Hach-Lange SI794 D User Manual - Manual (Page 70)

Zero calibration
Zero calibration
The sensors have a very low zero current. Therefore, a zero cali-
bration is only recommended for measurement of oxygen traces.
If a zero calibration is performed, the DO sensor should remain
for at least 10 to 30 minutes in the calibration medium in order to
obtain stable, non-drifting values.
During zero calibration, a drift check is not performed.
Zero current of a properly functioning sensor is notably less than
0.5 % of air current. The display (bottom: measured value, top:
entered value) does not change until an input current is entered
for the zero point.
When measuring in an oxygen-free medium, the displayed
current can be taken directly.
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