We talk a lot about measuring your pH and ppm strength when making hypochlorous acid, but ORP is another indicator of the disinfecting strength of your solution. ORP stands for oxidation reduction potential, or the oxidizing power of the chemicals once they have been added to a solution. Oxidation is the process of electron exchange--pairs of molecules, known as redox pairs, gain and lose electrons. The oxidant is the chemical that gains electrons, and the reductant is the chemical that gives up electrons.
In practice, this means that hypochlorous acid, which is a strong oxidant, pulls electrons away from the negatively-charged pathogen cells, breaking them down. When hypochlorous acid is added to water with a high amount of organic material, it begins pulling away electrons from the chemicals present there, thus reducing the ORP level as it consumes electrons. Low ORP can be an indicator of a less potent solution, either due to an initial low free available chlorine content, environmental factors such as exposure to heat/sunlight, or a high organic load.
ORP is an arbitrary form of measurement that does not distinguish between what type of oxidant is present in the solution. Thus, using ORP to determine whether or not you are making hypochlorous in your machine is not an accurate form of measurement, but it can be used in tandem with pH and ppm tests to determine the oxidizing strength of the solution you've made.
Testing ORP is not required for HOCl generation but it can be a useful tool. The recommended ORP range for hypochlorous acid is 800-900, but this can vary based on the factors listed above. For example, if you are adding HOCl to a hydroponic growing system you generally will have a lower ORP, such as 500-600.
Looking for more information on how to measure the chlorine in your system? Check out this blog post!
Rekalske, H. (n.d.). ORP vs. Free Chlorine: What ORP Is and When and How to Use It Instead of Free Chlorine. The Analyst, 20(1). https://www.awt.org/pub/?id=00D69B4E-E0EC-A6B9-C9B2-B72F08526A0D