
.
