ORP Controllers: When to Trust the Screen

Hey! It's Erin, and this is the PoolBoss podcast.

It's a Tuesday in July. You walk into the pump room on a community pool you picked up in the spring. There's a gray box on the wall, and the screen says seven hundred and twenty millivolts. That's a great number. You run your own test anyway, and your kit says zero point four parts per million of free chlorine. There are forty kids in that pool right now.

So which one is lying? Today I'm going to show you that neither one is, exactly. And once you understand why, you'll never look at one of those boxes the same way again.

Here's why this matters. A chemical controller is the one piece of equipment on the pad that makes decisions while you're not there. When it's right, it saves you a mid-week trip and keeps a busy pool safe on a Saturday. When it's wrong, it's wrong for days, and it looks perfect the whole time.

That costs you in three ways. First, chlorine. A controller that's chasing a number it can't reach will empty a drum in half the time it should. Second, the pool itself. On a commercial pool, an inspector with a test kit can close it on the spot, and it's your name on the service agreement. And third, your word. The property manager was told that pool takes care of itself. When it doesn't, you're the one standing in the pump room.

This whole episode is about one thing. The number on that screen. What it is, what moves it, and when to believe it.

Here's the plan. First, what ORP actually measures, and why it is not a chlorine reading. Second, what to do at the pad, in order, with the reason for each step. Third, that July pool, walked through from start to finish. And fourth, where this goes wrong, and the exact words to use with the property manager.

Part one. What the number means.

ORP stands for oxidation reduction potential. Forget the name for a second. Here's what's physically going on.

Chlorine kills things by pulling electrons off them. That's what oxidizing means. The probe in that flow cell has a small metal tip, usually platinum, and a reference built into the body. Water that's hungry for electrons puts a small voltage across those two. The controller reads that voltage in millivolts. So the screen is not telling you how much chlorine is in the water. It's telling you how hard the water is pulling.

Think of it like this. Your test kit counts how many workers showed up. ORP tells you how much work is getting done. Most days those two numbers move together. The whole reason a controller gets fooled is the days they don't.

The target most makers use is six hundred and fifty to seven hundred and fifty millivolts. Six fifty is the floor. Below that, the water isn't sanitizing fast enough, no matter what your kit says.

And here's how the box uses that number. Somebody gives it a setpoint, say seven hundred. When the reading falls below that, it switches the chlorine feeder on. When the reading climbs back, it switches the feeder off. That's all it does. It has no idea how much chlorine is in the pool, how many people are swimming, or what the weather is doing. It knows one voltage, from one small cup of water passing one probe.

So everything the controller does is only as good as that one reading. Keep that in mind, because every failure we talk about today is the same failure. The reading stopped matching the pool, and the box kept trusting it.

Now, three things pull those two numbers apart. And if you learn these three, you can reason your way through almost any controller problem.

The first one is pH. Free chlorine in water is really two things. There's the strong form, hypochlorous acid, and there's the weak form. pH decides the split. At a pH of seven point five, about half of your free chlorine is the strong form. At eight point zero, it's about a quarter. Same pool, same chlorine reading on your kit, and half the killing power is gone.

Your kit can't see that. The probe can. So when pH drifts up, ORP drops, and the controller does the only thing it knows how to do. It feeds more chlorine. But the pool didn't need chlorine. It needed acid. And liquid chlorine pushes pH up a little more every time it goes in, so the controller is digging the hole it's standing in.

The second one is stabilizer. Cyanuric acid protects chlorine from the sun by holding on to it. Chlorine that's being held isn't working. And here's the part most people don't know. Your DPD test counts that held chlorine as free chlorine. The probe doesn't. So on a pool with a lot of stabilizer, your kit says four, the screen says five eighty, and both of them are telling the truth. You've got plenty of chlorine, and most of it is parked.

The health codes know this, by the way. The national model code from the CDC sets the minimum free chlorine at one part per million on a pool with no stabilizer, and two on a pool that uses it. That's the code admitting, in writing, that the same reading on your kit is worth about half as much once stabilizer is in the water.

That's why a controller pool wants low stabilizer. Many operators keep it under thirty to forty on those pools, and none at all indoors. There's also a strange side effect. On a stabilized outdoor pool, ORP runs lower in the afternoon sun and climbs back after dark, with nobody adding anything. So if you take a reading at two in the afternoon one week and eight in the morning the next, you're not comparing the same pool.

The third one is the shape of the curve. ORP doesn't climb in a straight line as you add chlorine. It climbs fast at the low end and then flattens out. Going from half a part per million to one and a half moves the screen a lot. Going from three to six barely moves it at all.

Why does that matter? Because it means ORP is a very good alarm for too little chlorine, and a very poor alarm for too much. A controller can overshoot to eight or ten parts per million, and the screen will look only a little higher than normal. So never judge an overfeed by the screen. Judge it with your kit.

And there's a fourth thing, which is less a trick and more the point of the whole instrument. Dirty water. Sweat, sunscreen, urine, leaves. All of that uses up chlorine's working power before it gets to the germs. So on a packed Saturday, a controller will hold more chlorine in the pool than it does on a quiet Tuesday, to keep the same number on the screen. That isn't a fault. That's the controller doing its job. The more loaded the water is, the more chlorine it takes to reach the same reading.

It does have a limit, though. If your kit keeps climbing week after week and the screen is still only just making six fifty, the water is worn out. More chlorine won't fix that. A heavy shock or fresh water will.

One more thing about that kit, while we're here. It isn't a perfect ruler either. In one lab comparison, two well-known DPD kits tested the same one part per million sample. One read one point two. The other read two point four. So when I say compare the screen to your hand test, I mean the same kit, with fresh reagent, every time. You're watching for a change in the gap, and you can't see a change if your ruler keeps changing.

So, quick recap. The screen measures how hard the chlorine is working, not how much there is. High pH lowers it. Stabilizer lowers it. And it's sensitive at the low end and nearly blind at the high end.

Part two. What to do at the pad.

There's an order to this, and the order matters. Every step sets up the next one.

Step one. Read the screen before you touch anything. Write down the ORP, the pH the controller shows, and the time. You do this first because everything you're about to do will change the reading. Opening a valve, brushing, adding acid. You want the number the controller was living with while you were gone.

Step two. Take your water sample from the pool, away from a return, and test free chlorine and pH by hand. Away from a return, because the water coming out of a return was just dosed. It tells you what the feeder did a minute ago, not what swimmers are standing in.

Step three. Put the two side by side. This is the whole job. You're not asking whether seven hundred is good. You're asking whether seven hundred makes sense next to what your kit just said.

There are really only four outcomes. Screen in range and kit in range. Good. Log both and move on.

Screen low and kit low. The controller knows it's behind, and it can't catch up. That's a supply problem. So check the drum, then the feed pump, then the injection point. An empty drum is the most common cause and the cheapest to fix, which is why you look there first. Then the pump. A feed pump can lose prime when the drum runs dry, and it will run all day pushing air. Then the injection fitting, because liquid chlorine leaves scale that can plug it solid.

Screen low and kit fine. Now you know it isn't a chlorine shortage, so stop adding chlorine. Check pH first, because it's the fastest test and the most common cause. If pH is above seven point six, bring it down and watch the screen. Then test stabilizer. If that's high, no amount of chlorine will bring that screen up, and the fix is water replacement. And think about the temperature. Colder water reads lower too. A pool that held seven hundred in August can sit lower in November with nothing wrong.

On the stabilizer, the fix is simple arithmetic, and it helps to say it out loud to the manager. Nothing you can pour in removes it. Only fresh water does, and it comes down in proportion. Replace half the water, and you cut the stabilizer in half. So a pool sitting at eighty needs about half its water replaced to get to forty. Then find out where it came from. If someone has been topping up with tablets or stabilized shock on a pool that has a liquid feed, it will climb right back.

And if the screen is too high instead of too low? Test by hand first. If your kit agrees the chlorine is high, shut the feed off and let swimmers and sunlight bring it down. Don't pour in a neutralizer unless the pool is too high to swim in. Then find the reason. A feed pump relay can stick on. A setpoint can be left too high. Fix that, and note what you changed.

And the fourth outcome. Screen fine and kit low. That's the dangerous one, and it's the pool from the top of the show. The controller believes everything is fine, so it has stopped feeding. Suspect the probe, or the water going past the probe.

That brings us to step four. Check the flow cell. Is water actually moving through it? A clogged strainer or a closed sample valve leaves the probe sitting in the same cup of water all day. That cup holds its reading while the pool loses chlorine. You check flow before you check the probe because it takes ten seconds and costs nothing.

While you're there, look at where the sample line comes from. It should be drawing water from before the point where any chemical is injected. If it's tapped in after the chlorine goes in, the probe is tasting fresh chlorine instead of pool water, and it will read high every time the feeder runs.

Step five. Check the probe itself. There are two ways to do it.

The proper way is a test solution of known value. Most are around four hundred and seventy millivolts. You shut off the feeders, pull the probe, rinse it, and stand it in the solution. If it's off by more than twenty or thirty millivolts, clean it and try again.

Here's something worth knowing about that test. There's no reference solution up in the six-fifty to seven-fifty range where the pool actually runs. So you're checking the probe at four seventy and trusting it at seven hundred. That makes it a health check, not a calibration. It tells you the probe is alive and honest. It doesn't make the screen a precision instrument. Your hand test is still the judge.

The second way is quicker, and it's an old trick from the people who built these controllers. Turn the feeders off. Add a small amount of acid at the intake of the sample line, and watch the screen. Lower pH makes chlorine stronger, so a healthy probe should jump, and jump quickly. If the number just sits there, that probe isn't reading the water anymore.

And pay attention to speed either way. A healthy probe responds quickly. A probe that crawls toward the right number is on its way out, even if it gets there in the end. If cleaning doesn't speed it up, replace it.

Step six. Clean it the right way for what's on it. An oily film from sunscreen comes off with mild detergent or isopropyl alcohol. White scale comes off with a brief soak in dilute acid. Soft slime comes off with a soft cloth. Never scrape the tip, because that small metal surface is the whole instrument. And always follow that probe maker's own instructions, because they differ.

Step seven, and it takes five seconds. Write the install date on the probe body with a paint pen. Most probes last one to three years. The next tech in that pump room might be you, a year from now, with no idea how old it is.

Recap. Screen first, then your kit, then compare. Low and low is supply. Screen low with a good kit is pH or stabilizer. Screen fine with a low kit is the probe or the flow. And check flow before you spend money.

Part three. One pool, start to finish.

Let's go back to that pump room in July. Picture a tech who picked up three community pools in the spring. Each one has a controller, a drum of liquid chlorine, and a pH probe. The last company left no records at all.

Through May and June, the main pool looks great. The screen says seven ten or seven twenty every visit. And here's the first thing to notice. The tech is testing by hand, and the kit agrees, so nothing looks wrong. The screen number just gets glanced at. It isn't written down.

Then July arrives. Afternoon rain every day, which dilutes the water. Forty swimmers at a time. On a Tuesday, the screen says seven twenty, and the kit says zero point four.

So what does the tech think? The first thought is the obvious one. Low chlorine, add chlorine. And the tech does add some by hand, because swimmers are in the water and that can't wait. That part is right. But the tech doesn't leave it there, because of which outcome this is. Screen fine, kit low. If the controller were working, it would have seen that drop days ago and fed the pool. It didn't. So the real problem is still sitting in the flow cell.

Flow first. The strainer is clear and water is moving. So it's the probe. The tech pulls it. There's no date on it, but the manager finds an invoice, and it's four years old. The tip has a brown film. The tech cleans it and stands it in test solution. It still won't read right.

A new probe is a hundred and eighty dollars. It's the property's equipment, so the tech sends the manager a note with both readings and the price, and the manager approves it the same day.

With the new probe in, the screen drops to five sixty. That's the first true reading that controller has shown in weeks, and it matches the kit. The feed pump kicks on. By the next morning, the pool is holding three parts per million, and the screen says seven hundred.

Now here's the part I want you to sit with. How long had that probe been stuck? Nobody knows. The kit readings were in the log. The screen readings weren't. If both numbers had been written down every visit, the tech would have seen the kit sliding from three, to two, to one, while the screen sat at seven twenty and never moved. A number that never moves, while the pool changes around it, is the tell. That was visible two or three weeks before the Tuesday with forty kids in the water.

So after that, the crew logged both numbers at every stop on all three pools, along with the age of the probe. In August, the county inspector came out. The tech handed over a visit log with every stop, who tested, and both readings side by side. A hundred and eighty dollar part, and the difference between finding it yourself and having it found for you.

Recap. The stuck probe showed a perfect number. The fix was cheap. And the thing that would have caught it early cost nothing. It was a second number written next to the first.

Part four. Where it goes wrong.

Five mistakes cover most of it.

One. Raising the setpoint to fix a cloudy week. Somebody bumps it to eight hundred to push more chlorine in, the pool clears up, and nobody turns it back down. Now it overfeeds all season. The setpoint isn't a throttle. The right way to set it is in clean water. Get pH to seven point four or seven point five, get free chlorine to one or two parts per million, let the pump run, and read the screen. That reading is the setpoint for that pool. Then leave it alone. If you inherit a pool, write down the setpoint on your first visit, so you know if somebody changes it.

Two. Trusting a brand new probe on day one. A new probe needs time in the water before its reading settles. Don't judge it in the first few minutes, and don't change the setpoint because of what it shows that afternoon. Check it against your kit on your next visit.

Three. Ignoring the pH probe. The controller has two probes, and the pH one drifts as well. If it reads high, the controller feeds acid the pool doesn't need. Real pH falls, the chlorine gets stronger, and the screen looks wonderful while the water turns corrosive. So your hand pH test is checking a second probe. Calibrate that one with buffer solution on the schedule the maker gives you.

Four. Not knowing what the overfeed timer is set to. Most controllers will lock a feeder out if it has run too long without reaching the target. A properly sized feeder shouldn't have to run more than about an hour straight on a pool, or about fifteen minutes on a spa. If that alarm keeps tripping, don't just clear it and drive away. It's telling you the feeder can't keep up, or the controller is chasing a number it can't reach. That takes you right back to pH and stabilizer.

And five. Believing a dead-looking number. A reading stuck near zero usually means an electrical short. A reading pinned way up near twelve hundred usually means a broken connection or a cracked tip. Neither one is water chemistry, so don't dose for it.

Now, the pool you inherit. Treat somebody else's controller as untested until you've checked it yourself. Give it twenty to thirty minutes on your first visit, before you trust a single number. Run a full hand test and compare it to the screen. Write down both setpoints. Look for a date on the probes. Confirm water is moving through the flow cell. Check the drum level and the feed lines for leaks or air. And test stabilizer, because if it's high, the manager needs to hear that a partial drain is coming, and it's far better to hear it in week one than in week ten.

Then find out who looks at that box when you're not there. The model code expects a person to look at the controller at the start of every operating day. You're there once or twice a week. So somebody at the property is supposed to be doing the other days, and often nobody has told them. Take two minutes with the maintenance person or the manager. Show them the screen. Tell them the range it should be in, show them what a feed alarm looks like, and give them your number. That one conversation gets you a phone call on a Thursday instead of a green pool on a Monday.

And settle who pays for parts before a part fails. The controller and its probes belong to the property, the same as the pump and the filter. Most service agreements cover routine labor and chemicals, with parts and repairs extra. If yours doesn't say so, add it at the next renewal. Some operators carry one spare probe on the truck for their controller pools, so the swap happens the same day the manager says yes.

Two edge cases. On a salt pool with a controller, the reading can sag while the cell is producing. The leading explanation is hydrogen bubbles from the cell reaching the probe, though nobody has proved it. What matters at the pad is that your hand test wins. And if the cell is scaled, it makes less chlorine at full output, so check the cell before you turn it up.

The second one is a pool that was just shocked. The screen will read high for a while. That's normal. If it's over eight fifty and it stays there, check that the feed pump isn't stuck on.

Know when to stop, too. If the controller itself is throwing a sensor error, or the reading jumps around with the probe sitting in a cup of still water, that's wiring or electronics inside the box. Unless you're trained on that unit, that's a call to the manufacturer's rep. Go to hand dosing and hand testing until it's fixed, and tell the manager in writing that the pool is on manual.

On commercial pools, know what the code expects. That same model code says a pool on an automated feed still gets a hand test before opening and every four hours while it's open. Without a controller, it's every two. It also says to record the ORP at the same time as the hand test. And it caps stabilizer at ninety. Your state or county may be stricter, and theirs is the one that counts, so ask your health department. But notice what even the model code is saying. The controller cuts the hand testing in half. It doesn't replace it.

And here's what to say to the property manager, because "the probe is bad" won't get you an approval. Put the numbers in it. "The controller read seven twenty. Our test read zero point four free chlorine. The probe is four years old and fails its check. Replacement is about a hundred and eighty dollars 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.

And if they tell you the pool takes care of itself? Here's what I'd say. "It does, as long as somebody checks that it's telling the truth. That's what I'm here for."

Alright, let's wrap it up. Here are your takeaways.

One. The screen measures how hard your chlorine is working, not how much there is. High pH and high stabilizer both pull it down, and it barely notices an overfeed.

Two. The comparison is the job. Screen first, then your kit, then ask whether the two make sense together. A screen that never moves while the pool changes is a probe that has stopped reading.

Three. Most of the fixes are cheap. An empty drum, a high pH, a clogged strainer, a probe that costs less than two hundred dollars. The expensive part is always the weeks nobody noticed.

And if you only do one thing this week? On every controller pool you service, write the screen reading next to your hand test, and put a date on the probe. If there's no date, write today's date and the word "found". In a month, you'll have a gap you can watch.

That's it for today. Thanks for listening to the PoolBoss podcast. I'm Erin. See you next week.