ORP stands for oxidation-reduction potential and it basically measures the effectiveness of the sanitizer (usually Chlorine) in your swimming pool. ORP can also be used for Bromine and Ozone as well as other Oxidizing sanitizers.
ORP is widely used in Eurpoe as the standard for safety in swimming pools. The US is slow to adopt this standard and still use PPM of Free Chlorine as the standard. ORP is a proven method of safety in water and has a long track record dating back over 40 years.
So do you need to test for it? It depends. If you are a pool service professional you should test for ORP at your commercial accounts and some heavily used residential accounts. If you are a homeowner and want to be 100% certain your pool is safe to swim in, then getting an ORP tester is not a bad idea.
You can basically keep your sanitizer effective by keeping a Free Chlorine level of 2-3 ppm, keeping the pH at 7.2-7.6 and having your Cyanuric Acid (Conditioner) at 30 ppm to 100 ppm. The main factor is the pools pH. Chlorine is much more effective at lower levels and if the pH is over 8.0 most of the chlorine in the pool will be ineffective. The fast acting form of chlorine HOCI works best at lower pH and it is converted to a slower acting sanitizer (OCI) at high pH levels. HOCI is 80 to 300 times more effective than OCI.
But keeping your pH at those ideal levels can sometimes be challenging. Certain sanitizers like liquid chlorine can raise the pH, low Alkalinity can cause the pH to bounce, not check the pH weekly and a Salt Water Generator will raise the pH. So testing the ORP is a good way to tell is the chlorine is working effectively in a pool.
A good ORP reading for a pool is from 650 mV to 750 mV. If you test tap water the ORP is in the 300 range, safe for drinking but it won't kill any pathogens. Studies have shown that an ORP level of 650 mV or higher will kill just about anything in a mater of seconds, even milliseconds. An ORP level under 500 mV will take one hour or more to kill the pathogens. So the ORP level is important in determining if a pool is safe to swim in.
PPM or ORP?
Technically ORP is not designed to replace a PPM chlorine test. Yes a low PPM will translate into a low ORP number and a high PPM will be reflected in a high ORP number, but it is always important to know the PPM of Free Chlorine in your pool. The DPD and OTO test kits will only show you the amount of Free Chlorine in your pool, and not the effectiveness of the chlorine. The ORP tester will test for the effectiveness of the chlorine or sanitizer in the water. There is a difference and it should be noted. Even with a Free Chlorine reading of 3.0 PPM the chlorine can be ineffective if the pH is really high and the ORP test will reflect this. A reagent test kit cannot determine if the chlorine actually is working in the water. There is also no way to translate the ORP reading into PPM as they test for two different things in the pool water.
Again let me stress that ORP is more important in a commercial pools and a residential pool that gets heavy use. But all the same if you feel better knowing your pool is 100% safe invest in an ORP tester.
I recommend the HACH brand ORP Tester. HACH has been making industrial and scientific water testing devices for over 60 years. To learn more about the HACH Testers visit their website:
To learn more about ORP watch this video:
Here are some good articles on ORP: