Yokogawa DO402 Dissolved Oxygen Analyzer - Manual (Page 72)

IM 12J05D02-01E
6-6 Calibration
6-4. Calibration method using manual calibration method
6-4-1. Preparation
a. Cleaning
Confirm that the readings of the analyzer are stable and measured with a clean sensor.
Otherwise move the sensor to a maintenance site and wash off any dirt on the sensor membrane
Move the sensor back to the process water and wait for the readings to stabilize.
b. Sampling
The manual calibration procedure involves adjustment of the readings to a reference standard.
Normally this reference is a laboratory measurement that analyses a sample that must be identical to
the sample measured with the on-line analyzer.
The following precautions must be taken into account:
• The sample must be analyzed directly after sampling to avoid change of dissolved oxygen
concentration or temperature after sampling.
• The reference standard must be analyzed with a laboratory meter that is calibrated prior to
sample analysis.
• The reference method must be calibrated with the same calibration data as specified in ISO
5814 to avoid in discrepancies.
Most important calibration data are:
Dissolved oxygen concentration at saturation conditions as function of temperature, barometric pres-
sure, altitude and salinity.
These data are included in this manual for reference and verification of correct calibration of refer-
ence analyzers. DO402G has taken all these variables into account during the semi-automatic cali-
bration methods.
c. Compensating the salinity of a sample.
Example of procedure for (salinity compensation) calibration.
When performed in consideration of the salinity, calibration should be done manually.
The following shows a procedure for salinity compensation. Perform the calibration referring to Table
6.1.
Assuming that the atmosphere is 1 atm, the solution temperature is 22.5 ºC, and the salinity is 30 g/
kg (3 wt % seawater equivalent):
1. Calculating the saturated concentration of water
According to Table 6.1, the solubility is 8.74 mg/l at 22 ºC and 8.58 mg/l at 23 ºC.
The solubility at 22.5 ºC is calculated as follows:
8.74 + (8.58 - 8.74) × (22.5 - 22) = 8.66 mg/l
2. Calculating the value corrected for salinity
0.0453 + (0.0443 - 0.0453) × (22.5 - 22) = 0.0448 mg/l
0.0448 × 30 = 1.34 mg/l
3. Calculating the saturated concentration of solution (to be calibrated)
8.66 – 1.34 = 7.32 mg/l
NOTE:
When salinity compensation is done, salinity compensation must be enabled in Service Code 04
to allow the instrument to show the saturated concentration (%) during sample measurements
correctly.