Yokogawa DO202 2-Wire Dissolved Oxygen Analyzer - Manual (Page 52)

IM 12J05C01-01E
Calibration Procedure 6-1
6. CALIBRATION PROCEDURE
Calibration of the dissolved oxygen analyzer is per-
formed in the following situations:
• When a new dissolved oxygen sensor is installed.
• When the membrane is replaced and/or the
electrolyte solution is replaced
• When the sensor has been disassembled and
reassembled for maintenance
• When the measuring error after cleaning exceeds
the acceptable deviation from reference method
• When error message E16 indicates the need for
maintenance
6-1. General
6-1-1. Calibration methods
There are four methods for calibration of the EXA
dissolved oxygen analyzer: air calibration, water
calibration, sensitivity and zero (offset) calibration.
Air calibration
This is the most common and easiest way to cali-
brate the analyzing system. The calibration is done
in AIR.CAL mode in maintenance level.
Water calibration
This is the traditional way to calibrate dissolved oxy-
gen analyzers in laboratory environment. . Accurate
results can be obtained if the calibration is per-
formed carefully. The water calibration may include
a zero calibration if so selected in service code 21.
Normally only a zero check is performed without
calibration, since a good sensor has no zero signal.
Water calibration is not possible in salt water unless
salinity compensation is activated in service code
04. The calibration is done in H2O.CAL mode in
maintenance level
Note: In normal industrial environments better
results are obtained with air calibration, which
is therefore the recommended method.
Manual calibration
This is a calibration method whereby the sensor is
not exposed to a calibration fluid, but is calibrated
by comparison with a reference method. This refer-
ence method is normally a laboratory analysis
of the water sample: either by Iodometric titration
according ISO 5813 or an electrochemical method
according ISO 5814.
The calibration is done in SENS/ZERO mode in
maintenance level.
6-1-2. Diagnostic functions performed during
calibration
The calibration is a semi-automatic calibration,
which means that the sensor output is used for
calculation of sensor parameters after the read-
ings have stabilized. The criteria for stabilization
are set in service code 20. If stable readings are
not achieved within one hour error E1 message will
appear on the display and the procedure is aborted.
The calculated sensor parameters are compared
with preset tolerances and if these conditions are
not met, Error E2 (Zero Error) or E3 (Sensitivity
Error) will be signalled.
Notes:
1. Error messages that appear initially indicate
inadequate settings of the diagnostic variables
in service codes 01, 20 and/or 22.
2. Patience improves the accuracy of calibration.
When the EXA prompts the user that the
preset criteria are met by showing CAL.END
message it only means that the minimum
criteria are met. When the signal still changes
after this initial acceptance, EXA will
automatically update the sensor characteristics
with the new information.