What is Dissolved Oxygen (DO)?
Dissolved Oxygen (DO) is the term used for the measurement of the amount of oxygen dissolved in a unit volume of water. Oxygen gets into water by diffusion from the surrounding air, by aeration (turbulent movement), and as a waste product of photosynthesis from plants.
How is Dissolved Oxygen Measured?
Galvanic DO sensors consist of two electrodes: an anode and cathode which are both immersed in electrolyte (inside the sensor body).
An oxygen permeable membrane separates the anode and cathode from the water being measured. Oxygen diffuses across the membrane. It interacts with the probe internals to produce an electrical current (more detail is shown below the DO sensor graphic). Higher pressure allows more oxygen to diffuse across the membrane and more current to be produced. The actual output from the sensor is in millivolts. This is achieved by passing the current across a thermistor (a resistor that changes output with temperature).
V = i * R,
V is output in Volts, i = current
R is resistance from thermistor in ohms
The thermistor corrects for membrane permeability errors due to temperature change. In other words, increasing permeability at higher temperature allows more oxygen to diffuse into the sensor, even though the oxygen pressure has not changed. This would give falsely high DO if the thermistor were not used.
Why is Dissolved Oxygen Measurement Necessary?
For aquaculture if the DO level falls too low the fish will suffocate. In sewage treatment, bacteria decompose the solids. If the DO level is too low, the bacteria will die and decomposition ceases; if the DO level is too high, energy is wasted in the aeration of the water. With industrial applications including boilers, the make-up water must have low DO levels to prevent corrosion and boiler scale build-up which inhibits heat transfer. A high DO level in water makes drinking water taste better, 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