Chlorine Still Reading Zero
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Do you have a problem with your swimming pool water where you are testing and measuring zero chlorine in the pool but you just added some so how can this possibly be? First, it is important to understand that there could be a few different things happening in this situation and you really do not want to make too many assumptions. For example, it is possible that the method of water testing that you are doing is invalid. You might have expired test strips or reagents for your water test kit. This could easily result in errors of measurement or entirely inaccurate measurements being shown. If you are struggling with a problem like not having any chlorine in your pool even though you have added an appreciable amount of chlorine already, then I would suggest that you double check your water test with a secondary source. Either a free water lab analysis from a local pool and spa store or a different test kit / test strips source.
Aside from errors in the measurement of the water you might also have a rare situation where you have added so much chlorine that the color of your test strips or reagents are actually bleaching from the sky high sanitizer levels. This definitely can happen, but normally you would be able to notice a strange color reaction with your test. For example, a test strips might turn a dark, vivid color before starting to lighted from the edges inwards. If we assume that you have double checked the water chemistry and the above situations do not apply, and you actually have a situation where you have added chlorine but are reading zero chlorine in the water, then here is what is happening with your pool:
You add chlorine to the water to sanitize and oxidize all the nasty stuff that ends up in your water. When you have 1ppm (one parts per million) of free chlorine in your pool we know exactly how much you have right now, and we know exactly how much it would take to increase your 1ppm to 2ppm, assuming we also knew the pool volume. However, when you have 0ppm of free chlorine in your water, you actually do not have zero. You have a negative number. This is your chlorine demand. Let's pretend I dropped a full garbage bag of floor sweepings into the pool. Gross. The chlorine reading is zero. I can not add the known amount of chlorine to increase my pool volume to 1ppm of free chlorine, because I am not at zero chlorine. I am at negative chlorine because my pool is full of disgusting junk that I dropped in there, and it is going to take a lot of chlorine to overcome the amount of debris load I have just added before I will start to read above zero on a water test.
Why you are reading zero chlorine - On a microscopic scale this is what is happening in your pool when you add chlorine and still measure zero. There was existing debris / organic contaminants within the pool, whether you can see them or not, and the chlorine that you did add was all used up in trying to clear out the contaminants in the water. This is why the solution to measuring zero chlorine in your water (after adding some) is simply to continue to add more. You do not want to increase dosages of chlorine, because at some point you will overcome the chlorine demand and suddenly you will go from zero to not zero, and you do not want to way overshoot your chlorine as now you would have too much! Simply add a suitable size dose of chlorine to your pool, enough to increase chlorine levels around 5ppm based on the volume of your pool, and then wait a few hours before retesting the water. If the chlorine level is still zero, then repeat this process until you finally overcome the chlorine demand and start to register free chlorine levels above zero with your tests.
This process will solve your problem of testing zero chlorine in the water but you should be aware that you should also test your total chlorine levels following this process. Total chlorine and free chlorine should read exactly the same. When total chlorine is higher than free chlorine this indicates you need to perform breakpoint chlorination to eliminate the spent chlorine (chloramines) which are referred to as combined chlorine. This is all made simple in the pool chemistry crash course that I wrote. Check it out and it will probably help you to understand this chlorine relationship between free, combined and total chlorine in your pool.
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Swimming Pool Steve is an award winning, second generation swimming pool specialist from Ontario Canada and one of the most trusted voices in the swimming pool industry. With over 20,000,000 views on the Swimming Pool Steve YouTube Channel, winner of the Pleatco Pool & Spa Industry Leadership award and author of hundreds of pool and spa articles both online and in print. Steve is committed to helping pool and spa owners as well as pool and spa industry workers learn more about the technical side of building, renovating, repairing and maintaining all types of swimming pools and spas. Follow Swimming Pool Steve on Facebook, Twitter and YouTube.
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