Dissolved Oxygen Measurement in Water with Oxygen Meters

DO in Water Basics

For aerobic aquatic species, adequate dissolved oxygen (DO) is of prime importance to their continued survival (even though some can survive in low DO environments). Since DO levels are directly related to good water quality, the two are highly interdependent. Many factors can affect DO levels, and an understanding of these levels in order to make informed decisions concerning wastewater treatment operations, hypoxic zones, aquaculture facilities or large-scale ecosystems is essential.

Dissolved oxygen refers to the level of free, non-compound oxygen present in water or other liquids. It is one of the most important parameters when assessing water quality because of its influence on the organisms living within a body of water. A DO level that is too high or too low can harm aquatic life and affect water quality.

Non-compound oxygen, or free oxygen (O2), is oxygen that is not bonded to any other element. Dissolved oxygen is the presence of these free O2 molecules within water. The bonded oxygen molecule in water (H2O) is in a compound and does not count toward DO levels. One can imagine that free oxygen molecules dissolve in water much the same way salt or sugar does when it is stirred.

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

DO meters, oxygen meters, measure the amount of oxygen dissolved in an aqueous solution. How do dissolved oxygen probes work? There are two primary types of DO 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. Oxygen sensors can be selected with many features including the type of sensing technology.

What is Dissolved Oxygen | Oxygen Meters | Dissolved Oxygen Definition | Dissolved Oxygen | Dissolved Oxygen Meter

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. Salinity and temperature can then be used in Henry’s Law to calculate what the DO concentration would be at 100% air saturation. However, it is easier to use an oxygen solubility chart. These charts show the DO 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 DO concentration.

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

Example:

80% DO measured

30 ppt salinity

10°C

.80 * 9.318 = 7.45 mg/L DO

>>> Review the Oxygen Solubility Table

Types of DO Sensors

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 Clark electrochemical sensors available are Polarographic and Galvanic. Additionally, YSI manufacturers two types of Polarographic sensors: Steady-state and the patented Rapid Pulse sensor.

Measuring Dissolved Oxygen with Optical or Electrochemical Sensors

Dissolved oxygen sensors, both electrochemical and optical, do not measure the concentration of dissolved oxygen in mg/L or ppm (parts per million which is equivalent to mg/L). Instead, the sensors measure the pressure of oxygen that is dissolved in the sample. To simplify the readings displayed by an instrument, the pressure of the dissolved oxygen is expressed as DO % Saturation. The instrument converts the dissolved oxygen pressure value from the sensor to % Saturation by dividing the sensor output in mmHg by 160 (the pressure of oxygen in air at 760 mmHg) and then multiplying by 100. Thus, a measured oxygen pressure of 150 mmHg would be displayed by a YSI instrument as 93.8 % Saturation (150/160 * 100).

The fact that the sensor measures oxygen pressure and not dissolved oxygen concentration is known to be true because a sample of fresh water can dissolve more oxygen than a sample of sea water at the same temperature and at the same altitude (or under the same barometric pressure); however, the sensor’s output signal is identical in both samples since the oxygen pressure is identical in both media.

The pressure of oxygen at sea level is 160 mmHg because oxygen is about 21% of the earth’s atmosphere and 21% of 760 (average sea level barometric pressure) is about 160 mmHg.

There are several factors that affect the measurement of dissolved oxygen. These variables include temperature, salinity, flow or stirring dependence, and barometric pressure. Download the DO Handbook to look at these factors more in depth.



Why Monitor for Dissolved Oxygen

Why do we monitor dissolved oxygen in water? Most aquatic organisms require dissolved oxygen in order to survive, but what exactly is it? Dissolved oxygen, or DO, is just like the oxygen humans use to breathe, but has been dissolved in water. Unlike the bonded molecules of H20, DO refers to the free oxygen that is present in water or other liquids.

Get your copy of
The Dissolved Oxygen Handbook

Pioneers in DO

How do you measure dissolved oxygen | Dr. Leland Clark

YSI commercialized the first practical dissolved oxygen sensor using the Clark Polarographic electrode in 1962 based on the work of Dr. Leland Clark in conjunction with YSI scientists. This technology became, and still is, the trusted global standard for dissolved oxygen measurement. In 1992, YSI released the patented Rapid Pulse® stirring-independent oxygen sensor which revolutionized the measurement of dissolved oxygen in long-term monitoring applications.


 

Optical Dissolved Oxygen

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In 2005, YSI released its first optical dissolved oxygen sensor that utilized luminescent technology. The wiped ROX® probe significantly increased customer value by reducing costs associated with maintenance, especially when used in long-term monitoring applications.

The ultimate in handheld optical dissolved oxygen measurement can be found on the ProODO®. The ODO® (Optical Dissolved Oxygen) luminescent sensor has no flow dependence making it the perfect choice for applications like deep water profiling where stirring is difficult or undesirable. No flow dependence also reduces the possibility of operator error due to inadequate stirring, ensuring high quality data.  However, stirring, or movement, of the optical DO sensors does improve response time.  YSI may be the only ones who will tell you that.

Essential D.O. Posts on the Blog

What is Affecting Your Dissolved Oxygen Measurements? Part 1 of 4

Dissolved Oxygen Meter Selection - More Than Sensor Technology

Environmental Dissolved Oxygen Values Above 100% Air Saturation

Nonload Parameters in Surface Water | Dissolved Oxygen

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