For more information on what ORP is or how it works in aquatics, please check out our previous blog post: Understanding Oxidation Reduction Potential (ORP)
Now that we know how ORP works, let’s discuss an alternative means of measuring free available chlorine in the water: The PPM sensor.
Instead of measuring the potential for a substance/solution to gain or loss electrons, a ppm probe is directly measuring the active disinfectant in the water. This is achieved through a newer technology compared to ORP: The 4-20 mA (milliamp) loop. By measuring the electrical current of a substance, this can tell us how much of that substance is in a solution. A value is assigned, and a scaling algorithm is used. If there is none of the substance in the water, that is assigned to 4 mA. A high end is chosen to represent 20 mA.
As substances are added to water, the electrical current changes and can be measured through this technology. Each sensor is designed to look for the specific electrical current a substance like chlorine imparts the water with. There are specific sensors for different types of disinfectants. One sensor is calibrated to chlorine, and another for bromine, for example. If free available chlorine (FAC) in a swimming pool is 0 ppm, then that is 4 mA. The high end varies by sensor, but the most common is assigned a value of 20 mA to 10 ppm FAC. Some math in the chemistry controller then scales this so that the end user just sees a ppm value.
By using this technology, we are able to accurately and directly measure the free chlorine in the water.
ORP Probes vs PPM Probes
If we can directly measure free chlorine, why are ORP probes still in use? Mostly because the technology behind it is still a relatively recent development. Millions of pools are already using ORP probes and have for decades, millions of products are being sold with ORP, so it’s the industry standard.
PPM technology does require slightly more maintenance than an ORP probe, but if the instructions are followed the probe will last just as long as an ORP probe and be easier to read. Many pool operators across the world do not truly know how ORP functions, and so end up calibrating or changing setpoints of their chemistry controllers on a daily basis.
PPM technology is superior to ORP just due to the ease of use. Instead of relying on an obtuse measurement that shows millivolts, the user just sees ppm. So if the hand reading is 3.0 ppm, then the controller should also just say 3.0 ppm. Easy. ORP is subject to change based on so many different factors that it just much more difficult to be accurate.
We at TMI call ours “True PPM” as it’s true there is an older form of a “ppm sensor”. Before 4-20 mA ppm probes existed, the only alternative to ORP was a large box on the way that contained reagents, which periodically ran a DPD test to determine the FAC. This was clunky, requiring that the reagents be stocked and the insides kept clean from the frequent leaks. Calling this a “ppm sensor” was far from the truth.
This is not to say that ORP doesn’t have its place. It’s simple and has been around for nearly 100 years now. We offer redundant systems that use both PPM and ORP technology to provide total security in case one sensor is not working as expected.
We are encouraging all commercial aquatic facilities to move from ORP to True PPM technology. If you are using an ORP controller and would like to discuss moving to True PPM, please reach out to us at tech@tmiaquatics.com.