Understanding the difference ORP and PPM
Pool professionals must understand how these key measurements work in pool applications to benefit from their strengths and limitations.
ORP and pH controllers are commonly found in commercial pools although there is a big grey area about how ORP and PPM actually work. But pool operators, especially novices, often misunderstand ORP as an actual measurement of sanitizer.
ORP (Oxidation - Reduction Potential)
Oxidation-Reduction Potential (ORP) measures the oxidizing properties of any sanitizer (chlorine) present in the water. When chlorine is free to oxidize, sensors generate a millivolt reading, expressed as ORP. If the chlorine isn’t free and available, a millivolt reading will not be generated.
It is important to remember that the “P” in ORP stands for the “potential” of the water’s ability to oxidize and is therefore dependent on pH when the primary sanitizer is chlorine. CYA levels can also affect the readings taken by an ORP sensor, as can salt systems.
ORP is probably the most common method used by chemical controllers to measure sanitizer level, and the most inexpensive type of system. ORP is a qualitative measurement that is constantly changing, especially outdoors. This is primarily due to varying cyanuric acid (CYA) levels and sunlight intensity, but also from pH, combined chlorine and salt-chlorine generators. As these factors fluctuate, ORP measurements will change throughout the day.
The ORP reading is a constantly moving target, it is best practice to monitor ORP in combination with chlorine residual measured in parts per million (PPM) this will give the assistant a complete picture of the chlorine within the pool(s) to comply with prescribed health regulations.
Fact: Sunlight DOES affect ORP readings. That’s why when taking a ORP reading pool operators always see a distinct pattern at morning and afternoon in outdoor pool levels. ORP will go down as the sun rises because UV rays begin hitting the pool water, causing the chlorine to combine with CYA. At that time, the chlorine is less “free” and, therefore, has a lower oxidation potential at those two times of the day. This doesn’t mean the chlorine can’t do its job, because it will break free as needed to oxidize and sanitize. In the afternoon, ORP will again increase, because of the lower UV levels degrading the chlorine.
It is suggested that readings taken at these times take into consideration the above information, as if the levels are slightly higher, this may not always be a true representation of the available chlorine in the pool before adjusting set points or adding chlorine remover.
It is often difficult to obtain a satisfactory ORP reading in an outdoor pool stabilised with cyanuric acid. It may be necessary to limit the cyanuric concentration to 25 mg/L or even 20 mg/L to obtain a satisfactory ORP reading.
PPM (parts per million)
There are three primary methods that chemical controllers use to measure ppm: calculated ppm, selective membrane ppm and colormetric ppm.
Calculated ppm: Many controllers use this form to provide a free chlorine measurement. It is calculated based on the pool’s ORP and pH measurements. Because ORP continuously changes, the calculated ppm figure does provide an idea of how much free chlorine is available.
Selective membrane ppm: This system uses a special membrane through which only free chlorine ions can pass and provide direct readings. These are true readings of free chlorine and not indirect values derived from ORP and pH. The major advantage of the selective membrane ppm sensor is that it’s not affected by CYA and thereby provides constant readings of free chlorine at all times. Selective membrane sensors are good at measuring free chlorine but are the most expensive option. These sensors also allow the CYA level to be 30 - 50ppm without causing issues to the sensors or readings.
Colorimetric ppm: The term colorimetric 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 chemical and produces a visible color change in the solution. The darker the color, the more chlorine is present. DPD test kits, as an example, are familiar to those in the pool and spa industry. However, studies have shown that there can be significant variations in readings from these kits due to differences in color perception between individual users.
In swimming pools, an ORP of 700 to 720 mV allows for both a quick disinfection and for breakpoint chlorination (destruction of chloramines) where conditions permit.
The most important factor affecting sanitizer activity is pH, because it changes the concentration of the active form of free chlorine, hypochlorous acid, HOCl. As a result, chlorine and bromine become less effective at higher pH. This is why it is essential to maintain a stable pH value of 7.4 to 7.5
Section 4.4.1 (ii) of the Swimming Pool and Spa Pool Advisory document explains that pH affects the concentration of hypochlorous acid (HOCl) while the concentration of free chlorine remains the same. An increasing pH decreases the concentration of HOCl and hence its disinfection power. Similarly a decreasing pH increases ORP because the oxidative power of free chlorine increases.
ORP controllers provide the most cost-effective way to measure sanitizer, as long as operators understand how ORP measurements are obtained and remember that the measurements fluctuate. It is a very useful indicator of how the sanitizer is working, allowing operators to investigate factors that might be compromising its effectiveness.
By understanding how these readings are generated and the factors that affect them, operators can better adjust water chemistry and provide better reporting.