IC Controls Ltd.

ppb Dissolved oxygen measurement


Courtesy of IC Controls Ltd.

Dissolved oxygen (D.O.) measurement is one indicator of water quality because oxygen is a common element contributing to corrosion reactions. This corrosion reaction provides the need for low level D.O. control since dissolved oxygen is responsible for costly replacement of piping and equipment by corrosive attack on metah.

what is Dissolved Oxygen?
Dissolved axygan (D.O.f is the amount of oxygen gas dissolved in a given quantity of solvent (usually water) at a given temperature and atmospheric pressure. It is usually expressed as a concentration in parts per million or mg/l For trace levels of dissolved oxygen, parts per billion is the unit typically7 used. D.O. can also be expressed as percent saturation, where saturation is the maximum amount of oxygen that can theoretically be dissolved in water at a given pressure and temperature.

Corrosion is a state of deterioration in metals caused by oxidation or chemical action starting at its surface. Hard and porous metal oxide corrosion deposits have little strength and form rapidly in the presence of water and oxygen. The most serious aspect of oxygen corrosion is that it results in pitting. Pitting is a localized form of corrosion by which metal oxide filled cavities or holes are produced in the material. Pitting is considered to be more dangerous than uniform corrosion because it is more difficult to detect and predict. Pits can produce system failures even though only a relatively small amount of metal lias been lost. Rapid corrosion will progress inside a boiler water and steam system, for example, unless dissolved oxygen can be virtually eliminated. Unchecked corrosion eventually results in expensive repairs or equipment failures and subsequent replacement. For this reason, water with the least possible amount of dissolved oxygen is used. Therefore, in order to control trace levels of D.O. in Boiler feedwater supplies, on-line analytical instruments are employed to monitor and maintain the desired D.O. levels.

Dissolved Oxygen Sensor Design
For the continuous monitoring of dissolved oxygen concentrations, the most common sensor design utilizes a membrane. Membrane sensors typically share the following characteristics:

  • ELECTRODES - provide the reaction site needed for the reduction of oxygen molecules and generation of electrons
  • MEMBRANE - a gas permeable membrane separates the measurement cell from the sample and allows only dissolved oxygen to diffuse into the cell
  • ELECTROLYTE - provides an electrical path to complete the current loop between

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