MODEL | SECTION 7.0 |
| CALIBRATION - DISSOLVED OXYGEN |
SECTION 7.0
CALIBRATION - DISSOLVED OXYGEN
7.1 INTRODUCTION
As Figure
The zero standard is necessary because oxygen sensors, even when no oxygen is present in the sample, gener- ate a small current called the residual current. The analyzer compensates for the residual current by subtracting it from the measured current before converting the result to a dissolved oxygen value. New sensors require zeroing before being placed in service, and sensors should be zeroed whenever the electrolyte solution is replaced. The recommended zero standard is 5% sodium sulfite in water, although
The purpose of the
Oxygen sensors generate a current directly proportional to the rate at which oxygen molecules diffuse through a membrane stretched over the end of the sensor. The diffusion rate depends on the difference in chemical poten- tial between oxygen in the sensor and oxygen in the sample. An electrochemical reaction, which destroys any oxy- gen molecules entering the sensor, keeps the concentration (and the chemical potential) of oxygen inside the sen- sor equal to zero. Therefore, the chemical potential of oxygen in the sample alone determines the diffusion rate and the sensor current.
When the sensor is calibrated, the chemical potential of oxygen in the standard determines the sensor current. Whether the sensor is calibrated in air or
Automatic air calibration is standard. The user simply exposes the sensor to
analyzer calculates the partial pressure of oxygen. Once the analyzer knows the partial pressure of oxygen, it uses the Bunsen coefficient to calculate the equilibrium solubil- ity of atmospheric oxygen in water at the prevailing tem- perature. At 25°C and 760 mm Hg, the equilibrium solu- bility is 8.24 ppm.
Often it is too difficult or messy to remove the sensor from the process liquid for calibration. In this case, the sensor can be calibrated against a measurement made with a portable laboratory instrument. The laboratory instrument typically uses a
FIGURE 7-1. Sensor Current as a Function of
Dissolved Oxygen Concentration
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