10/01/2019 | Aquaculture | 10 MINUTE READ

Dissolved Oxygen Sensors: A Critical Measurement for Aquatic Environment Health

Dissolved Oxygen in Industrial Plants

Aquatic environment health is an essential component of making sure that all forms of life within the water remain healthy. Fish, bacteria, plants, and invertebrates all require a healthy aquatic environment in order to live and thrive. The best way to protect and take care of the aquatic environment is to monitor the dissolved oxygen in the water supply.

Dissolved oxygen refers to the amount of oxygen that has dissolved into the water, which can occur through aeration or by being diffused within the surrounding air. It’s also a waste product that’s produced from plants via photosynthesis. In order for fish and other life to remain healthy, it’s essential that they get the right amount of dissolved oxygen. If the health of the aquatic environment becomes poor, they may not receive the amount of dissolved oxygen that they need to survive, which can result in suffocation.

Within a sewage treatment plant or other industrial applications, the amount of dissolved oxygen in the water must be maintained at ideal levels. If ever the dissolved oxygen becomes too low or too high, the aquatic environment will start to deteriorate. If you want to make sure that the dissolved oxygen in your water is at the right level, you should take the necessary steps to understand and select the appropriate dissolved oxygen sensor for your system.

What Is Dissolved Oxygen? Why Is It Important to Evaluate Dissolved Oxygen?

aquaculture fish

Dissolved oxygen is a non-compound form of oxygen that can be found in water and other types of liquids. This type of oxygen is useful for determining what the quality of water is because of how important it is for the organisms that live in the water. When looking at the study of lakes, this form of oxygen is considered to be the second most important factor of a body of water. Only the water itself is believed to be more important.

Whenever dissolved oxygen becomes too high or too low, the quality of water and the aquatic life within can be harmed. Non-compound oxygen refers to oxygen that hasn’t become bonded to another element, which means that there are free molecules of O2 in the water. The many forms of life in water that can be affected by the dissolved oxygen level include fish, plants, invertebrates, and bacteria. These organisms are only able to breathe because of the dissolved oxygen.

Dissolved oxygen can reach the water in a variety of ways. For one, it could diffuse in the air before reaching the surface of the water. It could also be mixed in the water via man-made or natural aeration. Water aeration can be caused by a variety of things, which include the discharge of ground water, wind, or waterfalls. Finally, this type of oxygen can be produced through photosynthesis as a waste product from seaweed or algae.

When looking specifically at aquaculture, evaluating dissolved oxygen levels is important to properly assess what the quality of the water is. In the event that the dissolved oxygen in the water reaches a low level, it’s possible that fish and other life will suffocate. The importance of measuring dissolved oxygen levels depends somewhat on what it’s being measured with. When looking at a treatment plant for sewage, a low reading for dissolved oxygen means that the bacteria necessary for decomposing the sewage will die. If the readings are too high, energy efficiency will worsen significantly, which is why it’s very important that sewage treatment plants measure the dissolved oxygen levels of their water.

For industrial applications like boilers, low dissolved oxygen levels are necessary to keep scale and corrosion buildup at bay. Aside from companies, dissolved oxygen can also affect drinking water that’s consumed by customers. For drinking water, the dissolved oxygen levels must be high to improve the overall taste of the water. Because of how important the right dissolved oxygen levels are to practically all applications involving water, it’s recommended that you measure your DO levels with dissolved oxygen sensors.

How to Measure Dissolved Oxygen

Dissolved oxygen can be measured with one of the numerous dissolved oxygen sensors that we offer here at Sensorex. The method that’s used to measure dissolved oxygen depends on the type of sensor that you use, of which there are two kinds. You can either measure dissolved oxygen with a galvanic dissolved oxygen sensor or an optical dissolved oxygen sensor, both of which can provide you with accurate and precise results. The measurement process for both types of sensors is fairly straightforward and can be completed in anywhere from 1-2 minutes. The best aspect of using one of these sensors is that you don’t need much equipment to complete the measurement process.

To determine why the dissolved oxygen levels have changed, you might want to measure the salinity, temperature, and pressure of the water. These factors will dictate how much dissolved oxygen is in the water. Such compensations can be performed through manual methods or via automatic systems like data logging software or a dissolved oxygen sensor. While these sensors will provide you with the current temperature of the water as well as several additional readings, the salinity can be measured with a conductivity sensor. Most of these sensors can also be integrated with a PLC or similar control system.

Galvanic Dissolved Oxygen Sensors

Dissolved Oxygen Sensor with Digital Communication

A galvanic dissolved oxygen sensor is outfitted with two separate electrodes, which include a cathode and an anode. These electrodes are placed directly into an electrolyte solution, which is located inside of the main sensor body. These electrodes are separated from the water by a permeable membrane. This membrane allows oxygen from the water to enter into the sensor and diffuse before being reduced by the cathode. This process creates an electrical signal that’s sent to the anode before again traveling to the measuring instrument. The oxygen concentration will increase as the diffusion of oxygen decreases.

Eventually, the process will come to a conclusion, which means that you’ll be provided with a reading that’s sent to a dissolved oxygen instrument. The main benefit of using this type of sensor is that it has a quick response time and requires zero warm-up time. The cost of this type of sensor is around $500-$650 depending on the exact model that you choose. The main galvanic sensors by Sensorex include the DO6400 and the DO1200, the former of which comes with digital communication functionality.

Optical Dissolved Oxygen Sensors

Lumin-S Optical Dissolved Oxygen Sensor with Digital Communication

Optical dissolved oxygen sensors use a special dye to measure the dissolved oxygen levels. This dye will glow red when exposed to a certain wavelength of light. A permeable paint layer is placed directly over the dye, through which the oxygen from the water can pass. Once the oxygen molecules get through the paint layer, these molecules will interfere with the luminescence of the dye, which causes light to be emitted. The light that’s emitted is measured by a photodiode that converts the light into a standard electrical current.

The reading that you’re provided with can then be compared with a reference reading, which will allow you to calculate how much dissolved oxygen is in the water. While optical sensors require an ample amount of power consumption, they also come with high stability and only require maintenance every 1-2 years. If you want to purchase one of these sensors, the Lumin-S model comes with digital communication and is priced at $1,150-$1,300.

How Does Dissolved Oxygen Tell Us About the Condition of Water

aquaculture turtle underwater

The different types of dissolved oxygen sensors provide readings in different ways. When looking specifically at galvanic dissolved oxygen sensors, these sensors need to be moved in the water for you to obtain an accurate reading. This method for measuring dissolved oxygen is entirely dependent on the flow of water, which means that the sensor needs to be stirred until the readings stop increasing. When this occurs, you will be provided with a reading on the display at the end of the device. This reading will be provided to you as mg/l, which refers to milligrams per liter.

As for optical dissolved oxygen sensors, they will need to be connected to a separate interface in order for you to obtain the readings that you’re looking for. If you’re considering using this type of sensor, it’s known for providing very accurate readings even at low DO levels. It’s important to note that readings from an optical sensor can take around twice as long as the readings from a galvanic sensor.

The ideal dissolved oxygen amount depends on the sample of water that you’re testing. For water supplies that are available to the public for consumption, it’s typically required that dissolved oxygen amounts never reach below 3.0 mg/l. The ideal dissolved oxygen amount for most fish is around 7-9 mg/l. The majority of fish are unable to survive if the DO levels drop below 3.0 mg/l.

For industrial applications, the ideal dissolved oxygen ranges can vary significantly from plant to plant. With a sewage treatment plant, DO levels should typically range from 1-3 mg/l, which ensures that the bacteria will continue to decompose the waste. Readings above 3.0 mg/l will be less efficient than you want. Most DO sensors will provide you with a range that extends anywhere from 0-50 mg/l.

Benefits of Measuring Dissolved Oxygen

sewage treatment plant

No matter what type of industrial application you work with, the benefits of measuring the amount of dissolved oxygen in the water are numerous. If you manage or own a sewage treatment plant, maintaining the DO levels in the plant ensures that the decomposition of waste remains efficient. DO levels that are too low will cause helpful bacteria to die. On the other hand, high DO levels lead to inefficiency with the process and the amount of energy that you use. The same is true with every type of application. Ideal DO levels mean that you don’t need to worry about the quality of your water.

When the DO levels become too high or too low, you will be required to spend a significant amount of time correcting the problem. By regularly measuring the readings, you can make the necessary adjustments immediately. For something like water plumbing lines, higher DO levels cause ample amounts of corrosion, which is something that you want to avoid. By measuring dissolved oxygen on a regular basis, you will be able to keep the dissolved oxygen at the appropriate levels. The many applications that dissolved oxygen measurement can be useful for include aquatic environment health and fish farming, the transport of live fish, corrosion control, and wastewater treatment.

Using Dissolved Oxygen Sensors for Aquatic Health

Overall, having and maintaining an ideal dissolved oxygen level is critical for the aquaculture environment. While improper levels of dissolved oxygen can have different effects depending on what the water is being used for, the aquaculture environment will always suffer if the dissolved oxygen is too high or too low.

In order to avoid these issues and maintain an ideal level, you should consider select such highly beneficial dissolved oxygen sensors as the Optical Dissolved Oxygen Sensor or the Galvanic Dissolved Oxygen Sensor, both of which are available through Sensorex! If you’re interested in other water measurement sensors, check out the pH sensors and conductivity sensors that we offer.

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Posted by Dominic O'Donnell on October 1, 2019

Sensorex is a global leader in the design and manufacture of quality sensors for water quality and process applications. The company offers more than 2000 sensor packages for pH, ORP, conductivity, dissolved oxygen, free chlorine, chlorine dioxide, UV transmittance and other specialty measurements, as well as a full line of sensor accessories and transmitters. Its expert technical support engineers solve analytical sensor challenges with custom designs and off the shelf products.

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