Yokogawa DO402 Dissolved Oxygen Analyzer - Manual (Page 70)

IM 12J05D02-01E
6-4 Calibration
6-3. Calibration procedure using water calibration method
6-3-1. Preparation
Move the sensor to a maintenance site and wash off any dirt on the membrane. Lightly wipe off any
remaining water from the membrane with a soft tissue.
Prepare the necessary equipment and reagents to be used for the span and (if required) zero calibration.
Equipment for span calibration includes:
• beaker or bucket
• magnetic stirrer or other means of agitating the water
• Demineralized water or salty process water if salinity compensation is activated
• Air supply pump (aquarium pump)
• Glass diffuser to generate small air bubbles
NOTE:
It takes 15 to 30 minutes of aeration before it can be assumed that the water is fully saturated with air.
Reagent for zero calibration:
• Sodium sulfite
• demineralized water
The zero oxygen solution contains approximately 50 grams of sodium sulfite per liter.
6-3-2. Calibration operation (water calibration method)
If both zero and span calibration are used, it is better to start with span calibration in spite of normal
calibration sequence with analog instrumentation. Otherwise the sensor must be rinsed off carefully to
prevent ingress of oxygen absorbing sulfite in the aerated water.
NOTE:
It takes a long time to calibrate the DO30G sensor or other galvanic sensors and polarographic sensors
with sulfite-saturated water. Even after CAL.END is displayed, wait 40 to 50 minutes and then press
the YES key. If the YES key is pressed without waiting, the indicator may show a minus reading dur-
ing measurement. In an aerobic tank with high dissolved oxygen concentrations, a zero calibration is
not required if a zero check is performed, because the indicating error is so insignificant that it does not
affect the measurement results. There will be no problem if the electrodes are immersed in a zero solu-
tion for about 10 minutes and the indication falls below 1 mg/l. However, it is recommended to wait for
about 60 minutes before performing a zero calibration when a new sensor is installed, when the mem-
brane is replaced, or in a place where dissolved oxygen concentrations are low, such as in an anaerobic
tank. If the solubility does not fall below 1 mg/l even after waiting for about 10 minutes, remove the sen-
sor and clean it, and replace the membrane and/or replace the electrolyte solution.
Calibration may take longer than 10 minutes. Before starting the calibration procedure, check the setting
of Auto Return (*RET.) in Service Code 50. It is recommended to set the Auto Return to “0” (No return
to meas. from HOLD). After the calibration procedure has been completed, set the Auto Return to the
desired setting in Service Code 50.