Anybody who measures dissolved oxygen (DO) knows that temperature and pressure have a significant influence. But why?
The most important aspect is that temperature affects the solubility of oxygen in water. At atmospheric pressure and 20 °C, the maximum amount of oxygen that can be dissolved in water is 9.2 mg/l. It might seem somewhat contra intuitive, but at 25 °C this decreases to 8.3 mg/l, and at 50 °C is only 5.6 mg/l. At lower temperatures, solubility increases to 12.8 mg/l at 5 °C.
If, as is common in biotech and pharmaceutical processes, oxygen concentration is expressed in terms of an air saturation percentage, the maximum solubility corresponds to 100% at a certain temperature. If the temperature changes, the air saturation level also changes.
In amperometric sensors, oxygen diffuses through a membrane. This diffusion depends strongly on temperature. Correct compensation for this effect depends on the type of sensor used.
In the chemical industry and waste water applications, dissolved oxygen is expressed in mg/l. According to best practice, oxygen sensors are calibrated in fresh air at a fixed temperature. The raw sensor signal is than set to be equal to the maximum solubility of oxygen in water at that temperature. The sensor/transmitter must ‘know’ this value. When the temperature of the water changes, the sensor will give the correct oxygen concentration.
The solubility of oxygen in water is also dependent on pressure. If the pressure changes from 1 bar to 2 bar, twice the amount of oxygen can be dissolved. In other words, solubility is directly proportional to pressure.
For example, take a reactor or tank with oxygen-saturated water and a headspace of air at 1 bar, with 100% air saturation corresponding to 9.2 mg/l at 20 °C. If the pressure in the headspace is 2 bar and saturation is maintained at 100%, the corresponding concentration will rise to 18.4 mg/l.
To conclude, this brainteaser; if, in the above example, the headspace is compressed to 2 bar, the air saturation will drop to 50% but oxygen concentration will remain at 9.2 mg/l!
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