Understanding the Difference: ORP vs Dissolved Oxygen (DO)

orp-vs-dissolved-oxygen

Oxidation and reduction potential (ORP) and dissolved oxygen (DO) levels play an important role in water quality for natural systems and water treatment facilities alike. 

Even though both are important, the two characteristics and their crossover is not always well defined or understood. These concepts will be compared and discussed in this article as they relate to water, and the primary goal of having clean and healthy water—no matter the application (hydroponics, brewing, drinking water measurements, environmental sampling, and so on).

What is dissolved oxygen (DO) and why is it important in water?

Dissolved oxygen (DO) level is the available oxygen in a body of water for organisms to consume. In other words, the amount of extra oxygen that diffuses from the air into the water, or added by aquatic plants, and not the H2O oxygen part of the molecule. These dissolved oxygen levels can infer a great deal about the general quality of water, its ability to support life, and is used alongside other water characteristics for a more holistic picture. 

Note: 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. 

Why is DO important in water?

Similar to us humans, if fish and other aquatic organisms do not have enough oxygen they will not survive. To put this into perspective, the Environmental Protection Agency (EPA) states that 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.

Additionally, low DO levels are not only bad for fish life, but it also concludes that bacteria and other unwanted contaminants exist at unsafe levels (since they consume a lot of oxygen). If this is true, say an algae bloom occurs, the water becomes undrinkable for other living organisms as well.

As a result, proper monitoring of DO 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 probe. This particular kit from Atlas Scientific will provide everything you would need to begin measuring and monitoring dissolved oxygen. 

What is oxidation-reduction potential (ORP) for water?

ORP is a measurement of electron activity in a medium that relates to the relative amount of oxidant or reductant, for our purpose that medium is water. This characteristic is measured in millivolts by an ORP probe, with higher readings correlating to a more sanitary water system. 

ORP readings in the positive millivolts will correlate to an oxidative water state, or a more sanitary state, so negative readings (reductive state) will not be as common in a clean water quest. 

Elements like oxygen (yes, dissolved oxygen from above!) and chlorine contribute heavily to a higher ORP level in the water and the increased breakdown of unwanted contaminants. As a result, higher ORP levels can be correlated with higher levels of water sanitation. 

At a minimum, drinking water needs to exist around 650 mV to be properly sanitized and safe for drinking. This ORP potential for swimming pools should increase to around 700 mV to 750 mV for safe use. 

What is the relationship between dissolved oxygen (DO) and ORP?

This is where the confusion lies and two points must be cleared up:

  1. Dissolved oxygen is an isolated measurement dependent only on the DO content in water
  2. ORP measurement is not a direct measurement of dissolved oxygen but is heavily influenced by DO and other oxidants. 

Note: ORP measurements cannot be confused with direct parts per million (ppm) measurements of DO. 

In this way, both characteristics provide strong insight into overall water quality, but dissolved oxygen depends only on itself and ORP depends on dissolved oxygen and other oxidants like chlorine when taking a measurement. 

To take matters further, dissolved oxygen is usually measured to understand the ability of water to support aquatic life, whereas ORP is commonly measured to understand water sanitation levels. 

In this way, DO and ORP measurements are more suited to natural waterways and wastewater plants, respectively, but both measurements can be used in any scenario to understand water quality. And why not do both? Make it easy on yourself and use a consumer-grade ORP probe to accurately measure and monitor ORP (DO devices also range in grades from lab to industrial DO probes).

Should I test my water for ORP or DO?

Both! If you want the most holistic picture of water quality. However, if you have to choose one base it on your objective:

Other factors that affect ORP and dissolved oxygen (DO)

Temperature is a non-intuitive player in ORP and DO. As the temperature increases, the ability of water to hold dissolved oxygen decreases and therefore decreases ORP levels. For this reason, one must be aware of comparing ORP and DO measurements at different times of the day or different seasons.

Pressure and salt content (salinity) also affect DO levels, and in turn ORP levels. As pressure decreases so does DO, and as salinity increases DO will decrease. These factors are something to note if unexplained changes in DO or ORP occur, possibly during measurements over time or in a lab setting.

Summary

Oxidation-reduction potential (ORP) and dissolved oxygen (DO) levels are great measurements to determine the general water quality of any application. They provide insight into the overall sanitation level (positive ORP readings in the 650-750 mV range are ideal for sanitation) and life-supporting capability (above 3 mg/L DO), respectively. 

ORP meters and DO probes come in different grades from lab-grade ORP to industrial depending on the level of accuracy and intensity of the measurement environment. Choosing which measurement is right for you will come down to your goal: supporting aquatic life, sanitation, or both.

Additionally, if you’d like to understand more about your water quality check out Atlas Scientific’s industrial pH/ORP/temperature probe to conduct three measurements with one probe. 

If you are unsure exactly which ORP or DO 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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