What is a Dissolved Oxygen Meter and What Does it Report?

- Feb 06, 2018 -

What is a Dissolved Oxygen Meter and What Does it Report?

A DO meter measures the amount of oxygen dissolved in an aqueous solution. There are two primary types of dissolved oxygen sensing technologies available: the optical based sensing method which is commonly referred to as luminescent and the Clark electrochemical or membrane-covered electrode. Within these two types of technologies, there are slight variations available. For example, there are two types of optical sensors. Both types of optical sensors measure luminescence as it is affected by the presence of oxygen; however, one sensor measures the lifetime of the luminescence while the other sensor measures the intensity of the luminescence. The two types of electrochemical sensors available are polarographic and galvanic.

Dissolved oxygen is usually reported in milligrams per liter (mg/L) or as a percent of air saturation. However, some studies will report DO in parts per million (ppm) or in micromoles (umol). 1 mg/L is equal to 1 ppm. The relationship between mg/L and % air saturation has been discussed above, and varies with temperature, pressure and salinity of the water. One micromole of oxygen is equal to 0.022391 milligrams, and this unit is commonly used in oceanic studies. Thus 100 umol/L O2 is equal to 2.2 mg/L O2.

Calculating DO from % Air Saturation

To calculate dissolved oxygen concentrations from air saturation, it is necessary to know the temperature and salinity of the sample. Barometric pressure has already been accounted for as the partial pressure of oxygen contributes to the percent air saturation 7. Salinity and temperature can then be used in Henry’s Law to calculate what the DO concentration would be at 100% air saturation 10. However, it is easier to use an oxygen solubility chart. These charts show the dissolved oxygen concentration at 100% air saturation at varying temperatures, and salinities. This value can then be multiplied by the measured percent air saturation to calculate the dissolved oxygen concentration 7.

O2 mg/L = (Measured % DO)*(DO value from chart at temperature and salinity)

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