Dissolved Oxygen DO Measurement: A Guide from Sensor Selection to Calibration and Care

Dissolved Oxygen (DO) is a key parameter that reflects the state of aquatic ecosystems or the conditions in water treatment. In this article, you will learn about the different sensors available for determining DO and how they are used.

Dissolved oxygen (DO) plays a vital role in maintaining healthy aquatic ecosystems and ensuring effective water treatment processes. It refers to the amount of oxygen gas dissolved in water, crucial for the survival of fish, invertebrates, and aerobic microorganisms. DO levels are also key indicators in environmental monitoring and industrial applications, helping detect pollution and verify optimal conditions for biological and chemical processes.

Understanding Dissolved Oxygen Measurement

Measuring the DO concentration in a sample is done by submerging the sensor in it. In the classical measurement principle, oxygen can enter and leave the sensor through a selectively permeable membrane, which leads to an equilibrium of oxygen activity (concentration) between the sample and the sensor.

The sensor measures the partial pressure of oxygen inside the sensor. A consequence of this principle is that it does not measure the concentration of DO directly, but rather its partial pressure. Partial pressure is the pressure exerted by oxygen molecules in the gas phase equilibrated with the DO and can be expressed in percentage of air saturation.

For example, a stable solution in air is 100 per cent air saturated. If the oxygen saturation was above 100 per cent for clarity, the excess would leave the solution and enter the air. Were it below 100 per cent, the solution would slowly absorb oxygen from the air. Similarly, if the partial pressure in the solution is higher than in the sensor, oxygen will enter the sensor, and vice versa.

At equilibrium, the partial pressure inside the sensor is equal to that in the solution. However, this does not mean that the concentrations are equal. The concentration corresponding to 100 per cent saturation depends on the oxygen solubility in the solution. For water, the most common solvent, this solubility is well known, and the saturation can be converted into concentration (mg/L).

Types of Dissolved Oxygen Sensors

Galvanic DO Sensors: Galvanic DO sensors are electrochemical devices that measure DO levels by using two electrodes made of different metals immersed in an alkaline aqueous electrolyte. The sensor is sealed with a membrane that selectively allows oxygen to diffuse in. Due to the difference in electrode potentials, the sensor self-polarizes, generating a stable voltage without requiring external power or warm-up. When oxygen enters the sensor, it causes a chemical reaction where the anode is oxidized and consumed, while oxygen is reduced at the cathode. This electron flow produces a current proportional to the oxygen concentration in the sample. Galvanic DO sensors are designed for reliable oxygen measurement but require stirring of the sample to maintain accuracy and may need membrane replacement over time.

Polarographic Dissolved Oxygen Sensors: Polarographic DO sensors, sometimes called Clark-type sensors, consist of an anode made of silver and a cathode made of a noble metal such as gold or platinum, both immersed in an electrolyte solution. These electrodes are separated from the sample by a membrane that selectively allows oxygen to pass through.

The sensor requires polarization before measurement, during which a constant voltage is applied to create a positive charge at the anode and a negative charge at the cathode. When oxygen diffuses through the membrane, it is reduced at the cathode, generating an electrical signal proportional to the oxygen partial pressure in the sample.

The silver anode is oxidized and slowly consumed in the process, which results in the formation of a silver chloride coating on the anode surface that can degrade sensor performance over time. Proper sample stirring is necessary to maintain even oxygen distribution for accurate readings, and sensor maintenance can minimize coating buildup.

The similarities and differences between a galvanic and a polarographic DO sensor are summarized in Table 1.

Optical Dissolved Oxygen Sensors: Optical DO sensors use a special fluorescent dye embedded in a membrane at the sensor tip. This dye is excited by blue light emitted internally by the sensor. As the dye returns to its ground state, it emits red, fluorescent light, which is detected by an internal photo detector.

Oxygen molecules near the membrane surface quench this fluorescence by absorbing the dye’s excess energy, reducing the amount of emitted light detected. The sensor also includes a red-light source that reflects off the dye and serves as a reference to correct factors unrelated to oxygen quenching, such as dye degradation or temperature effects. Optical DO sensors offer advantages including no oxygen consumption, no need for stirring, immunity to interference from redox-active gases, no electrode wear or precipitate formation, and improved accuracy at low oxygen levels. However, they tend to be more costly and require more power during operation.

To learn more about the DO sensors, explore Dissolved Oxygen Electrode: Optical, Polarographic & Galvanic Determinations.

Choosing the Right DO Meter and Handling of DO Sensors

There are two types of DO Meters:

• Benchtop Meters: Ideal for laboratory use, benchtop meters provide high precision and typically employ galvanic or polarographic sensors.

• Portable Meters: Designed for field measurements, these meters prioritize convenience and ease of maintenance, often using optical sensors.

Optical DO sensors do not require any preparation prior to their usage. Electrochemical sensors, on the other hand, must be checked for membrane integrity. Additionally, it must be guaranteed that the electrolyte is properly replenished if electrolyte refilling is applicable. When using a polarographic sensor, the proper polarization of the sensor has to be ensured.

A measurement in water-saturated air should be used as the first point (this corresponds to 100% oxygen saturation). If the sensor is calibrated with just one point, the meter can only adjust the slope of the calibration curve by assuming a lack of signal at 0%.

For determining the offset, a second calibration point is required. Because the offset of most DO sensors does not deviate much from zero, a 1-point calibration is sufficient for many applications. For a second calibration point, an oxygen-free standard solution should be prepared (this corresponds to 0% oxygen saturation). For this purpose, Zero Oxygen tablets are dissolved in water in order to eliminate all the dissolved oxygen in it. With this second point, the offset can be determined. It is recommended to perform a 2-point calibration when measuring samples with an oxygen saturation below 10% or an oxygen concentration below 1 mg/L.

Tips for maintaining and storing DO sensors:

• Gently wipe the sensor dry using a soft tissue.

• Take extra care to avoid microbiological growth, especially when measuring biological samples.

• Store sensors in a safe environment with temperatures between 5°C and 45°C.

• Avoid rapid temperature changes to maintain sensor performance.

• For short-term storage of galvanic DO sensors: Rinse with deionized water, store in an appropriate storage solution.

• For long-term storage of galvanic DO sensors: Short-circuit the sensor to prevent deterioration from continuous self-polarization, keep in a cool place.

• Storage of polarographic DO sensors: To avoid long polarization times, keep connected to the instrument during short-term storage. For long-term storage, detach from the instrument to prevent continuous polarization and extend sensor life.

When storing electrochemical sensors for several months:

• Ensure the sensor is filled with inner electrolyte.

• Place a protective cap securely over the membrane.

• If stored for more than 3 months, replace the electrolyte before use.

• For storage longer than 6 months, remove the electrolyte completely.

• Avoid exposure of electrochemical sensors to gases like H₂S or SO₂ as they can tarnish electrodes.

Polarographic sensor cathodes develop AgCl coatings over time; this can be cleaned off mechanically. After cleaning, refill the sensor with fresh electrolyte to restore full functionality. The frequency of electrolyte replacement depends on exposure to oxidizing gases and sensor usage.

Optical DO sensors should be stored dry. Replace membranes in sensors with exchangeable membrane modules as soon as reduced performance is observed.

For further insights into DO sensors, including detailed specifications and operational guidance, consult specialized resources such as the Dissolved Oxygen Guide and Why Dissolved Oxygen Matters article.

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