Dissolved Oxygen (DO) Measurement: Process DO Analyzers
Dissolved oxygen (DO) measurement in water: ppb-level low-range DO analyzers, digital sensors and SWAS integration; boiler feedwater and treatment applications.
Measurement methods: Electrochemical Sensor
What is dissolved oxygen and why is it measured?
Dissolved oxygen (DO) is molecular oxygen present in dissolved form in water. In power plants and facilities using steam boilers, even very low levels of oxygen in feedwater cause corrosion on pipe and equipment surfaces; it is therefore monitored at ppb level and used to check the effectiveness of deaeration and oxygen scavenger chemicals. In wastewater treatment, oxygen control in aeration basins is fundamental to biological treatment efficiency. Oxygen control in process water may also be required in the food, beverage and pharmaceutical sectors.
Where is DO measured?
- Power plants: At the deaerator outlet, in feedwater and in condensate.
- Wastewater treatment plants: In aeration tanks.
- Water preparation and process lines: In quality control after oxygen removal.
Methods for measuring dissolved oxygen
Process DO measurement is generally performed with a sensor placed in a sample flow cell. In electrochemical (amperometric) sensors, oxygen passes through a permeable membrane and is reduced at the electrode; the resulting current is proportional to the amount of oxygen. In digital sensor systems, the sensor processes the measurement itself and transmits it digitally to the transmitter, which simplifies installation and sensor replacement. The measured value is read on a local display and transferred to the control system through a current output or Modbus.
Choosing a dissolved oxygen analyzer
- Low-level measurement at ppb or high-range measurement
- Sample temperature, flow and airtight line design (critical at low level)
- Sensor maintenance kit and membrane replacement interval
- Use within a SWAS or in a standalone panel
DO measurement with Tetra
Tetra supplies Thermo Orion process analyzers in Türkiye and provides installation, supply of reagents and consumables, commissioning, calibration and service.
Frequently asked questions
What unit is dissolved oxygen measured in?
In wastewater and natural waters, dissolved oxygen is given in mg/L (ppm) or percent saturation. Low-level applications such as boiler feedwater and condensate use µg/L (ppb). The choice of measurement range directly determines the sensor type and sample line design.
How is a dissolved oxygen sensor calibrated?
DO sensors are generally calibrated using water-saturated air or air-saturated water, taking temperature and atmospheric pressure into account. In low-level measurements, the accuracy of the zero point also matters and is checked according to the manufacturer's procedure. Calibration frequency is determined by sensor type and observed drift.
Why can high values be read in ppb-level DO measurement?
Leaking connections in the sample line, valves that admit air or permeable hoses can let air into the sample, showing falsely high values. Low-level measurement therefore uses stainless steel lines, leak-tight connections and a constant sample flow. On newly commissioned lines, it can take time for the value to stabilize.
What is the difference between amperometric and optical DO sensors?
In amperometric sensors, oxygen passes through a membrane and is reduced at the electrode, and the resulting current is measured; the electrolyte and membrane are renewed periodically. In optical (luminescence) sensors, oxygen quenches the signal of a luminescent dye; they require no electrolyte and are less sensitive to flow velocity. Measurement range, maintenance capabilities and sample conditions are assessed together when selecting.
Products measuring this parameter
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