What is a Dissolved Oxygen Sensor? And How to Use It?

In the ecosystem, oxygen plays a vital role in human survival, and for aquatic life, oxygen in the dissolved form is necessary for their environment. Dissolved oxygen (DO) is significant in industries; DO is required in wastewater treatment plants during the aerobic digestion process. Also, DO level is continuously monitored in steam power plants to avoid corrosion inside the equipment. Therefore, it is crucial to monitor this vital parameter; for that, dissolved oxygen sensors are widely used equipment for measurement.

Dissolved oxygen (DO) sensors have wide industrial applications and usage; for example, environmental monitoring, wastewater treatment plants, pharmaceutical and process industries, aquaculture, laboratory use, and many more. A DO sensor measures oxygen in water; there are two means: optical measurement, which uses luminescence, and the electrochemical method, which relies on chemical reactions at electrodes. In order to effectively use these sensors, handling must be done carefully; further calibration and maintenance are required to perform as per the proper procedure and recommendations.   

1. Understanding Dissolved Oxygen (DO)

1.1. What is Dissolved Oxygen?

Dissolved oxygen is the quantity of oxygen dissolved in one unit of water. Oxygen enters water in three different ways:

• Photosynthesis through plants underwater. Plants naturally release oxygen as a byproduct and absorb carbon dioxide in sunlight.
• Aeration: Aeration is the process by which water's oxygen content increases through turbulent movement.
• Diffusion: It is the movement of particles from a high concentration to a low concentration. In the atmosphere, oxygen content is 21%; when the oxygen concentration in water is lower than this value, oxygen from the atmosphere diffuses into the water surface.

1.2. Importance of Monitoring Dissolved Oxygen

Emergency! Emergency! When humans are collapsing due to a lack of saturation. The same goes for aquatic life due to inappropriate levels of dissolved oxygen. A sufficient level of DO is required for growth and sustainability. Low DO means increasing pollution and algae growth, while high levels cause gas bubble diseases in fish that affect biodiversity. Industrial processes like wastewater treatment can't afford to overlook this parameter; DO allows the development of aerobic bacteria that help to decompose organic waste. In addition, fish farms preserve DO to provide a healthy environment for growth and productivity. For these reasons, monitoring DO is essential using a dissolved oxygen sensor.

1.3. Units of Measurement for Dissolved Oxygen

Why Units? Ever thought of it? Imagine how data will be communicated, compared, and analyzed without it. Defined units provide a standard way to compute and decide based on these results. Units like milligrams per liter (mg/L), parts per million (ppm), and percentage saturation (% saturation) are regularly used to measure the level of dissolved oxygen in water. The number of milligrams of oxygen in a liter of water is mg/L. 1 mg/L is referred to as one ppm at standard temperature and pressure. These units measured the absolute amount of oxygen, whereas % saturation is relative to the maximum amount of oxygen in water at a specific temperature and pressure.

Solubility of oxygen in water works on Henry's law of partial pressure, which states that the amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid. For example, if the partial pressure of oxygen is low in the atmosphere, then its solubility in water decreases, decreasing % saturation. DO is maintained and monitored according to its environment; in lakes and rivers, it ranges from 4 to 10 mg/L, in dead zones it is less than 2 mg/L, and in aeration tanks in waste treatment plants it is between 1 and 3 mg/L.         

2. What is a Dissolved Oxygen Sensor?

2.1. Definition and Purpose of a DO Sensor

Sensors are equipment used for detection purposes. A dissolved oxygen sensor measures one of the key indicators of water quality: the oxygen concentration in water. The primary function of a water dissolved oxygen sensor is to monitor and detect the accurate amount of oxygen in water to verify that the environment is safe and processes are working correctly.

In wastewater treatment plants, real-time monitoring of DO is done using advanced IoT sensors, which provide information and record data for analysis and appropriate decision-making. Modern DO sensors are also used for environmental monitoring to maintain sustainable ecosystems.

2.2. Different Types of Dissolved Oxygen Sensors

What are the options available? Before making any selection, it's important to know what choices are available and if this will be the best choice according to the environment. The dissolved oxygen sensors have two main types: electrochemical and optical sensors.

There are two subtypes of electrochemical sensors: galvanic and polarographic. The working principle is the same: when the sensor is introduced into water, a redox reaction (reduction-oxidation) occurs between oxygen and the electrode inside it, generating a measurable current directly proportional to the dissolved oxygen concentration in the sample. The significant difference between the two sensors is that galvanic doesn't require any external current to start the reaction, while polarographic does require an external current. On the other hand, an optical sensor uses light to detect DO; high luminescence means high concentration. If the glow quenches, it means a high amount of oxygen.

The galvanic sensor has a quick response and requires no external power, but it needs frequent maintenance and can give improper results at low flow. The polarographic sensor is best suited for laboratory usage and has high accuracy. Maintenance of the membrane and warm-up are some cons of this equipment. Optical sensors are expensive but have low maintenance and long-term stability.

2.3. Key Components of a Typical DO Sensor

The components of a dissolved oxygen sensor depend on its type: an optical or electrochemical sensor. However, the majority of sensors have the following key components:

• Housing: The whole equipment needs to be covered with protected housing, usually of stainless steel, to avoid internal damage and harsh environmental conditions.
• Sensing Element: It is the fundamental part that interacts with dissolved oxygen.
• Membrane: Used to protect against the entry of contaminants and only allows oxygen to pass through it. These membranes are present only in electrochemical sensors.
• Electrolyte (for electrochemical): A solution that acts as a conductive medium between anode and cathode.
• Optical Window: Is a part of the optical sensor that transmits and provides shielding to the luminescent sensor layer.
• Temperature Sensor: Oxygen solubility depends on temperature; therefore, a temperature sensor is placed to detect the water temperature.

3. Dissolved Oxygen Sensor Working Principle (Electrochemical Types)

3.1. How Electrochemical DO Sensors Work (General Overview)

A redox reaction occurred when water with dissolved oxygen came in contact with the sensor. At the anode, an oxidation reaction occurs that releases electrons, and at the cathode, reduction (gaining of electrons) takes place to form water. During the flow of electrons towards the cathode from the anode, a measurable electric current is produced, directly proportional to DO concentration, which is then processed to display readings in mg/L or ppm.

3.2. Polarographic DO Sensors: A Closer Look

Polarographic dissolved oxygen sensors provide precise and accurate readings; therefore, they are widely used in laboratories and industrial applications. A constant voltage of 0.6~0.8 volts is applied to start the redox reaction.

Research is ongoing to replace noble metals with nanomaterials in dissolved oxygen probe as cathodes and anodes. Still, gold and platinum metals are irreplaceable choices as cathodes, while silver is used as an anode. The membrane in the sensor selectively allows diffusion of oxygen molecules at the cathode without contaminating the electrolytic solution. The oxygen that enters the sensor generates an electric current, reflecting the amount of dissolved oxygen in water. The following are the reactions that take place at the anode and cathode during this process:

At the anode, when silver is oxidized, it releases an electron. The reaction is given by      

Ag→ Ag + (e−)

At the cathode, oxygen is electrochemically reduced to water, which is given by

O2 ​+ 4 (H+) + 4(e−) → 2H2​O

3.3. Galvanic DO Sensors: A Self-Powered Approach

Does it produce its voltage? How? Voltage is generated through a spontaneous reaction; due to dissimilar metals, a noble cathode and reactive anode in an electrolytic solution, a natural potential difference is created in the galvanic dissolved oxygen sensors. The flow of electrons started when oxygen passed through the sensor membrane and reached the cathode, where it underwent reduction, while the anode oxidized. The anode is made of either lead (Pb) or zinc (Zn), which can be easily oxidized, while the cathode, which has good conductivity, is made of gold (Au) or silver (Ag). The self-powered feature is an advantage that allows these sensors to be used remotely, and the design is simplified compared to polarographic sensors.

3.4. The Role of the Electrolyte

In a dissolved oxygen sensor, the electrolyte solution is a medium between the anode and cathode that facilitates redox reactions. It ensures the reaction is continuous and sustained to give an accurate concentration of dissolved oxygen. Without this solution, the flow of electrons will be stopped as the ion exchange process is blocked.

4. Dissolved Oxygen Sensor Working Principle (Optical Type)

4.1. How Optical DO Sensors Work (General Overview)

When the word quench is referred to, it means to cool down, dampen, reduce, or stop. In optical dissolved oxygen sensors, the fluorescence is quenched when dissolved oxygen is present in the sample. This DO sensor has a fluorescent dye and a sensing layer; the dye gets excited by a specific light wavelength from a built-in light source. After excitation, the dye emits light at different wavelengths, and in the presence of dissolved oxygen, the intensity of this fluorescence is reduced.

4.2. The Fluorescent Sensing Element

When introduced to an external light source (blue or violet), the light is emitted by a fluorescent sensing element, which is the heart of the optical dissolved gas sensor. This element consists of luminescent dye, which, when excited with external light, enters the excited state and then returns to the ground state by releasing light of a longer wavelength (red or green). Oxygen, having a quenching property, reduces energy before light emission when interacting with the excited dye. As the quantity of oxygen rises, the fluorescence quenching will increase, and the intensity of light emitted will be lower.

4.3. The Optical Measurement System

The fluorescence is measured by an optical measurement system consisting of a light source, a fluorescent dye, and a photodetector. The changes in the level of fluorescence tell about the levels of dissolved oxygen. The light-emitting diode (LED) emits light with a particular wavelength, mostly blue or violet, which is directed towards the fluorescent dye. The dye gets excited and emits fluorescence, which a photodetector detects. A photodetector measures and analyzes the emitted light in two ways: by measuring the intensity (high intensity means low quantity of dissolved oxygen) and fluorescence lifetime (measuring the delay between excitation and emission; short delay means less oxygen). Based on the optical measurement system, the dissolved oxygen sensor has low maintenance costs, as no electrolyte or chemical is present for physical deterioration; it provides accurate and reliable readings, and the process is physical, so no oxygen is consumed. This means it is independent of flow and can be used in stagnant water.

5. How to Measure Dissolved Oxygen Using a Sensor?

5.1. Calibration of the DO Sensor

Humans monitor health using their health records and checkups, just like instruments have calibration cards that safeguard accuracy. Calibration is the alignment of the sensor with the reference value. Calibration ensures that the dissolved oxygen sensor provides realistic values. There are two standard methods for the calibration of DO sensors.

• Air Calibration: This method is used for both electrochemical and optical sensors. The sensor came into contact with saturated air, which has a fixed/maximum amount of oxygen at a specific temperature and pressure.
• Zero Oxygen Calibration: This method is the opposite of air calibration, where the sensor is exposed to a solution (water with sodium sulfate) with no dissolved oxygen traces. Hence, calibration is done for zero oxygen.

Calibration must be done by the experts following the manufacturer’s instructions provided in the manuals to avoid any damage to the sensor or inaccuracy in results.

5.2. Sensor Deployment and Immersion

Sensors must be carefully installed in industries to get accurate results, and in laboratories, they should be adequately immersed in the sample to perform their intended function. Avoid sensors interacting with air bubbles or debris. Take the sample or place the sensor where it represents actual conditions. For electrochemical dissolved oxygen sensors, proper water flow is essential for oxygen consumption to give accurate readings.    

5.3. Reading and Interpreting the Output

The final step is to read, analyze, and interpret the data about the quality of water provided by the dissolved oxygen sensor. The output obtained is in milligrams per liter (mg/L), which shows how much oxygen is dissolved in one liter, or parts per million (ppm), which shows the oxygen level compared to its standard value at a specific temperature and pressure. The reading of DO is drastically affected by temperature and salinity; the higher the temperature, the less dissolved oxygen is present, and the higher the salinity, the lower the oxygen solubility. Modern sensors with built-in temperature sensors must be used to avoid this issue.

6. Factors Affecting Dissolved Oxygen Measurements

The dissolved oxygen sensor is not giving accurate results; contact the vendor for calibration or replacement. Wait, before taking any action, you have to check the following factors first:

• Temperature: Yes, temperature affects the results. It must be checked whether the sensor is compensated for temperature to give accurate results.
• Salinity: When sampling brackish water, correction should be done during calibration or while analyzing data for salinity. It reduces the solubility of oxygen in water.
• Pressure: Estimates of % saturation of dissolved oxygen can be affected by increases and decreases in pressure.
• Fouling of the membrane/optical window: Timely maintenance is required to avoid fouling in the membrane or optical window, which can cause reading fluctuations and drifting.
• Flow rate (for electrochemical): Reliable results are obtained when a minimum water flow across the membrane occurs, as oxygen is consumed during the redox reaction in the electrochemical DO sensor.

7. Practical Example with RK500-04 Dissolved Oxygen Sensor

The Rika RK500-04 uses the fluorescence principle to measure oxygen levels in water. The type of sensor allows it to provide high accuracy, low maintenance, and stable performance over a long period. The inline sensor can reach 90% of its final reading in less than 100 seconds (T90<100s).

Rika RK500-04 is a practical instrument with applications spanning aquaculture, chemical processing, environmental monitoring, and biodegradation. Its temperature compensation with high-pressure resistance of 0.3MPa makes it suitable for varying temperatures and high-pressure environments. Moreover, the IP68 rating makes it durable and waterproof for challenging work environments. Here are some of the key features of the sensor that makes it an ideal choice:

Conclusion

Checking the water quality? One of its key parameters is dissolved oxygen. Dissolved oxygen is measured using sensors, either electrochemical (polarographic or galvanic) or optical. These sensors are widely used in wastewater treatment plants, aquaculture industries, environmental monitoring, and laboratories.

Unlike any other sensor, dissolved oxygen sensors require timely calibration and maintenance (per vendor instructions) to perform their intended function for an extended period. Understanding the sensor’s working process and factors affecting these values, like temperature, flow rate, salinity, and fouling, is vital to gaining valuable insights from the obtained data.

In conclusion, a sensor's reliability depends on the person using it. With knowledge and awareness, the DO sensor can help industries and environmental agencies make decisions to ensure sustainable environments and processes.  

Frequently Asked Questions (FAQs)

• Can a dissolved oxygen sensor be used in both freshwater and saltwater environments?

Yes, the DO sensor is feasible for both environments. However, the salinity factor, especially for saltwater environments, needs to be accounted for, as it affects the solubility of oxygen, resulting in improper results.

• What is the lifespan of a dissolved oxygen sensor?

Optical DO sensors have a greater lifespan of around 2 to 5 years than electrochemical DO sensors, which last 1 to 2 years. Lifespan greatly depends on the usage, the environment in which they are operating, and maintenance. In harsh conditions, frequent maintenance and regular calibration can increase equipment life.

• How often should I replace the membrane of a dissolved oxygen sensor?

The membranes are the component in electrochemical dissolved oxygen sensors that need frequent replacement. Depending upon the conditions, they need replacement after 3 to 6 months, when you observe slow response time, drifting, and dirt on the member.

Tip: When the calibration is performed, it is recommended that you check the healthiness of the membrane; if it is found degraded, perform both tasks in one go.

• Can dissolved oxygen sensors be used in wastewater treatment applications?

These sensors are widely used in wastewater treatment plants in the aeration process. To support the growth of aerobic bacteria, dissolved oxygen needs to be within limits; therefore, DO sensors are placed in aerobic treatment tanks to monitor their level. Both types of dissolved oxygen sensors can be used for wastewater treatment applications.