What Are the Different Types of Water Dissolved Oxygen Sensors?

Did you know that traditional oxygen sensors, such as electrochemical DO sensors, have a limited lifespan of ~3 years, whereas modern optical DO sensors can last for several years? The massive difference in lifespan is owing to their working mechanism and maintenance requirements. Understanding these differences between sensor types is crucial for selecting the right detector.

Monitoring dissolved oxygen (DO) levels is crucial in maintaining water quality. In applications such as aquatic life, wastewater treatment, aquaculture, and industrial processes, accurately measuring DO is critical. Knowing DO allows precision aeration control, compliance with regulations, and ecosystem monitoring.

There are mainly two types of DO sensors: Electrochemical and Optical. This guide will explore each type, detailing its principle of operation, advantages, limitations, and typical applications. It also includes a short guide on how to select the right sensor for your specific needs. Continue reading to learn all about dissolved oxygen (DO) monitoring!

1. Basics of Dissolved Oxygen Measurement

Before we dive into the engineering and chemistry of DO sensors, let's first understand what DO actually is!

♦What Is Dissolved Oxygen?

Oxygen is present in water, which is why aquatic life can survive underwater. Dissolving a gas, such as oxygen, into water requires a process called diffusion. For a gas like Oxygen to dissolve, as per Henry’s Law, the partial pressure of that gas must be greater in the air than in the water itself.

Since the atmosphere is approximately 21% oxygen, there's always a partial pressure of oxygen exerting force on the water's surface, driving the molecules into the water. We can measure the amount of DO in water in milligrams per liter (mg/L) or parts per million (ppm).

♦Why is Measuring Dissolved Oxygen Important?

A healthy aquatic environment typically has a DO concentration of 5-6 mg/L or more. In cases such as industrial processes, where metal piping is used, high dissolved oxygen levels can cause accelerated corrosion. This is a significant problem in industries like power generation, manufacturing, and oil & gas, where metal pipes and boilers are used to transport water.

To get accurate results, we need to understand challenges to the measurement of DO in water. The values which may be correctly read may seem inaccurate in some cases. While in some cases it may not be entirely correct. The following conditions can challenge the reliability of results:

• Drifting: After calibration, sensors can start to drift from their actual value. In electrochemical sensors, it can be caused by the degradation of the electrolyte and membrane. For optical sensors, it can be due to the slow degradation of the fluorescent dye.
• Fouling: The sensor's ability to detect can be obstructed by biological, chemical, or physical phenomena. Fouling creates a barrier between the water and the sensor, preventing oxygen from reaching the sensing element.
• Temperature: The temperature of water can affect the solubility of oxygen in water and sensor efficiency. That's why modern sensors feature temperature compensation.
• Salinity: Higher salt content causes a lowering of the DO value. Therefore, a sensor should also have compensation for salt content in the water.
• Atmospheric Pressure: The concentration of oxygen in water changes with altitude as atmospheric pressure increases. As we mentioned, particle pressure can fluctuate, resulting in changes in oxygen levels.
• Chemical Interferences: Certain chemicals can interfere with electrochemical or optical DO sensors. Therefore, consider the presence of chemicals that can disrupt readings.

2. Overview of Dissolved Oxygen Sensor Technologies

Sensors can provide live value, typically with a response time of 30 to 60 seconds. It is the time taken by the sensor to read 90% of its final stable reading after a sudden change in DO concentration. The time is the same for the two main types of oxygen-sensing technologies:

• Electrochemical sensors (amperometric, polarographic, galvanic).
• Optical sensors (fluorescence/luminescence).

The choice depends on the application and the degree of accuracy required. Here is a short table with details of both types:

In the next section, let's delve deeper into these sensors, their working mechanisms, and what makes one sensor better than another in specific applications.

3. Electrochemical DO Sensors

3.1. Principle of Operation

The electrochemical process is a chemical reaction that generates electricity as a byproduct. The amount of electrical current is directly proportional to the gas produced in the chemical reaction. It is the basic principle on which all electrochemical sensors work.

To understand electrochemical DO sensors, imagine two electrodes, a cathode and an anode, dipped into an electrolyte solution. The cathode is typically made of platinum or gold, whereas the anode is usually composed of silver or lead. The electrolyte solution generally is potassium chloride (KCl) or potassium hydroxide (KOH), as it does not interfere with the chemical reaction with oxygen. A permeable membrane separates the sample liquid containing oxygen from the electrolyte solution. It allows oxygen to diffuse through the membrane. The rate of this diffusion is directly proportional to the amount of oxygen in the water.

Reduction at the Cathode Causes Gain in Electrons

O₂+2H₂O+4e⁻ →4OH⁻

Oxidation at the Anode Causes the Release of Electrons

4Ag+4Cl⁻ →4AgCl+4e⁻

The current flow is measured using an ammeter, which is directly representative of the oxygen in water.

3.2. Polarographic DO Sensors

As the name suggests, it requires an external voltage to start the polarization process. A voltage in the range of 0.8V causes the oxygen reduction action at the cathode. It provides accurate results but is typically recommended for use with a large sample size. As it uses external power, there is no anode consumption when no power is supplied.

• Warmup Time: It takes 10-15 minutes to start providing stable value due to the polarization.
• Maintenance: Requires frequent calibration

3.3. Galvanic DO Sensors

Galvanic-type electrochemical DO sensors are self-powered. This means that the two different electrodes, the cathode and the anode, produce a sufficient voltage difference to drive a reaction.

• Warmup Time: No external power and warm-up. Provides instantaneous readings.
• Maintenance: The anode is consumed continuously, resulting in a lower lifespan and requiring it to be kept in a "zero DO" environment.

4. Optical DO Sensors

4.1. Principle of Optical / Fluorescent DO Sensors

The most efficient and widely used DO sensors are based on an Optical / Fluorescent working mechanism. These sensors utilize the fluorescence quenching phenomenon to measure DO.

• The sensor consists of luminiscenet dye and a sensor body that contains an LED and a photodetector. The LED shines blue light onto the dye, causing it to reach an excited state. When excited electrons reach their stable state, they emit light.
• In the case when the oxygen absorbs into the luminescent dye. The blue light excites the electron, but oxygen absorbs some of the energy from the electrons, causing a reduction in the emission of light when they return to their stable state.

Measuring the change in light through the output of the photodetector provides a direct value of the oxygen present in the medium in contact with the luminiscence dye.

Their installation process is also very convenient and easy, as shown here:

4.2. Advantages of Optical DO Sensor Over Electrochemical Types

Optical DO sensors offer numerous advantages over electrochemical type sensors. They are stable, accurate in stagnant/low DO water, low-maintenance, require minimal calibration, and have a long lifespan. Here are the top reasons why optical DO sensors are given preference over electrochemical sensor types:

• No oxygen consumption → works in stagnant water.
• No stirring required → accurate in still samples.
• Minimal calibration & low drift → holds calibration for months.
• Low maintenance → only cap replacement every 2 years.
• No warm-up time → instant operation.
• Better long-term stability → resistant to H₂S and biofouling.
• Higher accuracy in low DO → reliable in hypoxic environments.
• Longer lifespan → sensor body lasts 5–7 years.

4.4. Applications of Optical DO Sensors

Optical DO sensors are much more robust, offering a wide range of applications. They are convenient to maintain and easy to set up, making them the ideal choice for:

• Aquaculture
• Wastewater
• Research Labs
• Environmental Monitoring
• Drinking Water Treatment

 

5. Choosing the Right DO Sensor

Selecting the right DO sensor requires careful evaluation of the application that you have. Consider this short step-by-step guide to finding the ideal DO sensor:

Step 1: Consider Applications

For applications that require continuous, long-term monitoring, optical sensors are often preferred due to their stability and low maintenance requirements. For quick, on-the-go checks, galvanic electrochemical sensors are ideal as they provide instant readings without a warm-up time of up to 15 minutes..

Step 2: How Accurate You Need Your Readings

While both types can be highly accurate, optical sensors provide superior long-term stability and are less prone to signal drift. For applications requiring a swift response to rapid changes in DO, some electrochemical sensors offer a slightly quicker reaction time, reacting in as little as 10-30 seconds. Optical sensors typically respond in 30-60 seconds. 

Step 3: Can You Tolerate Regular Maintenance

If your application can handle frequent upkeep, the lower initial cost of an electrochemical sensor may be a good trade-off, as these require regular cleaning and electrolyte replacement. For a hands-off, low-maintenance approach, an optical sensor is the better choice, as they only need a sensor cap change every 1-2 years.

Step 4: Consider Your Budget

An electrochemical sensor has a lower initial purchase price. However, when you factor in the long-term cost of replacement parts and maintenance time, an optical sensor can have a lower total cost of ownership over its lifespan, which can be several years.

Step 5: Evaluate the Environment

For environments with chemical interferences like chlorine or hydrogen sulfide, an optical sensor is generally more robust, as these chemicals do not affect its measurement principle. Ensure the sensor has temperature and pressure compensation for accurate readings in varying environments, as a change in atmospheric pressure can alter readings by as much as 0.5 mg/L.

Conclusion

The ability to measure DO accurately, consistently, and quickly without chemical interferences depends on the type of DO sensor picked. As an engineer or scientist, you need to consider the pros and cons of each type to make an educated decision. Electrochemical-type DO sensors are great as they are simpler and cheaper, but they require frequent maintenance and have higher drift. On the other hand, Optical sensors are low-maintenance, highly stable, and long-lasting, making them ideal for long-term monitoring.

The robustness of optical sensors makes them the most widely used type of DO sensor. Manufacturers like Rika provide highly accurate sensors, such as their RK500-04 model. It offers:

• High level of accuracy, with a resolution of 0.01 mg/L and an accuracy of ±0.5% FS.
• Quick response time of less than 60 seconds.
• Integrated design with no external transmitter.
• Probe is made from 316L stainless steel or titanium alloy for use in corrosive environments like saltwater.
• An IP68 ingress protection rating for durability.
• Automatic temperature compensation over a wide range and an RS-485 output.

If you are interested in finding a wide range of DO sensors with a robust design, then consider visiting the RIKA website to explore all the options.

Frequently Asked Questions (FAQ)

Q1: What industries use dissolved oxygen sensors?

Industries, whether they require oxygen or not, can utilize dissolved oxygen sensors to provide live values of DO. Industries such as culture, fisheries, wastewater treatment, drinking water treatment plants, and various sectors (including beverage and pharmaceuticals) all utilize DO sensors for real-time, live water quality monitoring.

Q2: What is the effect of temperature on dissolved oxygen measurements?

Temperature can hinder the accuracy of DO sensors—the solubility of oxygen decreases as the temperature increases. Therefore, sensors will need to be recalibrated for new temperature conditions. Modern DO sensors come with temperature compensation, which makes them capable of operating in any temperature condition. However, working in extreme conditions (hot effluents, cold lakes) requires careful calibration.