What Is a Dissolved Oxygen Sensor?
Dissolved oxygen (DO) is the amount of molecular oxygen held in solution in water, usually expressed in ppm or mg/L. In activated sludge treatment, micro-organisms can only break down the organic load if the oxygen level in the aeration basin stays within a defined band. Too little oxygen reduces treatment efficiency; too much air means the blowers are consuming energy for no benefit.
A dissolved oxygen sensor is the analytical measuring element that provides the process data needed to manage this balance. The sensors covered on this page are amperometric: a measuring cell with electrolyte and a gold cathode behind a gas-permeable membrane generates an electrical signal proportional to the oxygen concentration. The sensor does not produce an output signal on its own — its raw signal is processed by a compatible analytical transmitter, converted into a measured value and passed on to the control system.
Within process instrumentation, the DO sensor is one of the core elements of liquid analysis. Used together with pH, ORP and conductivity measurements, it gives a complete picture of the biological and chemical state of the treatment process. In plants where aeration accounts for a large share of total energy consumption, a dependable dissolved oxygen measurement has a direct influence on both process quality and operating cost.
How Amperometric Dissolved Oxygen Measurement Works
Amperometric dissolved oxygen measurement is based on measuring the current generated when oxygen is electrochemically reduced at an electrode surface. The measuring chain has four basic steps:
1. Oxygen diffusion through the membrane: Dissolved oxygen in the process diffuses through the gas-permeable membrane at the sensor tip into the measuring cell. The membrane keeps the process liquid and contaminants away from the electrode while letting oxygen pass.
2. Electrochemical reduction at the cathode: Inside the cell, oxygen is reduced at the gold cathode immersed in the electrolyte. The current produced by this reaction is proportional to the amount of oxygen crossing the membrane, so the current is a direct measure of the dissolved oxygen concentration.
3. Temperature compensation: Both the gas permeability of the membrane and the solubility of oxygen in water change with temperature. A Pt-100 resistance thermometer integrated into the sensor body measures the process temperature, and this value is used to correct the reading automatically.
4. Signal processing in the transmitter: The sensor current and temperature signal travel over an EMI/RFI shielded cable to a compatible analytical transmitter. The transmitter applies the calibration data and temperature correction, produces the measured value in ppm and passes it to the control system.
Because the measurement continuously consumes oxygen at the membrane surface, sufficient sample flow is needed in front of the membrane. In aeration basins the agitation created by air bubbles usually provides this flow; in flow cells and open channels the minimum flow conditions must be taken into account.
Key Features
· Wide ppm-level range: The sensor covers a 0–20 ppm (mg/L O₂) measuring span — the working band needed for aeration basin control.
· High measuring accuracy: ±0.2 ppm accuracy at 25 °C and repeatability of ±0.5% of reading provide reliable data for stable control loops.
· Fast response: At 25 °C the sensor reaches 90% of the final reading in under 20 seconds in the 0–2 ppm range, so changes in aeration can be followed quickly.
· Integrated temperature compensation: A Pt-100 RTD in the sensor body corrects the effect of temperature changes on the reading automatically.
· Simple air calibration: The sensor can be calibrated in air on site, without preparing additional reference solutions.
· Tool-free maintenance: Membrane and electrolyte changes need no special tools; spare membrane assemblies, O-rings and electrolyte solution are supplied with each sensor.
· Dual-direction process connection: Forward- and rear-facing 1-inch male NPT connections allow the same sensor to be used for immersion or flow-cell installation.
· Flexible cable options: Integral EMI/RFI shielded cables in several lengths, or a plug-in connector that prevents cable twisting and speeds up sensor replacement.
· Comprehensive mounting accessories: A handrail mounting assembly, flow-through tees, a low-flow cell and a jet spray cleaner adapt the sensor to different site conditions.
Technical Specifications
The values below are typical technical data for the amperometric dissolved oxygen sensor described on this page. Final values should be confirmed at project stage for the selected configuration, cable and mounting accessory.
Parameter | Technical data |
Measuring principle | Amperometric, membrane-covered measuring cell (gold cathode) |
Measured variable | Dissolved oxygen (as O₂) |
Measuring range | 0–20 ppm (mg/L) |
Accuracy | ±0.2 ppm (at 25 °C) |
Linearity | 2% (typical) |
Repeatability | ±0.5% of reading (at 25 °C) |
Response time | <20 s to 90% of final reading (0–2 ppm, at 25 °C) |
Process temperature | 0–50 °C |
Process pressure | 101–549 kPa absolute (0–65 psig) |
Temperature compensation | Automatic, integrated Pt-100 RTD |
Process connection | 1-inch male NPT (forward- and rear-facing, two connections) |
Wetted materials | Engineering plastic (modified PPE), FKM, EPDM, PTFE and silicone |
Electrolyte volume / life | Approx. 25 mL / approx. 4–6 months |
Cable | Integral EMI/RFI shielded cable (4.6 m; 7.6 m; 10 m; 15 m; 30 m) or plug-in quick-connect option; maximum cable length 91 m |
Sample flow | Flow-through mounting: 3.8–19 L/min · Open channel: 0.3 m/s · Low-flow cell: 7.6–19 L/h |
Weight | Approx. 0.5 kg (shipping weight 1.5 kg) |
Signal output | The sensor produces a raw signal; the measured value and control-system output are provided by a compatible analytical transmitter |
The temperature and pressure limits of the low-flow cell accessory (70 °C, 722 kPa absolute) are higher than those of the sensor itself; the system limit is always set by the sensor values.
Key Advantages
· Energy-efficient aeration control: Continuous, accurate oxygen data helps match blower capacity to actual demand and avoid over-aeration.
· Stable biological treatment: Keeping the oxygen level within the target band helps maintain treatment efficiency in the activated sludge process.
· Low maintenance effort: Tool-free membrane and electrolyte changes and calibration in air shorten maintenance time.
· Suited to dirty media: A smooth membrane retainer and timer-controlled jet spray cleaning support measurement continuity in fouling-prone applications.
· Flexible installation: With handrail immersion, a flow-through tee or a low-flow cell, the same sensor can serve different measuring points.
· Reliable signal transmission: The EMI/RFI shielded cable design helps preserve signal quality around pumps and blowers where electrical noise is high.
· Traceable calibration: Optional calibration and loop calibration certificates make audits easier in plants where measurement quality must be documented.
Application Areas
The primary application for amperometric dissolved oxygen sensors is the aeration basin of municipal and industrial wastewater plants. The sensors can also be used on sidestream sample lines and in open-channel measurements.
Municipal Wastewater Treatment
In activated sludge aeration basins, dissolved oxygen is the main feedback signal for aeration control. A handrail immersion assembly places the sensor directly in the basin, and the reading can be passed via SCADA to blower and valve control.
Industrial Wastewater Treatment
In the biological treatment units of food, chemical and other industrial plants, the sensor helps keep the oxygen balance under a variable organic load. For wastewater with a high fouling risk, a jet spray cleaner and a smooth membrane retainer can be selected.
Sidestream Sample Lines
Where the sample is drawn from the process line and routed to an analysis point, the sensor is fitted in a flow-through tee or a low-flow cell. This arrangement gives easy access to the sensor and controlled flow conditions.
Open Channels and Basins
Continuous dissolved oxygen monitoring is possible in open channels as long as there is enough flow velocity in front of the membrane; in basins, the agitation created by air bubbles generally provides adequate flow.
How to Select a Dissolved Oxygen Sensor
Choosing the right sensor and accessories determines not only measurement accuracy but also maintenance frequency and operating cost. For a technical assessment we recommend clarifying the following:
Process and Sample Conditions
· Expected dissolved oxygen range (ppm or ppb level?) — ppb-level measurement requires a different sensor type
· Minimum / normal / maximum process temperature
· Process or sample line pressure
· Tendency of the sample to carry suspended solids, oil, biofilm or scale
· Components that require chemical compatibility with the wetted materials
Installation and Flow Conditions
· Installation type: basin immersion, flow-through tee, low-flow cell
· Flow velocity or sample flow in front of the membrane
· Clearance required to remove the sensor (90–150 mm depending on tee type)
· Cable distance between sensor and transmitter
Maintenance and Operation
· Planned membrane and electrolyte change interval
· Need for automatic jet spray cleaning and the cleaning medium (water, air or cleaning solution)
· Preference for a plug-in connector for fast sensor exchange
· Calibration certificate and documentation requirements
System Integration
· Compatibility with the analytical transmitter to be used or already installed
· Signal and communication type required at the PLC / DCS / SCADA (via the transmitter)
· Whether additional parameters such as pH, ORP or conductivity will be monitored at the same point
If the sensor is to be used in an area classified as a potentially explosive atmosphere, hazardous-area suitability should be assessed separately at project stage.
Calibration Certificates and Connection Standards
The documentation options and connection standards available for this sensor are summarized below.
Standard / approval | Scope and description |
Calibration certificate | Optional certificate of calibration (without test data). |
Loop calibration certificate | Optional certificate, with test data, confirming that sensor and transmitter were calibrated together. |
Electronic calibration certificate | Optional certificate, with test data, confirming calibration of the sensor against a factory reference instrument. |
NPT (National Pipe Thread) | The process connection is a 1-inch male NPT thread; the mounting accessories also use NPT threaded connections. |
For projects that require hazardous-area or functional-safety certification, suitability should be assessed case by case together with the TLY Enerji engineering team.
Dissolved Oxygen Measurement Solutions from TLY Enerji
At TLY Enerji we treat dissolved oxygen measurement in wastewater plants not as a stand-alone sensor purchase but as part of aeration control. Our engineers review process and sample conditions, check the sensor's suitability for the measuring point, mounting type and fouling risk, and define the right combination of accessories such as handrail mounting, flow cells or jet spray cleaning.
We support our customers throughout the project: selecting the sensor together with a compatible analytical transmitter, integrating the measurement into PLC, DCS or SCADA systems, preparing technical documentation, commissioning and maintenance planning. Planning membrane and electrolyte intervals around the actual process conditions, defining spare-part needs in advance and briefing site teams are also part of this approach — so the measuring point delivers reliable data from day one, with maintenance that is predictable.
Frequently Asked Questions
What is a dissolved oxygen sensor?
A dissolved oxygen sensor is an analytical instrument that continuously measures the amount of oxygen dissolved in water or wastewater. The result is usually expressed in ppm or mg/L. In wastewater treatment plants it is used to control the oxygen level in the aeration basin, helping biological treatment run efficiently and with balanced energy use.
How does an amperometric dissolved oxygen sensor work?
Oxygen from the process passes through a gas-permeable membrane into a measuring cell filled with electrolyte, where it is electrochemically reduced at a gold cathode. The current produced by this reaction is proportional to the oxygen concentration. An integrated Pt-100 temperature element corrects for temperature, and the transmitter processes the signal to give a value in ppm.
How is a dissolved oxygen sensor calibrated?
The amperometric sensors described on this page can be calibrated in air. Calibration is carried out through the calibration menu of the connected analytical transmitter and does not require reference solutions to be prepared on site. Where documentation is needed, the sensor can be supplied with an optional calibration, loop calibration or electronic calibration certificate.
How often should the membrane and electrolyte be replaced?
Electrolyte life is typically around 4–6 months, but the actual replacement interval depends on process conditions, the degree of fouling and the sensor's maintenance history. Membrane and electrolyte changes can be done without special tools, and spare membrane assemblies, O-rings and electrolyte solution are supplied with the sensor.
Can the sensor measure at ppb level?
No. The sensor described on this page measures in the 0–20 ppm range and is designed for ppm-level applications such as wastewater aeration. Applications that require ppb-level dissolved oxygen measurement, such as boiler feedwater, need a different sensor type. TLY Enerji carries out a separate technical assessment for such requirements.
Does the sensor need sample flow to measure correctly?
Yes. In amperometric measurement, oxygen is continuously consumed at the membrane surface, so the sample in front of the membrane must be renewed. In aeration basins the agitation from air bubbles is usually sufficient. For flow-through mounting a sample flow of 3.8–19 L/min is recommended, about 0.3 m/s in open channels and 7.6–19 L/h in the low-flow cell.
How can the sensor be protected in fouling applications?
Where heavy suspended solids or biofilm growth are expected, a smooth membrane retainer can be used. In addition, a jet spray cleaner operated by a timer and a solenoid valve can periodically clean the membrane surface with water, air or a cleaning solution, helping to extend maintenance intervals.
How is the sensor connected to the control system?
The sensor is connected to a compatible analytical transmitter with an integral EMI/RFI shielded cable or a plug-in quick-connect cable. Standard cable lengths go up to 30 m, and the total cable length can reach 91 m. The transmitter passes the measured value to the PLC, DCS or SCADA system; the signal and communication type depends on the selected transmitter.