Information on ORP Operation
What is ORP?
ORP (Oxidation Reduction Potential) is a way to measure how clean water is and its ability to deactivate contaminates. The higher the ORP value the cleaner the water, whereas the lower the ORP value the dirtier the water.
How ORP probes are used in Swimming Pools
ORP probes are used to indirectly measure the amount of sanitizer such as chlorine or bromine in the body of water. ORP value is fed into the controller and the controller uses the value to determine if more sanitizer is needed and if so the dosing equipment is activated.
What affects ORP readings
Chlorine: Chlorine residual in the body of water will cause ORP readings to be higher than if there was not any chlorine present. Additionally, more chlorine will cause this reading to go higher.
pH: Since ORP measures the cleanliness of the water pH affects this Value. Lower pH readings will cause the ORP to climb whereas higher pH readings will cause the ORP to drop. Why is this? Chlorine loses much of its oxidation power at pH levels above 7.8. Anytime the pH is higher than 7.8 in a pool or spa the chlorine is much less effective at responding to contaminants in the water.
Bathers: As more people enter the water the contaminants rise causing ORP values to drop. Observant operators will notice the ORP values to be higher at night or early morning when no one is swimming vs peak hours.
Cyanuric Acid: Higher cyanuric acid levels will have adverse effects on ORP probes. This causes two different issues 1) lower ORP values vs pools with no cyanuric acid. 2) the possibility of causing the probe to become contaminated quicker causing more sluggish readings. It is recommended that if using automation equipment to not let CYA get above 25PPM.
Other Impacts: Temperature of the water, sunlight exposure, secondary sanitation systems such as UV or Ozone.
Probe Operation
Operating an ORP based controller requires you to focus on 2 things:
- Stable pH
- ORP setpoints
Stable pH
As we see in the ORP chart, it is important to provide your controller with a stable pH. If this doesn’t happen we can easily over or undershoot our ideal chlorine level.
Keys to having Stable pH
Alkalinity: It is incredibly important to maintain your alkalinity levels between 80 and 120ppm. If below 80 you can experience what is known as pH bounce. pH bounce does exactly what it sounds like it causes the pH to swing dramatically when small changes occur to it. This can cause you to overshoot pH setpoints despite putting very little acid in or causing your pH to skyrocket when adding basic chemicals like liquid chlorine or calhypo. On the high side of the alkalinity spectrum the water can experience pH lock. This is where the pH is locked in place at a high value and takes much more acid than normal to lower pH.
pH setpoints and feed Settings: It’s important to ensure that your feed settings are set well in order to avoid overshooting or undershooting your pH setpoint. We want it to be able to maintain pH levels very rigidly. If you are overshooting your pH setpoints look into using time cycling or proportional feed modes. If you are undershooting make sure the pH dosing equipment is adequately sized and dial back any feed settings if not set to run as on/off.
ORP setpoints
ORP setpoints are the most important part of using your ORP probe. You may be asking if
Continuing from where we left off:
ORP setpoints are the most important part of using your ORP probe. You may be asking if the chart shown before is not meant to be used to create setpoints how do I find them? While it takes a bit more work than finding setpoints for pH probes or Chlorine probes the process is fairly straightforward.
- Adjust alkalinity to between 80 and 120.
- Manually adjust pH using dosing calculations to be exactly where you want it to be, typically between 7.4 or 7.5.
- Manually adjust your chlorine to be exactly where it should be.
- Look at your controller and record the ORP value. This will be used as a setpoint for your controller.
ON/Off points and Deadbands
Be cautious about making on and off points and deadbands too wide. As a general rule of thumb, 10-15 mV usually equals roughly 1ppm depending on pH. If your on/off points or deadband is much wider than 15mV, you will experience drastic swings in residual chlorine.
Changing over time
Always keep an eye on your ORP level using the same process mentioned above for finding your setpoint. Over time, the ORP setpoint will rise and fall depending on the time of year, use of the pool, and cleanliness of the probe. If you notice the chlorine is dropping slightly too high, make setpoint adjustments in 10mV increments until you are seeing the desired chlorine residuals again.