Constant High Chlorine Readings

We have had our 15,000 gallon in-ground pool for over 10 years and I've always done the pool maintenance myself. It's fairly easy to keep it under control, but I've always wondered why I never get a low chlorine reading. I typically put 2 or 3 tablets in the skimmer basket and I try to shock the pool once or twice per month. Even if the pool goes without chlorine for a week (meaning the tablets in the skimmer basket dissolved), I've never seen a reading below 5ppm. Ditto for the free chlorine reading - I've never seen the level adjust after I've let the sample sit for a few minutes.

Could it be that the normal range of Chlorine for our pool is just outside of the measurement range of our test system and that it's fluctuating at a slightly higher level?

I'm not completely sure that I understand the question. Are you saying that your pool never goes below 5 ppm for chlorine? What other reading are you referring to when you say "I've never seen a reading below 5 ppm. Ditto for the free chlorine..." ?

What is your normal source of chlorination, what are you using to test your water with, and what is your CYA level?

Janet

Sorry about the confusion in the post...

I've never had the pool go below 5ppm for chlorine.

I meant to say that the free chlorine and combined chlorine have never read below 5ppm.

I'm using a test kit from Leslie's Pool Supply (Basic 4 Test Kit from the Pool Shop). I take the free chlorine reading within 2 - 3 seconds of adding the solution and then the combined chlorine reading after 1 - 2 minutes. Both of these readings come in above the top end of the range (5 ppm).

The normal source of chlorine is 3" tablets placed in the skimmer basket. I shock the pool 1 - 2 times per month.

My test kit is only good for Cl, pH, Alkalinity, and Acid Demand, so I haven't measured CYA.

How long have you been using the trichlor tabs? They are stabilized and will increase your cya. Continuous use of them can cause the cya level to get too high. The kit we recommend is the Taylor K-2006 which will measure cya and everything else you need to test. You need to know your cya reading.

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Originally Posted by matthewnadeau

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I meant to say that the free chlorine and combined chlorine have never read below 5ppm.

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Just wanted to clarify: Total chlorine = free chlorine + combined chlorine. The goal is to get your total and free chlorine equal, which would mean that your combined chlorine (the chloirine actively fighting something in the water) is zero.

I'm willing to bet that your CYA level is very, very high--which is probably why you're not losing much chlorine. If you keep your chlorine higher than 5 at all times, though, it looks like it'll work for your pool. Do you close this pool in the winter? If so, you might want to consider a partial drain/refill to lower your CYA levels before closing. If you have a bad algae bloom over the winter, while the pool is closed, it can break down the CYA into ammonia and create a HUGE chlorine demand when you start up again in the spring.

When you shock it, what are you shocking it with?

Janet

Quote:

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Originally Posted by matthewnadeau

I'm using a test kit from Leslie's Pool Supply (Basic 4 Test Kit from the Pool Shop). I take the free chlorine reading within 2 - 3 seconds of adding the solution and then the combined chlorine reading after 1 - 2 minutes. Both of these readings come in above the top end of the range (5 ppm).
This is an OTO test and really can only measure total chlorine. It does not accurately measure free chlorine, only TOTAL chlorine. You need a DPD or an FAS-DPD test to measure FC and CC. We recommend FAS-DPd testing because it is easier, has more precision, and does not involve color matching.
The normal source of chlorine is 3" tablets placed in the skimmer basket. I shock the pool 1 - 2 times per month.

My test kit is only good for Cl, pH, Alkalinity, and Acid Demand, so I haven't measured CYA.

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It is very possible that your CYA is extremely high, which is why your chlorine never seems to drop. I would suggest investing in a good test kit (Taylor K-2006, which uses FAS-DPD chlorine testing) and you might find that :
1) your current chlorine testing is no where accurate
2) Your CYA is higher than you think.

These are educated guesses but they would explain your dilemma.

I had my water tested at a local pool store to test the accuracy of my home test kit. Here are the results:

Free Cl = 3 ppm
Total Cl = 3 ppm
pH = 7.6
Alkalinity = 90 ppm
Calcium = 220 ppm
Cyanuric Acid = 80 ppm
Phoshpates = 1000 ppm

I was expecting to see a much higher CYA reading, but the tech that helped me said that this level (80) isn't cause for too much of an alarm. The phosphate reading was very high, so I added a "Phos Free" product that they sold.

The high phosphates may explain why I occasionally get a light algae on the side of the pool even though I have Cl in my system, but I don't think that it explains why my Cl readings are always high.

The reason that you "occasionally get a light algae on the side of the pool" is because you're not keeping your chlorine levels high enough to compensate for your CYA level. With a CYA of 80, you need to keep your chlorine levels in the 5-10 range, and go up to 20 ppm when you shock. By letting the chlorine drop below 5, you're opening the door for the algae to come in.

This is the pitfall that most people don't understand about trichlor tabs. They are easy, fairly trouble free--until the stabilizer gets so high that the chlorine can't compensate, then they start having algae problems that they throw all kinds of stuff "at" to try to fix--such as phosphate remover--when all they really need to do is keep adequate chlorine levels. The chart at this link http://www.poolforum.com/pf2/showthread.php?t=365 will explain better where the chlorine levels need to be kept in relation to the CYA levels.

In order to clear the green, you need to shock your pool up to 20 ppm, and hold it there until the water clears, the algae is gone, and you don't lose any chlorine when testing at sundown and again at sunup. Keep the filter running 24/7 during the process, and brush the pool at least once a day. After those conditions are met, let your chlorine drift back down to the 5-10 range, but never below 5 ppm. I would also stop using the pucks, because your CYA is going to continue to rise as long as you use them, and when the CYA rises, so must your minimum chlorine.

Janet

Well, I guess my first question (to myself) is "why am I using a test kit that only goes up to 3 ppm on the Cl reading"? I always assumed that as long as I was above the "ideal" range of 1.0 - 1.5 ppm that I would be OK.

Sounds like I need to get myself a new test kit and ensure that my Cl is kept at a higher level and then also use liquid Cl along with the tablets.

Thanks for your help.

Yeah, the "ideal" range of 1.0 - 1.5 is for an inside pool without CYA, or an outside pool with a very low stabilizer level--but then the chlorine doesn't stay in the pool for long. Since the advent of stabilized tablets, the folks who write the guidelines either have forgotten to update the ranges, or never figured out the CYA/chlorine relationship to begin with--but it's been proven over and over again that as the CYa level rises, so must the chlorine level to be as effective.

When you manage your pool, a good test kit will save you tons of time, money, and aggravation. We recommend the Taylor K-2006 kit. It can be found online at several different places, but if you get it here http://www.amazon.com/gp/product/B0002IXIIG?ie=UTF8&tag=poolbooks&linkCode=as2&camp %20=1789&creative=9325&creativeASIN=B0002IXIIG"><b >Ta%20ylor%20Complete%20FAS-DPD%20Pool%20Water%20Test%20Kit%20K-2006</b then the Poolforum gets a commission from the order. In the meantime, here's a link http://www.poolforum.com/pf2/showthread.php?t=10073 that describes how to force your kit to measure higher chlorine levels. It's only gonna give you a ballpark, but it's better than nothing until you can get a good test kit.

I would stop using the tablets in your pool at this point--the CYA will continue to rise as long as you're using it, and when you get to CYA of 100 or so then it's very, very difficulty to maintain a clean, clear pool even with elevated chlorine levels. It's your call, though....

Janet

Thank you for the recommendation on the test kit. Unless I'm reading it incorrectly, it looks like the Taylor kit only reads Cl levels up to 5 ppm. Are there kits that read higher, or do I need to dilute the sample to get a higher range?

The Taylor K-2006 is a FAS-DPD kit and can read levels way higher than 5. More like at least 50 and probably much higher.

Be careful! The Taylor K-2005 kit is a straight DPD test kit that only measures FC to 5ppm. The K-2006 is the one with the FAS-DPD test that measures to (at least) 50 ppm.