Understanding ORP Measurements for Chlorine in Pools


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Chlorine and swimming pools are constantly lumped together in discussions around backyard pools, swim classes, or hotel stays. But what does chlorine actually do for swimming pools? 

In this article, we will discuss the role that chlorine plays in swimming pools, water in general, sanitation level, and how it can be understood with an oxidation-reduction potential (ORP) measurement. 

These concepts will be related primarily to swimming pools, since chlorine is usually desired for sanitation, but ORP readings and chlorine will apply to many other water areas like hydroponics, drinking water measurements, environmental sampling, and so on.

What is oxidation-reduction potential (ORP)?

ORP is a measurement of electron activity in a substance (in this case pool water). This characteristic is measured in millivolts by an ORP sensor kit, 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 natural environment. 

Elements like oxygen 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. 

It is recommended that pool water reside somewhere between 700 mV and 750 mV for ample sanitation; whereas drinking water only needs to exist around 650 mV for adequately sanitized water. 

Note: ORP measurements are not a direct measurement of chlorine. It only provides insight into the general sanitation level of the water, since chlorine is not the only contributing factor to ORP levels. In this way, ORP measurements cannot be confused with direct parts per million (ppm) measurements of chlorine. 

What is an ORP measurement in a swimming pool?

Similar to pH, which measures hydrogen ion concentration, ORP measures the overall strength of electron activity or the potential of a chemical to disinfect and deactivate bacteria in water. In the case of a swimming pool, the higher chlorine and dissolved oxygen concentrations will correlate to a higher ORP level, since these elements inherently contribute to the loss of electrons among other substances—known as oxidation.

What is pool oxidation and how does chlorine disinfect?

To start, bacteria, algae, and other harmful contaminants will have free electrons hanging around thus making them reactive and toxic to humans. In this scenario, the chlorine will enter a chemical reaction with these contaminants and take electrons from them. Once this reaction is complete these toxins will become virtually harmless and dissolve in water or become more easily removed by filtration, making it safe for swimming. 

It is important to note that active chlorine percentage is affected by the pH level of the water. A slightly basic range of pH from 7.2 to 7.8 will be ideal for chlorine’s ability to disinfect, but pH higher than this will significantly reduce the effectiveness of chlorine. If you’d like to learn more about pool disinfection, muriatic acid, and measuring pH in your pool check out our post on how to add muriatic acid to a pool.

Why are ORP measurements important in pool water?

ORP is affected by any substance that exchanges electrons. In pool water, this action is mostly performed by chlorine or potassium monopersulfate, but dissolved oxygen is also a very important characteristic of healthy water sources be it natural, hydroponic tanks, wastewater effluents, etc.

Organisms that break down contaminants and other unwanted substances floating in water rely on dissolved oxygen to do so. Since swimming pools will normally not have natural organisms living in them, the waters rely on chlorine to do all the dirty sanitation work for the pool. To state again, a higher ORP reading will equate to a more “sanitized” water and should increase as chlorine is added to the pool water. 

In this way, it is extremely important to have a basic understanding of ORP measurements because if they decrease significantly you could have bacteria build-up in the pool or an extreme loss of chlorine levels. 

Monitoring ORP levels in your pool will allow for decreased downtime by noticing changes before they become too drastic; as a result, you can enjoy your pool for more days out of the year. Make it easy on yourself and use a consumer-grade ORP probe to accurately measure and monitor ORP.

What factors decrease ORP levels in pools?

Since chlorine is a large contributor to higher ORP levels in pools, the reduction in the chlorine concentration will heavily reduce ORP readings. As mentioned above this could be due to increased pH levels, which would deactivate a lot of the chlorine levels in the pool, leaving it susceptible to bacteria and algae growth.

If bacteria and algae are left to their own devices they will also use up dissolved oxygen in the water. Losing dissolved oxygen, also a large contributor to high ORP, will decrease the level of ORP as well.

Temperature is also a non-intuitive player in ORP. 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 measurements at high noon versus at twilight—as ORP levels should be lower around noon when the temperature is higher.

How does an ORP probe work?

As discussed above, an ORP probe works similarly to a pH probe but instead of measuring hydrogen ions, it measures the relative amount of electron activity in millivolts (mV). 

An ORP meter operates using a reference solution inside the probe (usually potassium chloride, KCl) and compares that to the test sample to determine an ORP level in mV. These probes measure how strongly electrons are transferring in the solution and equate that to an overall voltage—positive voltage being oxidizing, and negative voltage being reducing. 

As shown in the figure above, each ORP probe comes equipped with a storage cap to place on the probe tip when not in use. ORP probes cannot be allowed to dry out otherwise it will decrease their intended measurement use and accuracy. This is not a worry for indefinite submersion and measurement setups. In the case of intermittent testing, be sure to keep the probe tip wet with ORP storage solution.

Additionally, calibration is extremely important and straightforward with the use of an ORP calibration solution. Atlas Scientific provides 225 mV calibration solution as this voltage lands in a common oxidative state (since most water samples will be in the positive voltage range). To learn more on this topic check out our article on ORP meter calibration.


Oxidation-reduction potential (ORP) is a solid measurement to determine the general water quality of a swimming pool. It provides insight into the overall chlorine level and level of sanitation (positive ORP readings in the 650-750 mV range are ideal for sanitation). ORP meters come in different grades from consumer to lab grade to industrial depending on the level of accuracy and intensity of the measurement environment. 

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 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.

ORP Probes & Sensors

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