MODEL SOLU COMP II
SECTION 6.0
CALIBRATION
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6.3 CALIBRATION — DISSOLVED OXYGEN
6.3.1 Purpose
As Figure 6-1 shows, oxygen sensors generate a current directly proportional to the concentration of dissolved oxy-
gen in the sample. Calibrating the sensor requires exposing it to a solution containing no oxygen (zero standard)
and to a solution containing a known amount of oxygen (full-scale standard).
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 subtracts the residual current from the measured cur-
rent and converts the result to a dissolved oxygen reading. New sensors require zeroing before being placed in
service, and sensors should be zeroed whenever the electrolyte solution is replaced. The recommended zero stan-
dard is 5% sodium sulfite in water, although oxygen-free nitrogen can also be used.
The Model 499A TrDO sensor, used for the determination of trace (ppb) oxygen levels, has very low residual cur-
rent and does not normally require zeroing. The residual current in the 499A TrDO sensor is equivalent to less than
0.5 ppb oxygen.
The purpose of the full-scale standard is to determine the slope of the calibration curve. Because the solubility of
atmospheric oxygen in water as a function of temperature and barometric pressure is well known, the natural
choice for a full-scale standard is air-saturated water. However, air-saturated water is difficult to prepare and use,
so the universal practice is to use water-saturated air. From the point of view of the sensor, air-saturated water and
water-saturated air are identical. The equivalence comes about because the sensor really measures the chemical
potential of oxygen. Chemical potential is the force that drives oxygen molecules from the sample through the
membrane into the sensor. Sensor current is proportional to the rate at which oxygen passes through the mem-
brane, so current is really determined by the chemical potential of oxygen in the sample. Because oxygen in air-
saturated water is in equilibrium with oxygen in water-saturated air, the chemical potential of oxygen in both phas-
es is the same. Whether the sensor is in air-saturated water or water-saturated air, the driving force pushing oxy-
gen into the sensor is the same, so the sensor current is the same.
Automatic air calibration is standard. The user simply exposes the sensor to water-saturated air. The analyzer
monitors the sensor current. When the current is stable, the analyzer stores the current and measures the baro-
metric pressure and temperature. The temperature element is part of the dissolved oxygen sensor. The pressure
sensor is inside the analyzer. From the temperature, the analyzer calculates the saturation vapor pressure of water.
Next, it calculates the pressure of dry air by subtracting the vapor pressure from the barometric pressure. Using
the fact that dry air always contains 20.95% oxygen, the 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 sol-
ubility of atmospheric oxygen in water at the prevailing temperature. At 25°C and 760 mm Hg, the equilibrium sol-
ubility 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 also be calibrated against a measurement made with a portable laboratory instrument. The laboratory instru-
ment typically uses a membrane-covered amperometric sensor that has been calibrated against water-saturated air.
FIGURE 6-1. Sensor Current as a Function of Dissolved Oxygen Concentration
