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 is Dissolved Oxygen Measured?
Galvanic DO sensors consists 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. While interacting with the probe internals to produce an electrical current (more in detail below the DO sensor graphic), oxygen diffuses across the membrane. Higher pressures allow for more oxygen to be diffused across the membrane and for more currents 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
Due to temperature changes occuring, the thermistor corrects the membrane from permeability errors. Although the oxygen pressure has not changed, increasing the permeability at a higher temperature will allow for more oxygen to be diffused into the sensor. If the thermistor were not used, this would result in giving a false reading of a high DO.
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. With industrial applications, including boilers, the make-up water must have low DO levels in order to prevent corrosion and boiler scale build-up which 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