How to Fix Your Chlorine Lock
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.
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.
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:
- Pool size: 15,000 gallons/Total Chlorine: 5/Free Chlorine: 3/Product used has 47% available Cl
- Pool size: 25,000 gallons/Total Chlorine: 3/Free Chlorine: 0.5/Product used has 57% available Cl
- 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.