Correct Levels For Vinyl Inground Pool Using Chlorine

by Jane
(Quakertown, PA)

I have been to many pool stores and seem to get different answers to what are the correct levels for Free Chlorine, Total Chlorine, Alkalinity, PH, Cyanuric Acid, Calcium Hardness, etc.

What should the correct levels be for a vinyl, inground pool (20 x 40) using regular Tri-Chlor 3" tablets? I do understand that Tri-Chlor tablets raises your Cyanuric Acid (stabilizer) level.

Tri-Chlor also lowers your PH.

Would the levels differ if I would change to Calcium Hypochlorite tablets? I do understand that Calcium Hypo raises your Calcium Hardness. Calcium Hypo also raises your pH.

I strip test my pool daily, and I test with a Taylor liquid test at least once a week (depending in weather).

Pool Size: 20 x 40 retangular
Filter: Waterways DE
Last readings:

Free Chlorine - 3

Total Chlorine - 5

pH - 7.6/7.8 (should be lower as we just had rain)

Alkalinity - 90

CYA - 50

Calcium Hardness - 250

My pool water is crystal clear. I do not have any problems, but would like to know the correct guidelines.

Currently I am using the chlorine product "Sustain" made by PPG. I will probably be changing to regular Tri-Chlor tablets next season as the Sustain is very expensive.

The Sustain is a three part chlorine which you have 3" pucks (Calcium Hypochlorite), 1" pucks for shock (Calcium Hypochlorite) and a third ingredient called "Summer Shield" which you put in at the beginning of the year and this "Shield" is supposed to gather some of the chlorine and hold it in reserve until it needs to be released when dirt gets into pool or algae starts to form.



Thanks for the question Jane. And very nicely written.

The free chlorine is how much chlorine you have in the pool to combat the algae, organic matter, bacteria, etc...

Total chloirne is the toal amount of chlorine in the pool. Some of it is being used up to kill the bacteria.

There's another reading that important as well. It called the combined chlorine, or chloramine reading. This is part of the chlorine equation. But for simplicity purposes, the two things I concern myself with are the free chlorine and chloramine level.

There's all of these mathematical equations that a pool owner can do, and it can get pretty confusing.

Concern yourself with the chlorine level and the combined chlorine. The combined chlorine (CC) is how much bacteria, organic matter is in the pool. Every pool store knows this reading and should be testing for it.

A test strip won't test for this. You need a good dropper kit like the Taylor FAS-DPD K-2006 kit. This is the one I've used since 1999, and the Oregon Dept. of Health uses when it tests commercial pools.

Pool Water Testing

Water Testing Kit

Your readings should be this:

Chlorine 1.5 - 3.5ppm

CC 0 - 0.6ppm

pH 7.6 - 7.8ppm

Alkalinity 80 - 100ppm. 120ppm is the top you'll want to go.

CYA 30 - 50ppm

Calcium Hardness 150 - 250ppm.

This is mostly for plaster pools. Water wants calcium and will take it from wherever it can get it. Plaster can be ripped out of the floor and walls if the calcium level is not satisfied in a plaster pool. For a vinyl pool, 150ppm is fine.

Total Dissolved Solids (TDS) 0 - 1500. Anything over 1500 needs a partial water change.

The chances of getting a green pool are slim if you keep these readings. Also, the Dept. of Health has come by at least 8 times since I've been the pool operator for the Y.

I keep the chlorine between 3.5 - 4.5ppm because it's a heavily used commercial pool, but that's the chlorine level they want. Each and every time I've perfectly passed their inspection.

I've also used these numbers when I had my pool route in Arizona and it works every time. These are real world numbers from someone who has actually taken care of, maintained, and cleared up hundreds of commercial and home pools.

The CC number let's you know when it's time to shock the pool. This should be rare bcause if the chemicals are kept at these levels, there shouldn't be any reason to shock the pool.

Shock the pool when it needs it, not because it's the weekend and you have nothing better to do.

Tri-chlor tabs can increase the CYA. For every 10ppm of chlorine added with Trichlor tabs you'll raise the CYA by 6ppm.

Stick with normal Tri-chlor tabs. Don't get fancy. Pool stores would love to sell you $500 worth of products.

Check this link out:

Do You Really Need All These Chemicals For A Pool Or Are They Just Trying To Get Your Money?

You can also use bleach to chlorinate your pool. It's the same thing as pool chlorine, just a watered down version. Both are sodium hypochlorite.

Cal. Hypo. pucks can raise your hardness, but it does take some time. Again, with a vinyl pool, you can start lower because having an elevated CH is not that important.

They can also raise the pH level, espcially if you use it for regular chlorine. It has a pH level of 12. It's best to shock with liquid chlorine. If you shock with chlorine granules, the pH goes through the roof.

Tabs will lower the pH, but again, it takes some time. If you use the pool on a regular basis, the splashing around burns off the carbon dioxide (CO2) and will cause a rise the pH level. If it get down to 7.2 - 7.4, there's still no worries. Anything under that and you'll want to raise it back up.

Swimming Pool pH Levels

Pool pH

Your pool is crystal clear because you're in the right chemical zone. With the exception of the CH being a bit high, keep doing what you're already doing. Remember the only way to reduce CH and CYA is a partial drain and refill.

Remember to start the CH low (if you have soft fill water) in the Spring, maybe even 50ppm. It will take some time to reach the 150ppm mark.

To post a reply, or if you have a similar question, you can see your post "Correct Levels For Vinyl Inground Pool Using Chlorine" on the Q&A page in the "Chemical Questions" category.

Swimming Pool Questions and Answers

Check back to this post for updates or answers.

Hope this helps and have a great rest of the Summer.


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