Fish Out of Chlorine

How to Fix Your Chlorine Lock

Posted in Uncategorized by Andrew Cockerham on May 27, 2008

This week’s Science and Tech Tuesday is a short chemistry lesson. One of the least exciting things to hear when you go to a pool store is, “You have a chlorine lock.” As noted in an earlier post, this means you’ll be spending anywhere from fifty to two hundred dollars before your pool is anywhere near ready to swim in. What I’ll try to do in this post is explain exactly how that money is going to fix your problem (If you’re unclear on what a chlorine lock is and why it’s bad, please see my earlier posts on the subject).

The important thing to remember is that there are many ways to approach this problem, and many variables which may not be covered in this explanation. Your pool professional will be able to help you better than I can, because he or she can ask questions about the condition of the water, previous attempts to solve the problem, etc. Basically, this is for informational and educational purposes only. So don’t sue. Now, to business.

We’ll start with the easiest case: a simple chlorine lock in a 10,000 gallon pool. Your numbers might look something like this:

Free Chlorine: 2.3 / Total Chlorine: 8

Subtracting the amount of free chlorine from the amount of total chlorine, we find that you have 5.7 ppm combined chlorine in your pool.  We need to oxidize that 5.7 ppm in order to break the chlorine lock. We have two choices: first, we can use a chlorine-based shock product to raise the chlorine level by 10 ppm per ppm of combined chlorine (in this case, we’ll need to raise the chlorine levels by at least 57 ppm).  This is called “breakpoint chlorination” and is relatively straightforward, simple, and effective.  There is a downside, however. If, for some reason, you don’t reach the breakpoint, you’ll have just made the problem worse.  So it’s best to aim a little high.

Breakpoint Chlorination

To solve your chlorine lock using breakpoint chlorination, we’ll need to do some calculations. Ideally, we’d be doing these in metric, but pool chemicals generally come measured in pounds, so we’ll just use those for now. OK, so one gallon of water weighs about 8.34 lbs. So we’ll multiply 10,000 (the number of gallons in your pool) by 8.34 to get the approximate weight of the water in your pool.

Then we’ll divide the result into 1,000,000, thus: 1,000,000/83,400=11.99, or about 12.  This number is the ppm each pound of added chemicals will add to your water. One pound of chlorine, in this case, will raise the chlorine level by about 12 ppm.

Next, to find out how many pounds of chlorine we’ll need in order to raise your chlorine level by 57 ppm, we’ll divide 57 by 12, which will give us 4.75.  So we’ll need to add at least 4.75 lbs of chlorine to break your pool’s chlorine lock.

Unfortunately, every product has a different amount of available chlorine, so we’ll need to do one more calculation. Take the number of pounds of chlorine we’ll need (4.75), and divide it by the percentage of available chlorine in the product you want to use. If, for example, you’re using HTH Poolife’s TurboShock (75% available chlorine), you’re figures will look like this: 4.75/0.75=7.31, or 8 to be safe.  So you’ll need to throw in at least 8 bags of TurboShock. 

In a single equation, these calculations will look like this:

(10*(Total Chlorine-Free Chlorine))/(1000000/(Number of Gallons*8.34))/Percentage available Cl in Product

It may look complex, but try plugging in the values, and you’ll find it’s quite simple.

Non-Chlorine Shock

Our second option is really only a first step.  That is, we’ll still probably have to use the first option, but we’ll need less chlorine. For this option we’ll use a non-chlorine shock like BioGuard’s OxySheen.  This product will oxidize the combined chlorine in your pool, but won’t add any more chlorine to the water.  On the up side, it’s not an “all or nothing” proposition: you can take half measures in this case.  In fact, since OxySheen is relatively expensive, you might want to take half measures. 

Let’s take the previous example pool: 10,000 gallons, 2.3 ppm free chlorine, 8 ppm total chlorine. Once again, we have 5.7 ppm combined chlorine present in the pool. 

In this example, weight is less important, so we’ll simply divide the number of gallons by 10,000, then multiply the result by 5.7 ppm. In this case, we’re left with 5.7, since 10,000/10,000=1.

Finally, we’ll multiply the last result by two, giving us the amount of product necessary to oxidize 5.7 ppm combined chlorine: 11.4 lbs.

Once again, in an equation, it would look something like this:

(((Total Chlorine-Free Chlorine))*(Number of Gallons/10000))*2=Required Lbs. of OxySheen

This equation works only for this particular product, although it could no doubt be adapted to fit any non-chlorine shock. 

Using this much OxySheen would oxidize all the combined chlorine in the pool, but we would still need to perform a superchlorination.  The difference, of course, would be that you’d need to add far less chlorine this time.

More next time on a chlorine lock couple with a chlorine demand…the beast of them all.  Oh, and here are a few exercises in case you’re interested in practicing this math:

  1. Pool size: 15,000 gallons/Total Chlorine: 5/Free Chlorine: 3/Product used has 47% available Cl
  2. Pool size: 25,000 gallons/Total Chlorine: 3/Free Chlorine: 0.5/Product used has 57% available Cl
  3. Pool size: 30,000 gallons/Total Chlorine: 9.5/Free Chlorine: 0/Product used has 75% available Cl

I’ll post answers tomorrow for any who are interested.




14 Responses

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  1. JACK FINNEGAN said, on May 28, 2008 at 3:25 pm

    My pool man says that I have ” chlorine lock ” Total chlorine is 5 ppm
    Pool is in ground concrete +- 40,000 gal capacity. When opened, water was filthy, heavy debris even with spring tension mesh cover. Water is now very clear, after changing DE FILTER many times. I used 6- ! lb bags of shock to this point. Please advise what your solution to this dilemma should be.

  2. fishoutofchlorine said, on May 28, 2008 at 5:12 pm

    OK, I’m operating under the assumption that your free chlorine is zero right now, and that you’re sure the total chlorine is no higher than five (some strips only read accurately up to about 5 ppm, so it’s possible the actual reading is higher). That said, here’s what I might recommend:

    Start by adding about 16 Lbs of a non-chlorine shock (I’m most familiar with BioGuard’s OxySheen, so I’m not sure on exact amounts for other products, but they will help the problem as well). That should either a.) bring your chlorine level down to a level where it can be read more accurately; or, b.) bring you down to 2 or 3 ppm combined chlorine (or it may do both–it’s hard to tell right now. At that point, use the equation from this post to calculate how much chlorine-based shock you’ll need for breakpoint chlorination. If you’d rather not, just re-test about 24 hours after you dump in the OxySheen, then post another comment with the new readings. This is something that has to be addressed over time, so let me know how it goes. Good luck!

  3. mc said, on May 30, 2008 at 6:54 am

    I have been in combined clorine for 3 days now I have added 24 gallons of liquid shock I did a test strip this morning no clorine reading and the water smells horrible.

  4. fishoutofchlorine said, on May 30, 2008 at 9:20 am


    It’s hard to suggest any course of action right now, since I don’t know how much combined chlorine you have in the water or the size of your pool. That said, I would definitely recommend using a granular shock if at all possible. The 24 gallons you added to the pool introduced less than 20 pounds of chlorine to the water. That apparently was not enough to reach the breakpoint required for chlorination. (I’ve dealt with pools where more than fifty pounds of chlorine was needed) A granular shock will give you at least 47% available chlorine, as opposed to liquid shock’s 10%.

    There’s a slight possibility that the chlorine level is so high that it’s bleaching out the test strip, but your comment that the water smells horrible suggests that’s not the case. If you can, take a sample of your pool water to a local pool store with a computerized analysis system. They’ll be able to measure all the variables more accurately.

  5. jeff said, on June 6, 2008 at 12:34 pm

    Just an FYI, you have a typo here: “If, for example, you’re using HTH Poolife’s TurboShock (75% available chlorine), you’re figures will look like this: 4.75/.075, =7.31or 8 to be safe. So you’ll need to throw in at least 8 bags of TurboShock. ”

    It should read “4.75/.75=7.31” Otherwise you might get someone adding 73 pounds of shock!

  6. fishoutofchlorine said, on June 6, 2008 at 8:08 pm

    Thanks for the tip! Yeah, 73 pounds would be quite something. Although I have seen pools where you would need that much… I’ve made the change, so now the numbers will agree with the results.

  7. Sandy said, on June 11, 2008 at 4:59 pm

    I have done a pool test and it shows my total chlorine at very low and free chlorine at 10+. What do I need to do. This is a 1800 gallon pool. It is one of the small blue self set up pools.

  8. fishoutofchlorine said, on June 11, 2008 at 5:15 pm

    It’s not possible to have more free chlorine than total chlorine (as a parallel, a baseball player can’t have more home runs than base hits; for another, if you have ten apples total, you can’t have eleven green apples–unless you forgot to count one of the apples the first time around). If, on the other hand, you’ve simply switched the figures (total: 10+, free: 0), start with some non-chlorine shock until you can get below 10 ppm. It’s very difficult to get a good reading above 10 ppm. Don’t use too much, since you have a very small pool. Once you get your total chlorine levels down to around 5 ppm, use the equations in the post.

  9. mel said, on June 11, 2008 at 6:24 pm

    I have a 30 foot round almost 30,000 gallons. The pool is clear however, our pool store says it needs about 50 pounds of shock additional. We just noticed that the filter seems to have a small crack it in so it is not filtering properly. We had a similar problem with the chlorine right before we closed last year and it need mega pounds of shock; however, the pool never had debris or was even cloudy. After we closed for the winter, we expected it to be cloudy. When we pulled the cover off it was crystal clear. We did have to add water to the pool (tap). Once it was at the appropriate level we did the required opening of 24 scoops of CLC. The pH is high which was expected but the chlorine is 0/0 total/free.

    Any ideas

  10. fishoutofchlorine said, on June 11, 2008 at 6:42 pm


    Thanks for your post. I have a few questions before I recommend anything:

    First, Was your pool store’s recommendation based on a chlorine demand test? You may have what is called a “chlorine demand,” which can occur in connection with a chlorine lock (a problem you don’t have), but is a separate problem. It can be caused by algae and other cloudy-making contaminants, but other cause may include fertilizer residue, pollen, even chlorine-removal chemicals… I.e., it can happen even with clear water. If your store performed a chlorine demand test, go with the 50 pounds they recommended. You’ll need that much before you can register any free chlorine reading. If not, read the rest of the questions.

    Second, How long has it been since you shocked the pool, and how long since you opened?

    Third, What percent available chlorine does your shock have?

    Fourth, How much CYA (Cyanuric Acid: Stabilizer) do you have in the water?

    Finally, as I’m sure you’re aware, you should fix your filter asap.

    Post back and we’ll go from there.

  11. Frank Bongiovanni said, on June 14, 2008 at 8:54 am

    Help! I have 22000 gal vinyl liner pool. Opened June 5th, no chlorine reading, but pool is clear, have not shocked since june 9th. Water tested ph 7.4 CYA about 60, CH 400, Alk 120. Cannot read chlorine on test strip. Taken to pool store four times appears to be .4 minus .3 = .1, waiting for results of chlorine deman test should be done today 6-14-08, I am leaving for three months, wife will be in charge. How can I fix this problem using your formula?!

  12. fishoutofchlorine said, on June 14, 2008 at 1:59 pm

    Fortunately, the problem appears to be essentially a chlorine demand, so you don’t need to worry too much about the numbers in this post. Combined chlorine levels below 0.2ppm are nothing to worry about, and can generally be fixed by your regular weekly maintenance shock. The chlorine demand test results should include directions for fixing the problem, but in case something goes wrong with the test, here’s what to do:

    Using a non-calcium shock (since your calcium hardness is so high already), add triple the usual maintenance amount (e.g., if you’re using something like Turboshock or BurnOut, use 6 bags instead of 2-3). Wait 1-2 hours and test the water. The strip should be dark purple: 10ppm or greater. If it is anywhere below very dark purple, add another 6 bags (or whatever amount you started with). Wait another 1-2 hours and repeat. Keep doing this every couple of hours until you can maintain at least 3-5ppm for 24 hours without adding any more shock. It’s very important that you or your wife are there to monitor the chlorine levels for at least the first day after you do this treatment. I don’t anticipate you’ll have to add more than the first two doses, but have a third on hand in just in case. You can always use it for regular maintenance later on.

    If this seems like overkill, it is. This is just what I usually recommend to customers who aren’t able to wait for the results of a proper test. The chlorine demand test should give you the approximate amount of chlorine you’ll need to overcome this demand. In that case, just follow the directions your pool store gives you, and keep an eye on the pool for the first day or so. Once you’ve maintained at least 3ppm for 24 hours, you’re out of the woods. Hope this helps, and that your pool stays beautiful all summer.

  13. Ken Almgren said, on June 18, 2008 at 8:38 pm

    I have a chlorine lock in a 40,000 gallon in grund black bottom pool in Maryland. Saturday shocked with 8 pounds of dry shock, Sunday did the same with 12 pounds, Monday did 14 pounds, and Tuesday night did 20 pounds. Cholrine reading is okay each morning but totally diappears during the day. Wednesday (tonight) I did another 20 pounds. Should I just forget it and dump 100 pounds in tomorrow night. This is costing me a fortune.

  14. fishoutofchlorine said, on June 19, 2008 at 11:31 am

    I’m going to suggest to you the same thing I suggested to Frank in a previous comment: pick a day when you can be home all day, then follow the instructions in the second paragraph of the comment before yours. The important thing is to check the levels every couple of hours and add more before you go down to zero. If you can maintain 3-5ppm for 24 hours, you should be fine for the rest of the summer if you just do your regular maintenance.

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