Bob Lowry Breaks Down TDS in Pool Water

If you’ve ever stared at a “perfect” test strip while the pool still acts weird, you’re probably missing one of the quietest drivers in pool water chemistry: TDS. We dig into what Total Dissolved Solids actually means, why a simple digital meter belongs on every service truck, and how TDS can swing your saturation index enough to change your real-world results. Along the way, we unpack the old-school truth behind TDS, what the meter is really reading (conductivity), and why it is still a valuable approximation for everyday pool maintenance decisions.

We also get specific about what’s building up in the water over time. Chlorine doesn’t just disappear, it becomes chloride. Many of the products you add carry sodium, and those ions stay behind. That matters even more in a saltwater pool, where you might be running 3,000 to 3,400 ppm salt before you count everything else. We talk through why a 5,000 to 6,000 ppm TDS reading can be normal with a generator running, but can turn into a chemistry headache when the cell is off and the salt is just along for the ride.

One of the biggest takeaways is a myth-buster: the saturation index can describe scale forming or plaster corrosivity, but it does not predict metal corrosion. Real corrosion shows up in places you can see, like salt splash-out that dries into concentrated salt on decks and equipment, or motors and hardware that fail even when the index looks “fine.” We also cover when high TDS is a drain-and-dilute conversation in non-salt pools (about 1,500 ppm over starting), plus the tricky world of color bodies, organic staining, and why chlorine alone sometimes cannot fix yellow or tinted water.

If you want fewer mystery problems and faster diagnoses, subscribe, share this with a pool pro friend, and leave a review so more techs start testing what actually builds up over time.

We break down what TDS really measures, why it affects the saturation index, and why so many pool pros still ignore it. We also clear up a costly myth: “balanced” water can still corrode metal, especially around saltwater pools.  
• why TDS testing fell out of routine pool service and why it matters  
• what a TDS meter actually measures through conductivity and why it is an approximation  
• what builds TDS over time, especially chloride and sodium from common pool chemicals  
• how saltwater pools push TDS higher and what happens when the generator is off  
• why the saturation index describes plaster risk, not metal corrosion risk  
• real-world clues of high TDS such as brackish taste, stubborn color, and “can’t fix it” water  
• how color bodies and organics can stain plastics and cleaners and resist chlorine  
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Transcript

Coaching Program And Setup

SPEAKER_01 0:24

Something that you should be testing for, and in the photo on your full route, but a lot of folks don't really understand TDS and Bob Lowry one package here for you in today's episode. Are you a full service pro looking to take your business to the next level? Join the Fool Guy Coaching Program. Get expert advice, business tips, exclusive content, and get direct support from me. I'm a 35-year veteran in the industry. Whether you're starting out or scaling up, I've got the tool to help you succeed. Learn more at swimmingfullearning.com. Let's talk about something that I can't remember who it was that mentioned this. I think it was uh Terry Arco was talking, he was in a group, and he mentioned checking the TDS in the pool in that group of pool pros, and no one in the audience knew what TDS stood for. And I think

What TDS Means And Why Test

SPEAKER_01 1:12

even today, you know, when you say, what's your TDS level? A lot of pool guys are scratched their head saying, I don't even know what that is. Why has it become such a foreign thing to test for? I know that you have to have a digital meter number one, which is something a lot of people don't carry with them. Why has it become one of those testing industries that's kind of ignored for the most part?

SPEAKER_00 1:34

I've been I've been asking people to check for TDS since 1977. It is just an instrument. It's electronic, so there's no reagent. It does not need to really be calibrated. It's just an instrument, it's a tool that you should have. It is important to know TDS when you are calculating the saturation index. And you need to put the soul level or the TDS level in there to get a correct saturation index level. And a change of 2,000 or 3,000 in TDS can have a significant change to the saturation index. I think it is important. But just to understand about TDS, years ago, before electronics got so easy to make, we used to have to do what was called a total dissolved residue. And what that was was you actually took a volume of water that was measured very carefully to a volume or to a weight, and then evaporated the water away from it. And the residue that was left, you weighted. And that's what was called total dissolved residue. Then they did the same test with pool water and called it total dissolved solids. But nobody, nobody, nobody wanted to do that kind of a test. Because you had to literally evaporate the water and see what was left behind. And you had to have a scale that could weigh measure in you know hundreds or thousands of a milligram, you know, so or you'd have to, you know, evaporate a significant amount of water to get a measurable amount of residue. So they found out that they could do, for the most part, a conductivity test. And what the meter is actually measuring is conductivity, and then they convert conductivity into parts per million with an algebra. And it is an approximation of the total dissolved solids. But there are things that you can add to the water that make a solid but don't but don't ionize. And in that case, it doesn't increase the TDS with your meter, but if you did an evaporation test, it would increase the the solids. Electronic test is an indication of what the TDS really is. But it stands for total dissolved solids or just total dissolved stuff that's in the water. And it's things that are dissolved. It's not things that you can see. Those are suspended particles or suspended solids. So these are not suspended solids, they're dissolved solids, which means you can't see them.

SPEAKER_01 4:33

And so is it kind of simplified to say that total dissolved solids are the junk left behind after some water evaporates out of there?

SPEAKER_00 4:41

Yeah, it's the same thing as if you kept using a teapot on the stove and boiled water and then used it and boiled water and used it, you would eventually see some stuff in the bottom of the kettle. That's dissolved solid.

SPEAKER_01 4:55

And so what does that consist of, basically? You know, in layman's term, the base level.

SPEAKER_00 5:00

The biggest constituent is probably chloride, because all chlorine that you add to the pool becomes chloride. The other big thing that's in there is sodium. Because, as you have guessed, many chemicals that you add to the pool start with the word sodium. And the sodium just comes apart as an ion and it's in the water and it's it's ionized and it's part of TDS. So when you add sodium chloride, you get TDS. When you add sodium bicarbonate, you get sodium. When you add soda ash, which is sodium carbonate, you get sodium. So every time you add something, you get sodium. When you add chlorine, you add sodium hypochlorite, so you get sodium and chloride. Everything you add to the pool eventually becomes a dissolved solid. Eventually.

SPEAKER_01 6:03

And so with a saltwater pool, your TDS level is going to be pretty high because of all the salt that's in there for the generator to operate.

SPEAKER_00 6:10

You don't need 3,000 to 3,400 parts per million of salt for the generator. And then your uh starting water, your storage water, has three to five hundred parts per million of TDS in it. And then the chemicals that you add to maintain the conditions in the pool, uh, those can add up to a maximum of a of 1,500. So if you have a a saltwater pool, it's possible

Salt Pools, LSI, And Metal Corrosion

SPEAKER_00 6:41

that you could have five to six thousand parts per million of TDS if you include the salt. And that's a lot. 1% is 10,000 parts per million. So when you think about 5,000 parts per million, it's a half of 1%. So when you get that high, you're bordering on what's called a solution and not just water anymore.

SPEAKER_01 7:06

With the salt water generator working, that's not a problem. But I've had in my route people that just turn off their generator because they're not working, they don't want to replace the cell. And with a TDS level of five or six thousand in there without the generator working, that becomes a pretty big problem in maintaining the pool and the chemistry.

SPEAKER_00 7:24

It does because it changes the saturation index. But understand, for you guys that are doing the saturation index and stuff, understand this about the saturation index. When we say that the water is corrosive or scale forming, we mean that it will form scale on anything. But when we use the word corrosive, it means it's corrosive to the plaster. It does not mean that it is equipment corrosion. And equipment corrosion is a whole different thing. It's water velocity, it's it's the amount of stuff that's dissolved in there, it's the on you know, the ionization constant of the water, and then that's what corrosion of metal is. And the saturation index does not predict corrosion of metal. And you've got a lot of salt in there, and the salt can get corrosive, period. And especially around the pool, and particularly if a guy's not going to use his chlorine generator, you should get rid of a lot of that salt that's in there because all the salt that splices out of the pool, when that water evaporates, it still leaves behind the salt. Then instead of 5,000 parts per million of salt, now it's 100% salt because the water is gone. And it's just like you went outside and poured some Morton salt all over your furniture and your deck, you know, and that stuff is corrosive.

SPEAKER_01 9:04

I gotta tell you that I was testing a solar surface skimmer for a company, and they made these motors, and these motors were not rated for, I guess when they tested it, they didn't realize that Americans use salt water generators in their pools. And after about two months, the motors totally corroded from being in my pool. And then I sent the manufacturer a picture of it, and I said, Hey, these motors aren't going to work here in America. And so they beefed them up, they're much better now. And when I take my robotic pool cleaner out of my pool, I noticed that when I leave on a deck to dry, it has all these white splotches on it from the salt in my pool. And so that's what you're talking about. You can notice, you can see the corrosion um right away in certain situations, like those aspects.

SPEAKER_00 9:47

So you see that there's the point exactly, is that you noticed that that motor corroded, but I would bet that your pool had a had a good saturation index. So the saturation index said everything was okay, but that motor corroded. And that's what I'm trying to talk about is that the saturation index is not an indication of corrosivity for metal.

SPEAKER_01 10:18

And so for a non-salt pool, what would be the maximum TDS you can get to before you have to drain it? Because I know that a lot of people have problems, like the pool's not holding chlorine or it's getting weird, having weird problems.

SPEAKER_00 10:31

In the classes I used to teach before we started the PCTI, I used to tell people that the TDS is kind of a catch-all for the things that we don't test for. And so it's all the stuff that we don't really check for. So when it gets high, you can have water that begins to taste a little brackish or salty. You can have clear water, but it has a color to it. And it's due to TDS. And it can be very difficult to get rid of those kinds of things. They

High TDS Symptoms And When To Drain

SPEAKER_00 11:06

can be even color bodies. You know, I've had in the past two years, in the past year, I've had three service techs call me up with serious problems with colored water. And they have tried everything. We even tried a lot of things that I suggested that didn't work either. Found out that they are color bodies, and getting rid of colored bodies isn't just a matter of throwing in some chlorine. Not all colored bodies can be bleached. And so getting rid of them, they are much smaller than the filter can remove, and chlorine's not powerful enough to destroy them. And so we've had to go to ozonation and other methods of getting those colored bodies out of the water. One of the big things that we did do that worked a lot was we we use one of these little solar-powered surface skimmers. I don't know the brand of it, but they actually are powered by the sun. You put them on the surface and they run around the pool picking up just what's on the surface of the pool. And we were, this guy that we did this test with, he was surprised at because he thought his pool was clean and his skimmers weren't working that good, which indicated he had a hydraulic problem. But he found all of these yellow flowers that had been falling into the pool and then they disintegrate and end up in the filter, and the color still empties into the pool. And when we put this skimmer on, the first day he used it, he opened it up and there was a whole bunch of these little yellow flowers in there. That was his problem. He was having yellow water, and he couldn't get rid of them. We tried a lot of things, including switching from sand to DE, using an ozonator, using UV, using a non-chlorine shock. We tried a lot of things till we finally got an ozonator and actually got his pool to the hydraulics in his pool to be better, so his own skimmer would work. But then they put in this this other skimmer, and that helped his problem a lot. Getting those flowers out as they were introduced seemed to be the ticket.

SPEAKER_01 13:22

Huh, that's interesting. So the TDS level of a pool, what would be the when you would drain a pool?

SPEAKER_00 13:28

What we say is a maximum of 1,500 parts per million over starting. You know, TDS are meters, but they don't really need any calibration. You buy one and you'll have it for 10 years. It's not, it's not something that needs replacement all the time. It's not, you know, it's just on your truck, but you need to know the TDS from every one of your pools.

SPEAKER_01 13:52

Yeah, I think that's important. What about like the I've had people send me pictures of the automatic cleaner? They pull it out of the pool, like a Polaris cleaner, and it's covered with this kind of a brown staining. Um, and you can see it on the walls and on the hoses of the cleaner. Is that caused by the TDS or is that something else?

SPEAKER_00 14:08

Well, yeah, it may be an organic stain. And organic means it would it used to be alive. And so you get organic stains under under many different conditions. Most often they turn up to be brown, but it can be leaves and things like that. And you get tannins and color bodies in there, and then they react with stuff and get oxidized and and form a precipitate, just like copper and iron do. But they tend to do that where water is kind of moving by and

Brown Stains On Cleaners And Wrap

SPEAKER_00 14:41

stuff like that. So we see it a lot of times that on the skimmer where the skimmer gets a stain on it, the tool vac and stuff like that get a stain on it. And those plastics are difficult to get the stains out. You can pour chlorine right on it, and the stain will stay right there. Uh, it can be difficult to get those stains off of those stains.

SPEAKER_01 15:01

If you're looking for other podcasts, just go to my website, swimming for learning.com. On the banner is a podcast icon. There'll be a drop down menu with over 1900 podcasts for you there to listen to at your leisure. And if you're interested in the coaching program, you can learn more at poolguycoaching.com. Thanks for listening to this podcast. Have a rest of your week. God bless.