Dissolved oxygen (DO) refers to the amount of oxygen dissolved in water. It is expressed as the mass of oxygen per liter of water, typically in milligrams per liter (mg/L). Dissolved oxygen exists in water in molecular form.
In water that are not polluted by oxygen consuming substances, mainly organic matter, dissolved oxygen is usually at or near saturation. For example, clear surface water typically has a dissolved oxygen concentration close to its saturation level. When the concentration of organic matter in water is high, the rate of oxygen consumption exceeds the rate of oxygen replenishment. As a result, dissolved oxygen levels gradually decrease and may even approach zero. Under such oxygen deficient conditions, organic matter decomposes anaerobically, leading to putrefaction and fermentation, which severely degrades water quality. Therefore, in water quality monitoring systems, dissolved oxygen sensor is among the most important instruments.
What is a Dissolved Oxygen Sensor?
A dissolved oxygen sensor is an instrument used to measure the concentration of oxygen dissolved in water or other liquids and convert it into an electrical signal.
How Does a Dissolved Oxygen Sensor Work?
At present, dissolved oxygen sensors mainly use two types of membrane electrodes: the optical fluorescence method and the polarographic electrode method.
The working principle of the fluorescence dissolved oxygen sensor is fluorescence quenching. Blue light illuminates a fluorescent material, causing it to become excited and emit red light. Oxygen molecules remove part of this energy through a quenching effect, so the lifetime and intensity of the emitted red light are inversely proportional to the oxygen concentration. By measuring the phase shift between the emitted red light and a reference signal, and comparing it with internal calibration data, the concentration of dissolved oxygen can be calculated.
The fluorescence method overcomes many limitations of traditional measurement techniques, such as complex operating procedures, oxygen consumption during measurement, high maintenance costs, and the inability to support continuous online monitoring. This measurement principle offers strong resistance to interference, high stability, and does not require chemical reagents, avoiding secondary pollution.
Fluorescence light source module
| Light source types | Features |
|---|---|
| Light emitting diode (LED) | Low operating voltage, high reliability, long lifespan |
| Incandescent light source | Wide spectral range, prone to heat generation, low efficiency |
| Semiconductor laser | High optical efficiency, susceptible to temperature changes, good monochromaticity |
| Fiber laser | Good tunability, high electro-optical efficiency, good stability, expensive |
The working principle of a polarographic dissolved oxygen sensor is based on an electrochemical reduction reaction. Dissolved oxygen at the surface of the gold electrode accepts electrons to form oxygen anions, which are then converted into hydroxide ions. The concentration of dissolved oxygen is determined by measuring the resulting change in electrical current during this process.
Polarographic dissolved oxygen sensors have a relatively simple structure and are cost effective while offering a full range of functions. They are widely used for dissolved oxygen measurement in wastewater treatment plants, drinking water facilities, monitoring stations, hydrological monitoring, aquaculture, and various industrial applications.
Comparison Between Optical and Polarographic Methods
Optical methods and polarographic methods represent two fundamentally different approaches to dissolved oxygen measurement. One relies on optical signal quenching, while the other is based on an electrochemical reduction reaction. Their different ways of sensing oxygen determine the measurement behavior, data characteristics, and sources of error.
1. Measurement principle
The polarographic method measures the electrical current generated when dissolved oxygen participates in an electrochemical reaction at the electrode surface. In essence, dissolved oxygen is converted into a measurable electrical signal. As a result, the measurement depends on the diffusion of oxygen molecules to the electrode surface. Dissolved oxygen acts both as the measured parameter and as a reactant in the process.
In contrast, the fluorescence method does not consume oxygen. It indirectly reflects oxygen concentration through the quenching effect of dissolved oxygen on the excited state lifetime or intensity of a fluorescent material. Here, oxygen functions as a quenching agent in an energy transfer process rather than as a reacting substance. This makes the fluorescence method closer to a non intrusive measurement approach.
2. Signal stability
In the polarographic method, the output signal comes directly from the reduction current of oxygen. The signal amplitude is typically high and shows a clear linear relationship with oxygen concentration. However, its stability strongly depends on the condition of the electrode, the composition of the electrolyte, and the integrity of the membrane. Any factor that alters the electrochemical reaction environment will directly affect the output signal.
For the fluorescence method, the measurement signal is derived from the optical system detecting changes in fluorescence decay characteristics. The signal does not depend on electrode chemistry, but rather on the stability of the light source, the optical path, and the condition of the fluorescent sensing layer. It generally exhibits high repeatability and low noise, although slow signal drift may occur over time due to aging of the fluorescent material.
3. Temperature influence
The polarographic method is sensitive to temperature, pressure, and flow conditions because these factors affect both the diffusion rate of oxygen and the kinetics of the electrochemical reaction. Changes in temperature not only alter the physical solubility of dissolved oxygen, but also directly influence the electrode reaction rate. Therefore, temperature compensation is essential for accurate measurement.
The fluorescence method also requires temperature compensation, but for different reasons. Temperature affects the excited state lifetime of the fluorescent molecules and the quenching constant, rather than the mass transfer of oxygen. In addition, the fluorescence method is largely insensitive to flow conditions. This characteristic arises from its fundamental measurement mechanism, not from structural or mechanical optimization.
What Factors Affect Dissolved Oxygen Sensor Measurements?
1. Temperature
The solubility of oxygen in water is strongly related to temperature. As temperature increases, oxygen solubility decreases. Dissolved oxygen sensors measure the partial pressure of oxygen, which must be converted to concentration (mg/L) based on temperature. Almost all dissolved oxygen sensors are equipped with an internal temperature sensor for automatic compensation. However, if the temperature sensor is inaccurate or damaged, the compensated dissolved oxygen concentration will also be incorrect.
2. Salinity and altitude effects
Oxygen solubility is also influenced by water salinity and atmospheric pressure. Higher salinity reduces oxygen solubility, and higher altitude lowers atmospheric pressure, which also reduces solubility. When measuring seawater, brackish water, or freshwater at high altitude, failure to apply salinity or pressure compensation will result in readings that are higher than the true value, since most sensors are calibrated for freshwater under standard atmospheric pressure. Advanced dissolved oxygen sensors provide manual salinity input or automatic barometric pressure compensation. These parameters should be set correctly before measurement.
3. Sample contamination
During sampling and measurement, contact between the water sample and air can introduce additional oxygen. For example, vigorous shaking of a sample bottle allows atmospheric oxygen to dissolve into the water, leading to artificially high readings. It is therefore recommended to use dedicated sampling devices, avoid agitation, and perform in situ measurements whenever possible by placing the probe directly into the water. If sampling is unavoidable, use narrow neck bottles, fill them completely without air bubbles, and measure immediately.
4. Chemical interference
Certain gases dissolved in water, such as hydrogen sulfide (H₂S), sulfur dioxide (SO₂), and chlorine (Cl₂), can penetrate the sensor membrane and participate in side reactions at the electrode. This may cause readings to be higher or lower than the actual value, or even poison the electrode. When measuring dissolved oxygen, it is important to understand the water chemistry in advance and select sensors or measurement methods with stronger resistance to interference. For example, optical methods are not sensitive to H₂S.
How to Calibrate a Dissolved Oxygen Sensor?
Calibration methods for fluorescence based dissolved oxygen sensors mainly include zero point calibration, air saturated calibration, and multi point calibration. The following describes each method in detail.
Zero point calibration
Zero point calibration places the dissolved oxygen sensor in an oxygen free environment. This can be achieved by using deionized water that has been boiled and then cooled in a sealed container, where the dissolved oxygen concentration is close to zero, or by adding an oxygen scavenger such as sodium sulfite to create conditions approaching 0 mg/L. After the sensor reading stabilizes, the output value is set to zero.
Air saturated calibration
In air saturated calibration, the dissolved oxygen sensor is exposed to air so that the fluorescent membrane surface is saturated with water vapor but no liquid droplets form. Under these conditions, the oxygen partial pressure is equal to that of the ambient air. Based on the current atmospheric pressure and temperature, the instrument automatically calculates the theoretical dissolved oxygen value at 100 percent air saturation. The fluorescence response under this condition is then used as the full scale reference point. This method is simple to perform and is particularly suitable for field calibration and routine verification.
Multi point calibration
For applications requiring high accuracy, a multi point calibration method can be used. Several standard solutions with different dissolved oxygen concentrations are prepared, covering the full measurement range of the sensor. Typical concentrations may include 0 mg/L, 2 mg/L, 4 mg/L, 6 mg/L, and 8 mg/L. The sensor is placed sequentially into each standard solution, and once the readings stabilize, the corresponding output values are recorded. These calibration data are then used, either through software or manual calculation, to establish a calibration curve that more accurately relates the sensor output to the actual dissolved oxygen concentration. Multi point calibration improves accuracy across the entire measurement range, but it is more complex and time consuming to perform.
This article was written by the Renke Technical Team, a professional engineering group specializing in the design, research, and manufacturing of environmental monitoring instruments. Renke is a trusted sensor manufacturer with over 15 years of hands-on experience in both hardware and software R&D. The company develops and produces a wide range of water quality sensors, including dissolved oxygen sensors, which are widely deployed in wastewater treatment plants, hydrological monitoring systems, environmental protection projects, and aquaculture operations worldwide. Backed by long-term field applications and continuous technological innovation, the Renke Technical Team provides reliable, experience-driven insights into water quality monitoring and sensor technology.
