Cannot Get A Free Chlorine Reading

by Robyn
(Columbus, OH)

We have a 32,000 gal. inground pool that we cannot get a free chlorine reading on. Alk. (good), ph 7.0, and stabalizer (good).

I realize pH is a little low too but can't seem to keep it any higher and still no chlorine reading. I use omni shock and recently just added 4 gallons of liguid chlorine to try and unlock whatever is happening here.

Pool is clear with no noticable signs of algea. Have had pool man check several times but they just keep telling me to add more shock. They can't figure out why pH won't stay up.

Should I keep adding sod. bicarbonate to get pH up before adding any more shock?


Thanks for the question Robyn

I'd like to have your complete chemical readings:

Chlorine, CYA (cyanuric acid/stabilizer), pH, Alkalinity, Calcium Hardness, and Metals (iron and copper). It makes troubleshooting much easier and the process of clearing up your pool will go much faster.

You can get this done at your local pool store.

With readings for the alkalinity and stabilizer as "good" I can't make an accurate diagnosis.

The pool guy keeps telling you to add more shock? I believe this is wrong advice. Your chlorine may be rendered ineffective if the stabilizer is either too high or low. The right reading is between 30 - 50ppm.

There's a relationship between the chlorine and CYA that must be met. The pool chlorine needs to be 7.5% of your CYA and that is 30 - 50ppm. As CYA goes up the chlorine level must go up to match it. If the CYA reading is 90 - 100ppm, then the chlorine must be 7 - 8ppm to match it.

Too much CYA and you'll need to do a partial drain the refill. There's no chemical to reduce the CYA. Too low CYA - you'll want to shock with Dichlor until the CYA get to 30 - 50ppm, then switch back to regular chlorine.

Concerning the pH, how much are you using? Are you putting it in the pool all at once, as you should, or over a period of time? Are you making a slurry and broadcasting it around the perimeter of the pool with the pump on? Are you allowing for one full turnover of the water, about 8 - 10 hours before taking another test?

All of these can affect you pH readings.

Swimming Pool pH Levels

Pool pH

To raise the pH level 0.2 per 20,000 gallons, you'll use 2.5 lbs. of sodium bicarb. 20 Mule Team Borax and soda ash will do the job as well.

Be sure to broadcast it around the pool with the pump on. Any water features such as slides or waterfalls should also be on.

If the pH of the fill water is low, around 7.2 or so, you'll need to make pH adjustments periodically. Most water municipalities run the pH between 7.2 - 7.4.

You used the words "unlock whatever is happening here." There's no such thing as chlorine lock, chlorine block, chlorine mock, chlorine dock, or anything else like that.

Maybe some pool guys think there's a little gnome at the bottom of the pool taking the chlorine away and putting it in a safe and "locking it up". The meaning of chlorine lock is, "I don't have a clue but I need to tell you something in order to make you think I'm smart."

This isn't against you, but the person(s) who told you this. It irritates me when so called "pool guys" tell pool owners this stuff.

Again, many times it's simply either too high or low CYA. You can pour 100 gallons of chlorine in a pool (the chemical companies would love if you did) but if the CYA is too high or low, and the pH and alkalinity aren't right, you're not going to get a chlorine reading after 48 hours. If you do, it won't stick. You need to have the CYA between 30 - 50ppm and the chlorine between 1.5 - 3.5ppm.

Once again, too high CYA above 70 - 80ppm, do a partial drain, refill, test, and balance everything out. Too low CYA, shock with Dichlor. Get the CYA to 30 - 50ppm, then switch to regular chlorine.

To post a reply, or if you have a similar question, you can see your post on the Q&A page in the "Chlorine" category.

Check back to this post for updates or answers.

If you need immediate assistance (within 24 hrs) or for emergency personal assistance, you can make a donation of your choice and I'll answer your questions by phone.

Contact Me


Hope this helps and have a great Summer.


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