What is Dissolved Oxygen (DO)?
Dissolved Oxygen (DO) is the amount of oxygen dissolved in a unit of water. Oxygen gets into water by: diffusing within the surrounding air, aeration (turbulent movement), and as a waste product from plants through photosynthesis.
How to Measure Dissolved Oxygen
Galvanic Dissolved Oxygen Sensors
Galvanic DO sensors consist of two electrodes: an anode and cathode. Both of these electrodes are immersed in electrolytes (inside the sensor body). An oxygen permeable membrane separates the anode and cathode from the measured water.
The permeable membrane allows oxygen from the sample water to diffuse into the sensor, where it is reduced at the cathode. This chemical reaction produces an electrical signal, which travels from the cathode to the anode and then into the dissolved oxygen measuring instrument. Consumption of oxygen at the cathode creates a pressure difference across the membrane that varies based on the partial pressure of oxygen in the sample. Therefore, as oxygen concentration increases, partial pressure and the rate of diffusion also increase, and the current to the instrument increases proportionally.
Optical Dissolved Oxygen Sensors
Optical dissolved oxygen sensors feature replaceable caps containing the sensing element: a special dye that will luminesce (glow red) when exposed to light of a particular wavelength. The dye is covered by an oxygen permeable paint layer, which allows oxygen molecules to interact with the dye, while protecting it from other sample constituents. Oxygen interferes with the dye’s luminescence (intensity and lifetime). The sensor emits light and measures the resultant luminescence with a photodiode. This reading is compared to a reference reading using light of a different wavelength. The measurement and reference values are compared to calculate dissolved oxygen in the sample.
Why is Dissolved Oxygen Measurement Necessary?
For aquaculture, if the DO level falls too low then the fish will suffocate as a consequence. In a sewage treatment plant, bacteria will decompose the solids. If the DO level is too low, then the bacteria will die and the decomposition will cease; if the DO level is too high, then the energy will be wasted through the aeration of the water. In industrial applications, including boilers, the make-up water must have low DO levels in order to avoid corrosion and boiler scale build up. Monitoring dissolved oxygen content is essential for ensuring process efficiency, because boiler scale build up inhibits heat transfer. A high DO level in water improves the taste of drinking water. However, high DO levels will increase corrosion in water plumbing and transport lines.
- Fish farming and aquatic environment health
- Live fish transport
- Wastewater treatment
- Industrial make up waters
- Corrosion control