How to Understand ORP Measurements For Chlorine in Swimming Pools

Written by Michael Dean
May 28, 2024

swimming pool chemicals

One of the main chemicals you deal with when maintaining your swimming pool is chlorine. But while you will have heard of and dealt with chlorine before, you may be less familiar with ORP. ORP, which stands for oxidation-reduction potential, is a measurement that shows how much chlorine and the level of sanitation are in the water.

ORP measurements can be confusing, so today, I’ll explain everything you need to know about them for chlorine in swimming pools. Let’s get into it!

Main Takeaways

  • ORP, or oxidation-reduced potential, measures the pool water’s ability to break down contaminants.
  • ORP measurements are expressed in millivolts (mV), with an ideal level ranging from 650 to 770 mV.
  • ORP measurements are important as they monitor sanitization power, maintain water clarity, protect swimmers, act as an early-warning system, and complement traditional chlorine testing.
  • Some factors that can lower ORP levels include: low chlorine levels, high pH levels, high CYA levels, warm water temperature, contaminants, and lack of circulation.

What Is Oxidation-Reduction Potential (ORP)?

Oxidation-reduction potential (ORP) is a crucial parameter in pool chemistry. In short, ORP measures the pool water’s ability to break down contaminants through a process called oxidation, which we’ll discuss in more detail below.

It’s a fairly nonspecific measurement that only approximately indicates the capacity or potential of the substance’s ability to reduce or oxidize. Interestingly, since many different chemicals affect sanitizer levels (beyond chlorine), ORP readings are also affected by CYA levels and pH levels, among other factors.

Although it is a relatively broad measurement, it is highly useful for determining the water’s sanitizing power, which reflects its ability to eliminate bacteria, pollutants, and other harmful contaminants.

You can measure your ORP with a sensor or meter, which detects electron activity in the water. If your reading has a higher ORP value, it means the pool water has a greater potential for oxidation; in other words, the water is more effective at eliminating contaminants. On the other hand, if the ORP value is lower, there is a reduced ability to maintain clean and safe water.

Readings are typically expressed in millivolts (mV).

What Is Pool Oxidation?

Before diving deeper, let’s define pool oxidation so you have a more well-rounded understanding of ORP.

It is extremely important that you understand pool oxidation to keep your pool clean and safe. Oxidation involves the use of chemical agents to break down and remove contaminants, such as bacteria, algae, sweat, oils, and other debris introduced by swimmers and the environment. This process ensures the water remains clear, hygienic, and free from harmful contaminants.

But how does oxidation work exactly? These harmful contaminants have free electrons that make them toxic to humans. In a pool, when the chlorine (or bromine or your chosen form of sanitizer) oxidizes these contaminants, it enters a chemical reaction and takes electrons from these contaminants. Once this process is over, the contaminants become harmless, and the filtration system takes care of the rest.

Why Are ORP Measurements Important in Pools?

While you should now understand the basics of ORP measurements, it’s time to dig deeper into why these measurements are so important to the safety of your pool. Why should you pay attention to ORP measurements?

Monitors Sanitization Power

As you should know by now, an ORP measurement offers a broad, real-time indication of how efficiently your pool sanitizer is doing its job in keeping the water clean. A high ORP reading (ideally between 650 and 770 mV) indicates that the chlorine is actively oxidizing contaminants and the water is well-sanitized, making your pool a safe space for your friends and family to swim in.

Maintains Water Clarity

ORP levels reflect the water’s ability to oxidize organic matter, such as sweat, oils, and other debris, all of which affect the look of your pool. High ORP levels mean these contaminants are being effectively broken down, leading to clearer, more stunningly blue water. Consistently high ORP levels can also help prevent the growth of algae, which can lead to cloudy water.

Protects Swimmers

Chloramines, or combined chlorine, are compounds formed when chlorine reacts with contaminants like urine and sweat. They are responsible for that tell-tale strong chlorine odor, which can cause eye and skin irritation. By maintaining optimal ORP levels, there will be fewer chloramines, leading to a more pleasant and safe swimming experience for everyone.

Early-Warning System

A drop or rise in ORP can be a sign of a looming issue before it becomes a more serious problem. By knowing exactly when your pool’s sanitation power decreases, you will know to check your chlorine, pH, and CYA to take prompt corrective action to prevent problems like a full-blown algae infestation.

Complements Traditional Chlorine Testing

While ORP is not a substitute for direct testing of the chlorine levels, which is measured in PPM via test strips or liquid drop kits, ORP measurements can help complement PPM. Combining both methods paints a clearer picture of your pool’s disinfection status.

Factors That Can Lower ORP in Pools

As mentioned, you want to maintain an ORP level of around 650 and 770 mV, with 700 to 750 mV being ideal. But it is inevitable that there will come a time when the level drops below the recommended range. But why does this happen, and what causes it? Understanding the factors that can lower ORP levels can help you better prevent this issue to maintain a healthier pool.

Low Chlorine Levels

Probably the most obvious factor that can lower ORP in pools is a low level of sanitizer. Since ORP measures the sanitization ability of the water, if levels of chlorine are low, ORP levels will also be low, as there’s less sanitizer to deal with the contaminants in the water.

To avoid this, try to maintain a chlorine level of 1 to 3 PPM.

High pH Levels

At high enough pH levels (above 7.8), chlorine becomes less efficient. This is because it converts into a form of hypochlorite ion, which is weaker at killing impurities. So, unfortunately, this affects overall ORP readings.

Make sure to maintain a pH level of 7.4 to 7.6 at all times, and ensure they do not go below 7.2 or above 7.8.

High CYA Levels

Cyanuric acid (CYA) is a stabilizer that protects chlorine against sunlight. While it’s useful for extending the chlorine’s lifespan during sunny, hot days, it can interfere with ORP readings if too high. And if it is too low, your chlorine will be depleted much faster than you think!

You should keep CYA levels balanced at around 30 to 50 PPM.

Warm Water Temperature

Surprisingly, water temperature also influences ORP levels. Warmer water can lead to decreased ORP levels, as higher temperatures can reduce the effectiveness and availability of chlorine. So, make sure to maintain a stable and moderate water temperature.

For this reason, I recommend measuring ORP levels during the heat of the day, when it’s warmer, rather than at night.


Too many contaminants in the water, such as algae, bacteria, sweat, oils, urine, and debris, can significantly lower ORP levels. These contaminants are more likely to overwhelm existing chlorine levels after a storm or pool party.

Regular cleaning and shocking the pool can help manage these nasty contaminants and maintain higher ORP levels as well.

Lack of Circulation

Proper water circulation and filtration are the heart of any pool system. They are essential for distributing chemicals and keeping the entire pool clean. Without them, some areas of your pool may have low ORP, while others may come back with a reading of ideal ORP levels.

Poor circulation can result from various issues, including malfunctioning pumps or improperly positioned return jets. So, make sure your pool water properly circulates throughout the system to maintain good ORP levels throughout the entire pool.

Difference Between ORP and PPM

Both ORP and PPM help monitor pool sanitation, but they assess different things.

Parts per million, or PPM, is a direct measurement of the concentration of a substance, such as chlorine, in the water. This means it gives you an exact reading of how much sanitizer is present in the pool. The results are not affected by external factors like pH, temperature, or CYA.

On the other hand, ORP is a broad measurement of the water’s sanitizing ability and how well it can oxidize contaminants—not the amount of sanitizer present in the water itself. Unlike PPM, ORP results can be influenced by external factors such as pH, CYA levels, and water temperature.

For the best pool care, I recommend using both PPM and ORP readings.

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ORP and Its Importance

Any pool owner knows that effective pool sanitization is the heart of pool maintenance. To help you keep an eye on this, ORP comes in super handy. While it may not indicate the actual PPM of the pool chemicals you use, it is a great indicator of your pool’s sanitizing abilities so that you can quickly act when the ORP levels are not high enough.

Any more questions about ORP or PPM that I can help you with? Let me know!

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