High Chlorine Level with no Chlorine Added in 6 weeks

I have been perplexed as to why I have high chlorine level (> 5 ppm) in my pool when I have not added any chlorine is over 6 weeks. Its probably been more like 8 weeks now.

I have a 10,000 gallon gunite pool, the CYA is at about 100 ppm. PH is about 7.4 - 7.5. Alkalinity is about 190. The water is crystal clear with no algae whatsoever.

I have tested with a OTO and its about 5ppm. I tested with a DPD and its almost 0 ppm. Leslies Pools has tested several times and they say its above 5ppm as well. I am not sure what they use (DPD or OTO) but I believe it is DPD.

Am I getting false readings using OTO? I tested on tap watter and the reading is 0 ppm using OTO.

Thanks in advance!

With a CYA level of 100 you want to run your chlorine higher. I've attached a chart that shows you should run min 8ppm for a CYA of 100. Your high CYA is allowing the chlorine to stay in the pool longer but is not effectively sanitizing your pool if you are around 5ppm, you will eventually get an algae bloom when you water really heats up. I learned this the hard way the first year I had my pool. I used pucks which add CYA and I was running 1ppm-3ppm of chlorine based on what I had read on the test strips and had real problems with algae that was 4 years ago since then, my CYA hovers around 100 and I have my chlorine at 10ppm and have never seen algae again.

Also with the OTO you are just checking Total Chlorine (Combined Chlorine + Free Chlorine). You really need to know your Free Chlorine.

Stabilizer . . . . . . . Min. FC . . . . Max FC . . . 'Shock' FC
=> 0 ppm ... . . . . . . 1 ppm . . . . . 3 ppm . . . . 10 ppm
=> 10 - 20 ppm . . . . 2 ppm . . . . . 5 ppm . . . . 12 ppm
=> 30 - 50 ppm . . . . 3 ppm . . . . . 6 ppm . . . . 15 ppm
=> 60 - 90 ppm . . . . 5 ppm . . . . . 10 ppm . . .. 20 ppm
=> 100 - 200 ppm . . 8 ppm . . . . . 15 ppm . . .. 25 ppm

Consider yourself to have luckily gotten enough residual chlorine to a) not have it breakdown, because all that CYA is slowing it down, and b) to have a residual to keep your pool clean!

Now you need to get your OWN test kit that can measure higher chlorine levels....You want something with FAS-DPD chlorine testing. That's Ben's kit, the PS-234, or Taylor's 2006 or Leslie's Chlorine FAS-DPD Service Test Kit. You can only get any of these on-line. You can instead just get the FAS-DPD test portion without the full kit. Both Taylor and Leslie's sell them on-line--I don't know if Ben does. All of Leslie's test kits are just rebadged Taylor kits anyway.

Or get a kit that goes to 5ppm of FC and hunt up the CarlD Shotglass Method in the forum!

morechoff,

My first question would be, "how much Cl did you put in 8 weeks ago"?

If I assume you use 1-2ppm daily in normal chlorine loss (most pools lose that or more) and you have not added in 50+ days and still have 5ppm or greater, you must've had a Cl level somewhere between 55 and 110!! (yeah, I can see why you would have no algae)

Secondly, "Why"?