Disappearing chlorine

My pool is an in ground concrete with 20,000 gallons built in 1974. I have had no major problems in maintaining chlorine levels till now. In Oct 2013 I drained and re-filled the pool and added the chemicals Leslies supplied me with to balance the water. My current readings are as follows: calcium hardness 250....cyanuric acid 45..........total alkalinity 100......ph 7.4........total dissolved solids 100..........I have two floaters in the pool each with 3" trichlor tabs. I started with 2 tabs and am now up to 3 tabs in each floater and yet I get a reading of less than 0.5 fac. I am using an OTO test on my chlorine test. The only time I get a strong reading is when I put in 2lbs of Leslies power powder.............and the pool is not being used! What can be the cause of this?

arjayh

Trichlor tabs dissolve really slowly. How many more tabs do you have left? Are they wrapped or unwrapped?

What are the ingredients of the power powder? Probably dichlor. A CYA of 45 is pretty good so you really don't want it much higher and the dichlor and trichlor both have CYA in them. I would not use any more dichlor. Whether or not to use any more tabs kinda depends on the answers to whether they are wrapped or not. You might want to read the guide about what to do with leftover chemicals. If you go to the section of the forum called 'Pool Chemicals and Pool Water Problems' you'll find a stickied thread at the top about just that.

You can just use bleach as your source of chlorine. Since your chlorine has dropped so low, I think it would probably be a good idea to shock your pool just in case something is brewing in the water that you may or may not yet be able to see. With a CYA of 45, you should be keeping your chlorine between 3-6ppm all the time and your shock level would be 15ppm. In a 20K gallon pool, each of the 121-oz jugs of 8.25% bleach will add about 4ppm of chlorine. You can use that as a reference to help you figure out doses of bleach to add based on your testing results. More about the relationship between CYA and chlorine can be found here:> http://pool9.net/cl-cya/

Speaking of testing, you need a better kit. The one we recommend is the Taylor K2006 or 2006C (better buy). You won't be able to buy it locally, but can get it through this link:> http://pool9.net/tk/

Hope this helps.

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To Watermom..........Thanks for your quick reply. Here are responses to your questions. My trichlor tabs are unwrapped and come in a plastic pail. I have about 25 left. The power powder is calcium hypochlorite and I throw in 2 pounds weekly for shock. Also in my weekly schedule I add 6 ounces of a polyquat based algaecide (which I learned about in your web site back in 2003). I also add a product called Phos-Free to keep phosphate levels at zero. As to test kits, I have two Taylor test kits...I use the K1000 for testing for chlorine as it is an OTO test as it does not hide high levels of chlorine as the Taylor DPD test does. On my own I only test for chlorine, ph & total alkalinity. I let Leslie's test for everything else.............When you asked whether my trichlor tabs were wrapped or not I immediately went t another pool supply store and bought four wrapped 3inch tabs and put them in my floaters.after a day and overnight I was disappointed to find it made no difference..........my chlorine level still read zero. In summer my pool filter runs from 7 am to 5 pm and temperatures are in the 90's and occasional 100 plus. You now know everything about my pool. I am totally stumped why the trichlor tabs are not producing a readable chlorine level
for testing.

arjayh

First of all, thanks for the subscription! We appreciate that!

The reason I asked if the trichlor tabs were wrapped or unwrapped was not because wrapped ones work better. We have had some discussion this summer about what to do about leftover chemicals. Your CYA is already high enough. So, if your tabs were wrapped, I was going to suggest that you not use the rest but instead save them til next year. Since they are unwrapped, that is trickier. There is a thread at the top of the "Pool Chemicals and Pool Water Problems" section of the forum talking about what to do with leftover chemicals that I want you to read. (You'll have to log out first to be able to see the rest of the forum, though.) After you read it, tell us what you want to do about all the trichlor tabs you still have.

With a CYA of 45, you want your chlorine to stay between 3-6 all the time. The fact that yours keeps dipping too low may mean that you stay on the verge of an algae bloom which will use up more chlorine. I want you to go ahead and shock the pool up to 15ppm this evening with bleach after the strong sun is off of the pool. Amounts to add are in my previous post. A couple of hours after you add it, test with your OTO kit. The level will be higher than the darkest yellow on your kit, but you can use the chart at the following link to help you interpret the off the scale colors:>
http://pool9.net/oto-chart/

Then, tomorrow morning within an hour of sunrise, I want you to test again with your OTO and again compare it to the color chart. See if you can determine if you have lost more than 1ppm of chlorine from sundown to sunup. That will tell us if you are fighting something in the water. (You need to be able to do all of you own testing. Pool stores are not known for accurate testing. I still think it is worthwhile to buy the kit I suggested above.This is a whole lot easier if you have a K2006 kit that can read chlorine levels higher than 5ppm.)

Report back what you find out overnight.

Also, I am moving your post into our subscriber section. That way, your post will get priority attention whenever we get busy on the forum which lately is all the time! ;)

Watermom.......I bought 4 gallons of 8.25% bleach which I thought would be enough to bring the fc up to 15, but I fell short.
At 8:45 pm on 7/09 I put the 4 gallons into the pool with the filter pump running. Then at 11:15 pm I tested and guessed the fc to be about a 9. It was a dark yellow without any tinge of orange which would suggest in the teens. I tested it again the following morning (7/10) and it was the same dark yellow ....which I guessed to be a 9. I tested it again at 3:30 pm and it was still the same 9 with ph at 7.6. Today...7/11 at 2pm it tested at 2. Does this pattern sound normal or does it appear I still have a problem? And if I shift from trichlor tabs to bleach it seems like I will be adding bleach daily. The cheap store brand bleach is $1.99 a gallon and the concentrate level is not stated on the label. I'm guessing it's about 6.0%. What do you recommend now?

If you went from sundown one evening to 3:30 the next afternoon without losing any chlorine that is good. But, then it is odd that within the next 24 hour time period, you lost 7ppm. Personally, if it were my pool, I'd do the overnight test one more time.

Here is your choice. If you continue to use the trichlor tabs, your CYA will continue to climb. As a result, you'll have to run higher and higher chlorine levels to compensate for the higher CYA. Or, you can use the tabs for a little while to bring your CYA up some --- maybe try 60ppm or so --- and then switch to bleach and see if that allows you to go to adding bleach every other day instead of every day.

.Watermom........I took your advice and did the overnite thing again. On

7/12 @ 7:45 pm I added six 121 oz jugs of 8.25% bleach.....
7/12 @ 10:45 pm my fc was apprx 15.

7/13 @ 6:30 am it was also at apprx 15
7/13 @ 4 pm same day it was apprx 10+..........

7/14 @ 4pm it was down to 4 (air temp was 102).

7/15 @ 1:30 pm fc was at 1.5

My first three readings of 15 - 15 & 10 were my best guess/approximations as it is difficult to be exact at levels well above 5 even though I used your chart.

My questions now are............is this a normal decline in this space of time and how can I switch to bleach if is going to require every other day attention. I may be gone for a week or two at a time. So I will have to go back to trichlor tablets to have my freedom............that is providing the tablets work which they have not been doing. I put two 3 inch tabs in each of two floaters today to see if the shock treatment allows the tabs to maintain a proper fc level hereon.

One last bit of info for you, I had been adding a product called Phos-Free to my pool. It's an organic product that eliminates phosphates from the water which is supposed lessen the risk of algae. Would appreciate your comments on all of this.

Thanks.

My guess is that the variability of OTO readings is attributable to inaccuracy reading OTO off scale and changes in temp and sunlight. To make an assessment of loss rate will require a K-2006 which will also give you a better idea about the CYA.

Chlorinating with bleach really needs daily attention - you might be able to stretch that to a couple days with high CYA and high FC but a week is pretty long. Perhaps you can work out a hybrid plan using bleach daily and loading up trichlor when you're away. It would require regular CYA testing and adjusting the FC targets as CYA level climbs.

As W-M said, OTO measurement is a rough estimate, not an accurate measurement of high chlorine levels.

Generally speaking, with CYA level < 50 ppm, treatment needs to be daily or continuous (feeder or SWCG). With CYA from 50 - 100 ppm, it can be every other day. From 100 - 150 ppm, it can be weekly, but you have to raise levels to 15 - 20% of CYA when dosing.

When the pool is running normally, chlorine levels need to be 10% of CYA. Under optimum conditions -- cool water, light load, good filtration -- you may be able to get away with 5%. Where loads are heavy (parties, or small pools), 15% is more appropriate.