ORP and pool controllers: should you trust one?

ORP stands for oxidation reduction potential. It's a voltage, read in millivolts (mV), that shows how strong the water's sanitizer is right now. Higher means the chlorine is killing germs and breaking down waste faster. Most commercial pools aim for 650-750 mV, and 650 mV is the floor most controller makers set.

A controller reads ORP from a probe and turns a chlorine feeder on when the number drops. Most units also read pH from a second probe and feed acid or CO2. On a busy pool, that means chlorine goes in all day as swimmers use it up. It keeps going between the once or twice a week you stop by.

So should you trust it? Trust it the way you trust a truck's gas gauge. It's right most of the time, and you still check it. The probe is the weak spot. It sits in pool water every hour of the day, and over 1-3 years it gets coated, drifts, and can stick on a number. The only way to know it's still telling the truth is to test the water by hand at every visit and compare.

Here's what the number on the screen usually means. These are rough bands for a chlorine pool with pH around 7.4-7.6. Your controller maker and your local health code may set their own numbers, and theirs win.

ORP and free chlorine measure two different things. Your test kit tells you how much chlorine is in the water, in ppm. ORP tells you how hard that chlorine is working. The same 3 ppm of free chlorine can give very different ORP readings, depending on the rest of the water.

pH is the first thing that moves it. Chlorine works best at lower pH. When pH climbs from 7.4 to 8.0, much less of your chlorine is in its strong form. ORP drops, even though your chlorine test hasn't changed at all. A controller sees that drop and feeds more chlorine, when the fix was acid.

Stabilizer is the second thing. Cyanuric acid holds on to chlorine to protect it from the sun, and that slows it down. A pool at 80 ppm stabilizer can read low ORP with plenty of free chlorine in it. The controller keeps feeding and never catches up. You end up with 8 ppm on the test kit, a low number on the screen, and a chlorine drum that empties twice as fast as it should.

A controller pool should run low stabilizer, or none on an indoor pool. Many operators keep it under 30-40 ppm on controller pools. If it has crept up, the only fix is draining and refilling to bring cyanuric acid down. Check your local code too, because some cap it lower still.

An ORP probe has a small metal tip, usually platinum, and a reference inside the body. Both wear out. Oils, sunscreen, scale, and biofilm coat the tip. The reference slowly goes bad. A worn probe reads slow, drifts, and can stick near one number no matter what the water does.

That's the dangerous one. A stuck probe that shows 720 mV tells the controller the pool is fine, so it stops feeding. The water keeps losing chlorine. The screen never changes. Nobody knows until someone tests by hand, or until a health inspector does it for you.

Most ORP probes last about 1-3 years, and a new one runs about $100-300. Mark the install date on the probe body with a paint pen. That way the next tech knows how old it is without guessing.

Here's a check routine that catches drift early:

Raise low ORP by fixing the water first and the probe last. Most low readings come from low chlorine, high pH, or too much stabilizer. Work through them in this order, and test by hand at each step so you know which one it was.

Take an operator running 52 pools around Fort Myers. In the spring, he picked up three HOA pools in one community. Each had a controller, a liquid chlorine feed, and a pH probe. The last company left no records.

In July, the main pool's controller showed 720 mV all week. His tech's test kit showed 0.4 ppm free chlorine. Afternoon rain was diluting the water every day, and the pool was full of swimmers. The controller wasn't feeding, because it thought the pool was fine.

The ORP probe was four years old and coated with a brown film. He cleaned it, and it still wouldn't read the test solution. A new probe cost $180. Once it was in, the screen dropped to 560 mV, which matched the test kit. The feeder kicked on. By the next morning the pool held 3 ppm and read 700 mV.

After that, he had his crew log both numbers at every stop on all three pools. That's the screen reading, the hand test, and the probe age. When the county health inspector came out in August, the operator handed over his own visit log. It showed every visit, who tested, and both readings side by side.

Log the controller's readings and your own hand test at every visit, with the time and the tech's name. The controller's memory doesn't prove what you did. Your log does. Most controllers only keep a few days or weeks of data, and it's the property's box, not yours.

On a commercial pool, the log is also your defense. Health codes vary by state and county, so ask the local health department what it wants and how often. Many want a hand test at set times, even on a pool with a controller. A record with your name and a timestamp is what shows you did it.

The screen reading is worth nothing if it lives on a sticky note. It belongs in the same record as the hand test, so you can see when the two start to split. Keeping a per-visit chemical record on every pool is what makes that split show up in week two instead of at inspection time.

In PoolBoss, you can add ORP as its own reading, in mV, with a target range of 650-750. The tech types it in from the app along with free chlorine and pH. A reading outside the range is marked on the visit and in the pool's reading history. PoolBoss doesn't connect to the controller. The tech enters what the screen shows, which also means somebody has to look at it.

That record is what protects you if something goes wrong. The case for why the written record is the thing that protects you is even stronger on a commercial pool, where an inspector can close it on the spot.

Treat a controller you inherit as untested until you've checked it yourself. On the first visit, spend about 20-30 minutes on it before you trust a single number.

Tell the property manager in writing as soon as a probe, a feed pump, or the controller itself needs money spent on it. On an HOA or apartment pool, the equipment belongs to the property. Your agreement should say who pays for parts. If you're still learning how these accounts work, read up on taking on commercial and HOA accounts before you quote.

Send a short note with the readings that show the problem. For example: "Controller read 720 mV. Our test read 0.4 ppm free chlorine. Probe is four years old and fails its check. Replacement is about $180 plus labor." A manager can approve that in one reply. Save the note with the visit, so the record shows you raised it and when.