FC Going Way High After Adding Liquid Chlorine

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Sanford35000

Sanford35000

Active member
Jun 23, 2021
38
Sanford NC
Pool Size
35000
Surface
Plaster
35000 gallon pool
CYA: 50
PH: 7.6
Alkalinity: 90
Calcium Hardness: 300

Tested water around 1PM yesterday and had 5 FC (I use the powder FAS DPD test kit).

Decided to wait about an hour and add 2 quarts of 10% liquid chlorine to have FC at about 6 by 6PM (2 quarts would normally add about 1.5ppm).

Tested water at 6PM and FC was at 13. This means that the 2 quarts of liquid chlorine added somewhere between 8 and 10 ppm (assuming no "burn off" or a "burn off" of 2ppm).

I also had this happen one day last week, but thought it was a "one off" and decided to ignore.

Just wondering if there are sometimes quality control issues with liquid chlorine which result in a super strength liquid. Any other possible explanations?
 
JamesW said:
Probably test error.

No way the chlorine would be about 7 times stronger than labeled.

Did you use the right sample volume?

Do you have tabs in the tab feeder?

View attachment 504508
Click to expand...

I've been using the test kit for 2 seasons now and know how to use it. I did nothing different. I also tested several times since the reading was so high and I didn't believe it.

There are no tabs in the feeder (tabs might add that much chlorine to 35,000 gallons in 4 hours if it contained 8 3" pucks and was wide open - I never tried it). I almost never use the automatic tablet feeder any more since finding this forum.

Does anyone know what kind of test results come back using old DPD powder and R-871 reagent? I'm using last year's test kit which might not be giving accurate results, but don't know why I seem to get normal readings most of the time.

I ordered fresh DPD powder and R-871 that will be here tomorrow.

I also still have 2 quarts left from the suspect gallon bottle of liquid chlorine which I will add using only 1 cup at a time once my FC level gets back down to a normal range.
 
Sanford35000 said:
I've been using the test kit for 2 seasons now and know how to use it. I did nothing different. I also tested several times since the reading was so high and I didn't believe it.

There are no tabs in the feeder (tabs might add that much chlorine to 35,000 gallons in 4 hours if it contained 8 3" pucks and was wide open - I never tried it). I almost never use the automatic tablet feeder any more since finding this forum.

Does anyone know what kind of test results come back using old DPD powder and R-871 reagent? I'm using last year's test kit which might not be giving accurate results, but don't know why I seem to get normal readings most of the time.

I ordered fresh DPD powder and R-871 that will be here tomorrow.

I also still have 2 quarts left from the suspect gallon bottle of liquid chlorine which I will add using only 1 cup at a time once my FC level gets back down to a normal range.
Click to expand...
Old reagents don’t seem to read much different for me. Really sounds like just a goofed up test, or maybe the cylinder wasn’t rinsed well. Try rinsing with a little liquid chlorine, rinse well, and dry off with a paper towel.
 
Last edited:
FC was down to 5.5 late this afternoon so I added 3 CUPS of the suspect super strength 10% liquid chlorine. I waited an hour, tested again, and the FC is up to 8. I usually need to use an entire gallon (16 cups) to raise the FC by 3, so this is absurd.

The new DPD test kit arrived and shows FC at 9. It’s possible that the new R-0871 dropper bottle puts out a smaller drop than the old bottle.

Either way, the bottle of liquid chlorine that I have appears to be super strength. I’m glad I didn’t add the entire remaining 1/2 bottle.

I have about 10 more bottles of the stuff from the same batch. If they’re all super strength I’ll be using about 1/5th as much as usual. I’m not complaining.
 
It’s just Pool Essentials Liquid Chlorine which I buy at Walmart. The lot number is shown in the photo below.

I’m going to use a bottle with a different date the next time I add chlorine, which might be late tomorrow afternoon or mid morning on Monday, and see what happens.

Also, I just checked my recycling bin and found 3 empty bottles that were from the same date. I think one of them was super strength (as I said in my original post, this happened to me one day last week), but the other 2 were “normal.”

This could turn into a major PITA if I have to start adding by cups and checking the FC an hour later just in case even one more bottle is supercharged.

BCA0DCBB-DC98-455F-B5AA-A9D10E5FC827.jpeg
 
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Please tell me what else could explain 3 cups of liquid chlorine adding 3ppm FC.

I really need to know.

It’s not a testing error. I’m testing correctly as I’ve been doing for the past 2 pool seasons.
 
Using the K-1515...
  • 10ml of pool water
  • 1 generous scoop of R-0870 powder
  • Holding the R-0871 bottle perfectly vertical, letting the drop fully form on the tip (don't force it) and adding one drop about every second to the test vial
  • Each drop counts as 0.5 FC (divid resultant drops by 2]
Manually swirling or using an automated stirrer?
 
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81EFF74E-5930-48B9-8D2C-D0E89B3725B1.jpeg

Major egg on face moment. Sorry guys. I just read the instructions for the DPD test.

1 DROP = .5PPM

I was thinking 1 drop = 1ppm. What can I say? It was a long winter. My chlorine has been half of what I thought it was for the past 2 weeks since opening. It’s a miracle that the pool is as clear as glass.

Oh well…figured out a way to save R-0871. I need to add some chlorine in the morning.

Sincere apologies for wasting your time.
 
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