10/22/2019 | ORP | 10 MINUTE READ

Calculating ORP Measurement in Water

backyard pool water

There are many different methods for measuring water, all of which are necessary for different applications. Some of the more popular methods for water measurements include measuring the pH of the water, measuring the conductivity, measuring the amount of dissolved oxygen, and measuring the chlorine dioxide in the water. Another very useful method for water measurement is ORP, which refers to the oxidation reduction potential. The main goal of this measurement is to assess a molecule’s ability to oxidize, which allows for the reduction of another molecule.

In many applications, this water measurement determines how clean water is and how well it’s able to break down other contaminants within the water. This form of water measurement can be used for many different applications, which mainly involve the monitoring of various processes that take place in water. When attempting to disinfect and sanitize the water with ozone, bromine, or chlorine, ORP can be used to identify how effective the disinfection is. Different applications will call for different ORP levels. The following will provide you with a comprehensive guide that details everything you need to know about ORP and its many applications.

What is Oxidation Reduction Potential?

water droplet splashing

ORP is a measurement that refers to oxidation reduction potential. When measured properly, the ORP levels of water will display the ability of one molecule to reduce or oxidize another molecule. The “oxidation” in this term indicates a loss of electrons, which means that the oxidizers in water will take electronics from some other molecules in the water. The “reduction” aspect of this term refers to the process of gaining electrons, which indicates that reducers will give electrons to a range of other molecules.

When ORP is being measured, the number that you’re provided with is displayed as millivolts, or mV. Oxidizers like nitric acid or sulfuric acid will always be displayed with a positive value of ORP. Reducing agents like magnesium, sodium, and aluminum always have a negative value for ORP. The oxidation reduction potential measurement is highly important for a wide range of applications to determine what reactions will occur in the water. It’s important to measure ORP levels for such applications as aquaculture, cooling towers, swimming pools, hot tubs, the disinfection of water, and the sterilization of water.

water sunlight

The importance of ORP depends mainly on what the measurement is being used for. When you’re disinfecting swimming pools or cooling towers, ORP measurement is necessary to effectively control the disinfection process and to make sure that it’s progressing smoothly. Keep in mind that ORP measurement tools are similar to dissolved oxygen sensors in that they obtain ORP levels by measuring the amount of dissolved oxygen that’s currently in the water. If a large number of contaminants are located in the water, the amount of dissolved oxygen will be low, which also means that the ORP levels are low. Higher ORP levels indicate that the water is able to effectively eliminate the foreign contaminants that enter it, which include carbon-based contaminants and microbes.

How is ORP Measured?

ORP is directly measured by using a high quality electrochemical sensor, which is typically referred to as an ORP sensor. These sensors are very similar to pH sensors in that they operate as a combination sensor that’s comprised of a reference electrode and a measuring electrode. The single measuring cell usually consists of a noble metal like gold or platinum, which refers to any kind of metal that’s able to resist chemical reactions. This cell will detect any changes in the ORP level. On the other hand, the reference electrode will provide you with a comparison signal.

Keep in mind that the measurement that you’re provided with doesn’t actually indicate the concentration of ORP in the water. Instead, this measurement is designed to indicate activity in the water. If you’re disinfecting a cooling tower, an ORP sensor can be used to determine if the disinfecting agent is breaking down the contaminants properly. However, there are times when the ORP sensor will provide you with a concentration reading, which occurs when there is only one active species in the water. If your pool water is comprised solely of chlorine and is very clean at the moment, the measurement that you receive will be the ORP concentration of the pool water.

When you’re thinking about selecting an ORP sensor for your cooling tower, pool, or other application, there are several different types of ORP sensors that you can select from, which include laboratory sensors, process sensors, and differentiation sensors. If you decide to obtain a laboratory ORP sensor, there are three subcategories of sensors available to you, which include basic, advanced, and research-grade sensors. The basic sensors are easy-to-use and are perfect for swimming pools and hydroponic applications. Advanced ORP sensors can be used for many different sample types and are ideal for wastewater and environmental sampling. Finally, research-grade sensors use highly sensitive electrodes, which makes them beneficial for use in corrosive or wet environments.

S272CD-ORP Online Process ORP Sensor with Digital CommunicationProcess sensors like the S272CD are designed for continuous monitoring and are durable enough to be used in moderately contaminated water, which is commonly seen in natural water resources and industrial source water. This particular sensor comes with an extended sensor lifetime and compatibility with several types of digital communication. As for differentiation ORP sensors, they use three electrodes to provide a more precise measurement. These sensors are designed to be used for tough wastewater and industrial applications that would typically contaminate the sensor. The inclusion of a solution ground electrode means that the measurements that you’re provided with from the sensor will be error-free.

Once you’ve selected the ORP sensor that meets your needs, you can begin to take readings of any sample of water. ORP is measured in millivolts, which comes with an abbreviation of mV. The way that you analyze the readings depends entirely on what application the ORP sensor is being paired with. If you’re involved in some form of aquaculture, the optimal ORP levels will be 150-250. On the other hand, the correct ORP reading for disinfection is 600. Complete sterilization will require a reading of 800.

Since these readings indicate how much activity is occurring, a high reading of 800 indicates that the oxidation agents are working effectively to break down contaminants in the water. Most ORP sensors available through Sensorex will display readings that range from -2,000 to +2,000 millivolts. However, oxidation only occurs when the ORP levels are in the positive range, which means that you’ll never be provided with a negative reading.

What Does the ORP Measurement Tell You?

power plant cooling tower

The measurement that you obtain from your ORP sensor can tell you a lot about the quality of your water. As touched upon previously, ORP sensors are designed mainly to indicate the level of oxidation activity in the water, which means that it won’t display the concentration of ORP unless the water is very clean. While you may want high ORP levels for your cooling tower or swimming pool, it’s also important that you check the pH levels to make sure that the acidity readings are correct. For pools and cooling towers, the chlorine levels will also tell you if the ORP is at the correct level. If the pool has high amounts of chlorine in it, the ORP level should be high as well.

The optimal ORP levels depend on the application. For instance, the disinfection and sterilization of water will require ORP levels of at least 600 or 800 respectively. On the other hand, swimming pool should have ORP levels that are higher than 400 ORP. Higher ORP levels indicate that the water is able to destroy contaminants. While there aren’t many problems caused by high ORP levels, you may need to add chlorine or some other oxidation agent to the water to make sure that your ORP readings increase.

Why is ORP Measurement Needed? How Do You Obtain an Optimal Level of ORP?

bacteria pathogen

An ORP measurement is needed because it allows you to measure a wide variety of processes. The most common application for ORP measurement involves the disinfection of water with strong oxidizers like bromine or chlorine. ORP can also be used to monitor such processes as the destruction of cyanide and chromate. The measurement of ORP has proven to be essential for the production of bleach as well. In most cases, an ideal ORP level will be a high ORP reading. An optimal level of ORP needs to be maintained so that you can be certain that a process is working as intended. If you’re trying to disinfect your pool, an ORP measurement that’s below 600 indicates that the disinfectant isn’t working correctly.

To obtain an optimal level in a cooling tower of 700mV-800mV, chlorine will need to be added, which ensures that any contaminants are effectively eliminated. If the ORP levels dip too low, problematic microbes like bacteria and algae will continue to build up and contaminate the water. You can also control ORP levels by increasing or decreasing the acidity of the water. Higher pH levels will cause ORP levels to be low. On the other hand, low pH levels indicate high ORP levels. If you find that your ORP is too low, you can add muriatic acid to the water to increase ORP levels and decrease pH levels. If your ORP levels are too high, consider adding sodium hydroxide and sodium carbonate to the water to increase pH levels.

Achieving Optimal Levels of ORP Measurement

When you want to obtain optimal ORP level, there are different steps that you can take depending on the application and the measurement that you’ve been provided with by the ORP sensor. If you need your water to be clean, it’s important that the ORP level is relatively high. If you’re using one an ORP sensor for a pool and find that the reading is too low because of contaminated water, all you need to do is add bromine or chlorine to effectively maintain ORP levels. Similar treatment can be performed for a wide variety of applications.

If you want to measure the ORP levels of your water, there are a range of different sensors that you can select from. If you want a sensor that’s durable and eliminates the potential for fouling, it’s highly recommended that you use a differentiation ORP sensor such as the SD7000 sensor. If you require a sensor for your swimming pool, the ORP1000 is likely the best option for you.

The best sensor for ORP measurement would be the SD7500 sensor, which is made for coating industrial and wastewater applications. This sensor is outfitted with three electrodes for precise measurements. If you require additional sensors to test the conductivity or dissolved oxygen in your water, you can learn more about the products that we offer by searching through our extensive catalog of water measurement sensors here at Sensorex.

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Posted by Dominic O'Donnell on October 22, 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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