Oxidation reduction potential and parts per million
The misunderstanding between ORP and ppm has occurred since chemical controllers were first introduced to the aquatics industry. If properly understood, pools/spas can be run efficiently with an ORP controller. For many facility operators, water chemistry is a small part of their job, but health departments want pool/spa water to have a specific amount of sanitizer in ppm. As a result, chemical controller manufacturers have developed equipment with sensors capable of measuring sanitizers in ppm, or quantity of chlorine, to help eliminate this problem.
Most controllers will measure the levels of sanitizer and pH in a body of water. The three most popular sanitizer measurements, however, are: ORP, amperometric ppm, and colourimetric ppm. Understanding the basis of each measurement can assist facility operators in managing pool and/or spa water.
ORP
ORP is a measurement of the oxidizing properties of the sanitizer present in a body of water. Most chemical controllers use a bypass mechanism that takes a small water sample from the circulation system, directing it into a small container that houses the chemical controller’s sensors. An ORP sensor comprises two electrodes, generally a platinum electrode in contact with the water, and a reference electrode inside an electrolyte chamber. A membrane separates the electrodes from the electrolyte solution. This membrane prevents the loss of the electrolyte solution and stops contaminants from entering the sensor housing. The output from the sensor is a voltage, typically expressed as millivolts.
ORP measures the waters’ ability to oxidize and is dependent on pH when the primary sanitizer is chlorine (Cl). Cyanuric acid levels can influence the readings taken by an ORP sensor, while salt systems can also affect ORP sensors and, in some cases, special metals must be used for the electrodes to prevent buildup on the sensor.
ORP is probably the most common and inexpensive method used by chemical controllers to measure sanitizer levels. This measurement, however, is the water’s ability to oxidize, not the actual amount of sanitizer. This is important to understand as it can be misleading for some inexperienced facility operators.
Some chemical controllers calculate the chlorine concentration based on ORP, pH, and temperature measurements. Therefore, it is important to remember the ORP sensor does not measure anything specific, and it will not differentiate between ozone and bromine.
Selective membrane ppm
The total amount of sanitizer in pool/spa water is measured in ppm. Like ORP, selective membrane ppm measurements are taken with a sensor that is housed in a small chamber located on a bypass from the main plumbing.
Selective membrane ppm provides direct readings of free chlorine in water. These are true readings, not indirect values derived from ORP. The ppm sensor is not affected by cyanuric acid and potassium monopersulfate (MPS), which assures constant readings of free chlorine at all times.
Colourimetric ppm
This method also measures sanitizer levels in ppm; however, the process is much different. The term colourimetric is used to describe a measurement system where reaction-based chemical indicators are used to detect the presence of a specified chemical. The indicator reacts with the specified chemical and produces a visible colour change in the solution. The stronger the colour, the more compound is in the solution. A DPD (N,N Diethyl-1,4 phenylenediamine sulfate) test kit is an example of a colourimetric measuring device used in the aquatics industry.
Studies have shown there can be significant differences in readings when using these test kits due to differences in colour perception between individual users. However, this method can be quite accurate when the eye evaluating the test is automated. Essentially, a colourimetric system, which is a separate unit from the chemical controller, automates a manual water test. The chemical controller can be programmed to send a signal to the colourimetric unit to test chemical levels in the water at defined intervals, or periodically, throughout each day.
During the testing process, a chamber is filled with a water sample from the pool or spa and a beam of light is sent through the test chamber where it is detected on the other side to establish a base reading. Then, reagents are added and mixed with the sample before a beam of light is once again sent through the test chamber. These results are then measured against the base reading to determine the amount of sanitizer in the water.
Colourimetric systems are some of the most accurate and expensive automated sanitizer measuring systems available. The sensors used by these systems also require flushing the water sample to waste, which can amount to 7.5 to 11.3 L (2 to 3 gal) of water per day.
ppm
This is the easiest sanitizer measurement for end users to understand as the system is easy to set-up and maintain.
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