Can't get a chlorine reading

[tdean]

I took a water sample to my local pool store, because no chlorine reading and green algae. The test results were as follow: Free Cholorine: 0.1ppm, Total Chlorine: 7.2ppm, Combined Chlorine: 7.1ppm, pH OVR 8.2, Hardness: 140ppm, Alkalinity 166ppm, Cyanuric Acid 15ppm, Copper 0, Iron 0.

I was told to add 2 lbs of Regal Oxy-Klear, then 25 lb of Super Shock-It, then 4 lbs of Regal pool stabilizer. I did all that and pool is crystal clear, but no chlorine readings. I called the local pool store and they said there is something blocking the chlorine and i need to double the amount of chlorine that I initially put it, which would be another 40-50 lbs. I should then get a reading.

Can i get a second opinion before I go and spend another $150 on shock for my pool? Thanks in advance for any assistance. It is an inground pool, about 16-18k gallons.

All the best!

[chem geek]

Was your CYA level higher last year when you closed the pool? Were you using stabilized chlorine (Trichlor or Dichlor)? If so, then if you let the pool go over the winter and had the FC get to zero, then bacteria could have converted some of the CYA into ammonia. This creates a huge chlorine demand. You do not need to use expensive non-chlorine shock to clear it. Bleach or chlorinating liquid will get rid of it much less expensively, though it could still take a lot so might be better to do a partial drain/refill.

You can do a bucket test to see how much chlorine it will take until you get an FC reading. Take 2 buckets of water and add chlorine to it and test FC after 30 minutes to an hour. Every 1/4 teaspoon of 6% bleach in 2 gallons is 10 ppm FC.

I describe what happens technically in this post and describe my own experience with this issue in this thread.

[kaptn]

so how does this bucket test actually work?

Do you take 2 gallons of pool water and put 1/4 teaspoon of bleach (6%) and then test it until you get what? How often do you test?

[chem geek]

You take a bucket (NOT exposed to sunlight) with 2 gallons of pool water and add 1/4 teaspoon of 6% bleach, mix, and measure the Free Chlorine (FC) within 10 minutes. If it has a reading near 10 ppm wait an hour and retest. If still near 10 ppm then stop. Otherwise, add another 1/4 teaspoon and repeat. This will tell you how much total cumulative FC you need before you get a reading. It's 10 ppm FC times the number of 1/4 teaspoons you needed to add until the FC started to hold.

Note that to measure 10 ppm FC you should use either a FAS-DPD chlorine test where you count the drops or, at a minimum, an OTO chlorine test that compares against intensity of yellow. A DPD chlorine test could bleach out so you'd need to dilute the water sample 1:1 with distilled/filtered water.