The Dissolved Oxygen Monitor – Measuring the O2 Concentration in Aquatic Environments
I already mentioned a few basic things about dissolved oxygen in my previous posts. Today we’ll take a closer look at the dissolved oxygen monitor (also called by certain manufacturers dissolved oxygen meter or dissolved oxygen analyzer).
What is dissolved oxygen?
I’m sure you already know that all aerobic (oxygen dependant) organisms – including those inhabiting liquid environments – need oxygen for their survival and harmonious development. Without oxygen, there would be no life in oceans, rivers and lakes. Fish and plants, invertebrates and water aerobic bacteria need oxygen for their respiration. However, in order to be used by all these organisms, the atmospheric oxygen needs to be “mixed” with water first. When the gaseous oxygen is physically distributed into a liquid solution, it is called dissolved oxygen.
Dissolved oxygen (DO) is the measure of the amount of O2 dissolved in a given liquid environment.
But how does oxygen get from the air to the water? Actually, it’s very simple. Most dissolved oxygen comes directly from the atmosphere by diffusion, being afterwards distributed by waves and currents. Dissolved oxygen is also a product of photosynthesis, being released by various algae and other aquatic plants.
Oxygen depletion – the dangerous decreasing of the oxygen saturation in water or air – is a burning issue nowadays, threatening the frail balance of our ecosystem. When organic or chemical waste is allowed to decompose in water, it consumes dissolved oxygen, directly affecting aquatic organisms.
What is a Dissolved Oxygen Monitor?
During the last decades, oxygen measurement in aquatic environments was facilitated by the invention of dissolved oxygen monitors (meters) – electronic devices which use special sensors to determine the oxygen concentration in water or another liquid.
Dissolved oxygen is one of the main indicators of water quality. The amount of O2 that can be dissolved in water depends on several factors: salinity, temperature and altitude (and therefore – pressure). Oxygen solubility increases as the temperature and the salinity are decreasing. Cold water can hold more oxygen than warm or hot water; fresh water can hold more oxygen than saltwater. Oxygen is absorbed better at low altitudes; its saturation in water decreases as the altitude increases.
Dissolved oxygen monitors, therefore, besides displaying the oxygen saturation, can also indicate water temperature and salinity, calibrating the results depending on these variable factors. Most models can display the dissolved oxygen levels in two different ways: as a percentage and as a ppm measurement. The user can toggle between these two display modes depending on his purpose.
There are three main types of sensors used in dissolved oxygen analyzers: galvanic, polarographic and optical. The surface of the sensor requires regular maintenance, because it’s necessary to remove any build-up accumulated on the membrane that could affect the accuracy of the measurements.
Many dissolved oxygen monitors come with an incorporated Auto-Cleaner: every two hours (or another time interval that you can program according to your needs) a blast of compressed air will clean the head of the sensor. This way, the monitor will be able to function without maintenance for many months to come.
Just like other types of oxygen monitors, dissolved oxygen meters come in portable and wall-mounted models.
Which are the main applications of dissolved oxygen monitors?
Because of the increasing pollution and the continual worsening of the air and water quality on a global level, these monitors started to have a wide range of applications:
- determining the quality of drinking water
- observing the oxygen levels in waste water treatment plants
- pollution control in rivers, lake and seawater
- fish farming
- measuring dissolved oxygen in ultrapure water in the semiconductor industry etc.
When necessary, the dissolved oxygen meter can even display the oxygen saturation in the ambient air.
The usual price of a dissolved oxygen monitor varies between $100 and $400; however, complex models can even reach $2000 and more.
