Dissolved oxygen in water seems unintuitive. We cannot breathe underwater so how can there be any oxygen there? Even though humans cannot survive underwater, aquatic life does depend heavily on the small level of dissolved oxygen in the water.
This phenomenon can be observed by simply setting a glass of water on the table and letting it sit for a few hours. Over time the oxygen will accumulate and form visible bubbles in the water, which were always there in the first place but become more evenly distributed (and invisible to the naked eye) as the water mixes and absorbs more water from the air.
For background, the amount of dissolved oxygen in water is usually measured as milligrams per liter (mg/L) or parts per million (ppm). This value can be converted into a percentage by dividing by 10,000. However, the percentages are usually pretty low so using mg/L or ppm is preferred.
The importance of dissolved oxygen in water
As stated above, aquatic life relies heavily on enough oxygen to survive just like you and me. If dissolved oxygen levels drop too low there can be a large loss of fish and plant population. Therefore, dissolved oxygen measurements can provide a great insight into water quality, especially when paired with pH, temperature, and other probe measurements.
As a rule of thumb, according to the Environmental Protection Agency (EPA), dissolved oxygen levels approaching 3 mg/L are in the danger zone for supporting common aquatic life, and then levels below 1 mg/L cannot support any aquatic life.
Dissolved oxygen levels vary for each organism, but levels around 8-9 mg/L will support all life (fish or plants) and approach the oxygen saturation level of water (which we will discuss later). This high level of dissolved oxygen should be closely monitored in hydroponic setups, as large changes can have detrimental effects on the plants.
Therefore proper monitoring of dissolved oxygen levels is extremely important to prevent loss of life. Whether you are considering hydroponics, an aquarium, or environmental work, you’ll want to have an accurate and reliable dissolved oxygen sensor. This particular kit from Atlas Scientific will provide everything you would need to begin measuring and monitoring dissolved oxygen.
Why would you want to increase dissolved oxygen in water?
Since we now know the importance of dissolved oxygen to aquatic life, if levels drop too low then you’d want to increase those levels and prevent loss of life.
Water has a property called the oxygen saturation point, or the point where no more oxygen can dissolve. Similar to putting sugar into water, at some point no more sugar will dissolve and will then pile up at the bottom. The same happens with oxygen, but it will float away instead.
The oxygen saturation point for water usually sits around 10 ppm or 10 mg/L but depends on a few factors: temperature, salinity, and pressure.
What affects dissolved oxygen levels in water?
First, the oxygen saturation point of water is affected by three key factors:
Temperature – as temperature rises, DO levels decrease
Pressure – as pressure decreases, DO levels decrease
Salinity – as the salt content increases, DO levels decrease
Using the graph above, as temperature goes up water cannot retain as much dissolved oxygen. But in the same environmental conditions, freshwater will be able to hold more dissolved oxygen than saltwater. These trends are important to keep in mind when dealing with hydroponic setups, aquariums, or environmental baselining.
Next, water is not usually at the saturation point and has oxygen gradients within the whole volume. Surface level water will have more dissolved oxygen than the bottom since oxygen is diffusing in from the surface. In still lakes and ponds, this can create a large dissolved oxygen gradient if no wind or wildlife come about to mix the water.
Wind creates natural circulation underneath the water’s surface as well as increases absorption by roughing the surface and increasing surface area. For this reason, a flowing river will usually have higher dissolved oxygen than a stagnant lake.
Bacteria and plant populations also have a noticeable effect on dissolved oxygen in water. Plants, as we know, produce oxygen and that oxygen can dissolve directly into the water, increasing dissolved oxygen levels. On the other hand, bacteria consume oxygen and produce CO2, thus reducing dissolved oxygen in the water system. In this way, algae blooms can not only be toxic but also use up the available oxygen for other organisms and decrease dissolved oxygen to dangerous levels.
Putting this all together, many factors can alter the dissolved oxygen content in water. Think hot climates, stagnant water, algae blooms. All these conditions are bad for the dissolved oxygen business and vice versa. Overall, instead of worrying about how many factors can adversely affect your water system, just monitor the water with an Atlas Scientific dissolved oxygen probe that can be submerged in salt or freshwater indefinitely.
How to measure dissolved oxygen in water
For background purposes, let’s first discuss how a dissolved oxygen probe works.
A cross-section of the dissolved oxygen probe is provided to show where (when submerged) oxygen molecules can diffuse through the plastic membrane. Once they diffuse through, the oxygen molecules will react with the cathode and create a small, but measurable voltage. This voltage is then read by a multimeter or analog to digital device as a dissolved oxygen level in mg/L. Fascinating!
Note: due to the operating principle of the probe, it technically consumes a very small amount of oxygen while measuring the current levels. This constant consumption will not largely affect the measurement, especially those taken within the first few seconds.
Now that you know how it works, all you have to do is calibrate the probe and submerge it in water.
To calibrate the probe, simply let the probe sit out in the open air for 5 to 30 seconds until readings stabilize. Once stabilized, issue the calibration command and the dissolved oxygen probe is ready to use! Re-calibration should occur between different samples as conditions change frequently—thankfully the calibration is super quick and easy.
One thing to mention, if you require dissolved oxygen measurements below 1.0 mg/L then you will need a calibration solution for an accurate reading. However, the only new calibration step is to submerge the probe in the zero dissolved oxygen calibration solution and adjust the system to read zero mV, as there will be no oxygen reacting with the internal cathode.
It is also recommended to replace the probe membrane and internal electrolyte solution every 1 to 2 years to ensure the highest performance of your device.
Once calibrated, submerge the probe in water and take a reading immediately. And that’s it!
Now you will be able to quickly measure dissolved oxygen levels and determine if they are becoming too low for plant growth, fish life, or even winemaking. Different setups can be achieved for continuous monitoring, with the most robust approach being direct attachment into water piping.
How to increase dissolved oxygen in water
Taking into consideration our discussion on what affects dissolved oxygen levels, there are multiple approaches to increase dissolved oxygen levels in your water setup.
The first step, always, is to use a dissolved oxygen probe to accurately measure the current state of water quality and then identify the level you need to reach (specific to each application, like hydroponics). In most cases, this will be the oxygen saturation level around 8 to 10 mg/L. Check out the Gravity Analog dissolved oxygen kit from Atlas Scientific to quickly cover this step.
For increasing dissolved oxygen in water think of the 4 A’s:
Add O2 Gas
Aerators can come in many different forms, the most common being an air stone that has consistent air pumped through it. It will add oxygenation directly but also create turbulence to increase surface roughness and oxygen absorption.
Agitators can also come in many different setups from large paddle wheels in a stagnant pond, to fountains, to underwater fans. Even yourself swimming through a pond technically contributes to water agitation and increases the dissolved oxygen levels as well as mixing. Let your mind run wild with the possibilities of creating mechanical water agitators from small electric motors.
Aquatic plants, as discussed previously, will produce oxygen during photosynthesis and add it directly to the water. Ample sunlight and nutrients will need to be a consideration if choosing this method for indoor applications.
Lastly, bubbling pure oxygen gas through water will significantly increase the dissolved oxygen levels. In this approach, however, be mindful of oversaturating the water which can also lead to aquatic life death. Be sure to have a dissolved oxygen probe when adding pure oxygen gas to water sources. Continuous monitoring is best for this approach and easy to set up with an industrial dissolved oxygen probe that screws directly into piping.
The size and scale of your operation should also be a consideration. If a small aquarium or hydroponic system is the goal, then a small air stone will do the trick. If it’s a large pond for fish and plant growth, large agitator systems like the paddlewheel and/or fountains should be considered (and they look cool too).
Overall, dissolved oxygen is an extremely important characteristic of water quality for any application (hydroponics, environmental sampling, fish farming, aquariums, brewing, etc) and should always be monitored. Be mindful of the environment (temperature, pressure, salinity) and other natural factors that influence dissolved oxygen. In the end, always use your calibrated dissolved oxygen probe (see the mini lab-grade probe) to accurately measure and monitor water quality to prevent unhealthy conditions.
If you are unsure exactly which dissolved oxygen device will best suit your needs, or you would like to learn more about other water measurements like pH levels and electrical conductivity, do not hesitate to reach out to the world-class team at Atlas Scientific.
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