Yokogawa 2-Wire Dual Channel Transmitter/Analyzer FLXA21 - Manual (Page 168)

<14. CALIBRATION OF DO (Dissolved Oxygen)>
14-3
IM 12A01A02-01E 5th Edition : Oct. 31, 2013-00
14.3 Manual slope calibration
Calibrate the sensitivity of the sensor with a solution of known oxygen concentration.
First, analyze the oxygen concentration of a sample solution and then calibrate the sensor
sensitivity to adjust to this value. From the actual measurement of salinity and temperature,
obtain the dissolved oxygen concentration by referring to Table 1 in Appendix 4 and enter it.
When salinity must be considered, perform the calibration manually.
Note the following:
• Quickly analyze the sample water to prevent the dissolved oxygen concentration or
temperature from changing.
• Before analyzing the sample water, analyze the reference with a calibrated laboratory
instrument.
• To eliminate any discrepancies, the laboratory instrument must have been calibrated with
the same calibration data as specified in ISO 5814.
Confirm that the reading of the instrument is stable and the sensor is clean. If the reading
remains unstable, set the sensor in the maintenance mode and wash off any stain on the sensor
membrane. Return the sensor to the sample water and leave it until the reading becomes stable.
l Salinity compensation
To compensate salinity, select “Enabled” in “Measurement setup” – “Salinity compensation” –
“Compensation” (see Section 13.2.4).
Perform calibration by referring to Table 1 in Appendix 4.
Example:
When the atmospheric pressure is 101.325 kPa, the solution temperature is 22.5°C, and the
salinity is 30 g/kg (3 wt% seawater equivalent):
(1) Calculate the saturated concentration of desalinated water.
According to Table 1 in Appendix 4, the saturated concentration is 8.74 mg/l at 22°C and
8.58 mg/l at 23°C.
The value at 22.5ºC is calculated as follows:
8.74 + (8.58 – 8.74) × (22.5 – 22) = 8.66 mg/l
(2) Compensate the value for salinity.
The compensation value for 30 g/kg salinity at 22.5°C is calculated as follows:
0.0453 + (0.0443 – 0.0453) × (22.5 – 22) = 0.0448 mg/l
0.0448 × 30 = 1.34 mg/l
(3) Calculate the saturated concentration of the solution (for calibration).
The saturated concentration for 30 g/kg salinity at 22.5ºC is calculated as follows:
8.66 – 1.34 = 7.32 mg/l
14.4 Temperature calibration
For the most accurate measurements, it is important to have a precise temperature
measurement. Measure the temperature with a high-precision thermometer and adjust the
sensor reading accordingly. For best accuracy, this should be done as near to the normal
operating temperature as possible.
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