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dissolved oxygen (D.O.) measurement

The fundamentals of electrochemical and optical dissolved oxygen measurement

Measure oxygen - WTW

Dissolved oxygen

Dissolved oxygen is present in virtually every liquid. Liquid dissolves oxygen until the partial pressure of oxygen in the liquid is in equilibrium with the air or gas in which it is in contact. The actual concentration of dissolved oxygen depends on a number of factors, such as temperature, air pressure, oxygen consumption by microorganisms in a biodegradation process or oxygen production by algae.

For example, at a temperature of 20 °C (68 °F) and an atmospheric pressure of 1013 mbar, saturated water contains about 9 mg/l oxygen. Ethanol can contain up to 40 mg/l, whereas glycerol only has about 2 mg/l. In all three cases the measured oxygen saturation is 100%. 

The oxygen concentration is important for the:

  • Living conditions for fish and microorganisms in waters
  • Degradation processes in wastewater treatment
  • Corrosion processes in pipelines
  • Shelf life of beverages, etc.

The determination of the oxygen concentration was formerly carried out by the WINKLER titration method. Today, electrochemical measurement is a recognized method in numerous standard procedures. During the last years the optical measuring of dissolved oxygen has become more important. For both methods a sensor and a meter is used.

In its simplest form a dissolved oxygen sensor contains a working electrode and a counter-electrode. Both electrodes are located in an electrolyte system which is separated from the sample by a gas-permeable membrane. The working electrode reduces the oxygen molecules to hydroxide ions. In this electrochemical reaction a current flows from the counter-electrode to the working electrode.The more oxygen present in the sample, the larger the current signal. The D.O. meter calculates the concentration of dissolved oxygen in the sample from this signal.

For optical measurements there are no chemical reactions involved. A special fluorescent dye is used instead, which is activated by light in the measuring membrane. In the presence of oxygen the fluorescence changes its character (quenching), this effect is used for the quantitative determination. The biggest advantage of this optical method is the extreme low maintenance requirement. Since December 2014 this method is implemented into the norm DIN ISO 17289:2014‐12 

D.O. concentration [mg/l] as a function of temeprature and air pressure (at 100% saturation)
D.O. concentration [mg/l] as a function of temeprature and air pressure (at 100% saturation)

WTW's D.O. measurement technology

Stationary Meters for Waste Water and more

  • Wastewater treatment plants
  • Drinking water plants
  • Fishfarming
  • Industrial applications

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