Understanding the difference between Chlorine ORP and PPM

If you're further along in your pool maintenance journey they you may start to hear or read things like ORP or Oxidation - Reduction Potential. Many years ago, ORP measurements were strictly in the realm of commercial pool operators but recent advances in technology have brought ORP technology for use in many home pools in Australia.

Warning: The following sections will contain a lot of technical terms, if you prefer to read a more user-friendly version, click here to jump to the bottom of the article for a more layman-friendly version.

In the past, ORP and PPM were measured either by pen and paper or through the use of really expensive probes and testing equipment. The recent advances in Technology has made testing for ORP and PPM much easier and Water TechniX has that packged into one convenient Digital Pool Water Test Meter.  

If you're a looking to do some in-depth testing or if you want to get to know your pool water better and fine tune your balancing, then this is the perfect tool for you.

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.

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.

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.

The lower the ORP, the lower the ability of the water (in our case chlorinated water) to break down contaminants. The higher the ORP, the higher the ability to break down contaminants, hence cleaner water.

ORP sensors measure the amount of Dissolved Oxygen in the water. What this means for us is that the more Dissolved Oxygen in the water, the higher the ORP level will be and our pool chlorine will be more effective. Another takeaway for this is that the less Dissolved Oxygen in the water, the higher the contaminants are.

In home pool applications, ORP sensors are used with automatic chemical dosing devices like the Astral Rola Chem automatic chemical dosing unit which utilizes ORP sensors to properly dispense chlorine. There's also the Astral Viron eQuilibrium EQ35 Salt Water Chlorinator which can utilize different add-ons for ORP, pH balance, and chlorine management for even more hands-off and accurate chlorine management for your pool.

For many home pool owners, ORP and PPM when it comes to chlorination isn't really a big factor since bather loads are under control and swimming pool sizes aren't that much. So if you're looking for something for home use without too much to think of, then a Water TechniX salt water chlorinator, or any other salt water chlorinator is more than enough to get your pool water clean and sanitised for use!

Worried about ORP and PPM? Test it quickly and easily with the Water TechniX Digital Pool Water Test Meter