Confusing #'s past couple of days & getting worse

[CheyDee]

For the past couple of weeks, I have been running FC and TC levels between 3 and 7, with a CC level always 0. The pH runs between 7.4 and 8.0. I add 10% bleach and 14% muriatic acid each evening, the exact amounts depending upon the actual numbers. On Mondays I run full sets of numbers.

On 4/3, however, the FC dropped all the way down to 1.5 (checked twice), the TC was 3, and CC = 1.5. The TA was 60, CH 400, CYA was 50, and temp was 74F. I was supposed to add 406 oz. of 6% bleach, but only had 328 oz., so I added that. I also added 46 oz. of 14% muriatic acid and, for the first time, added 2# of baking soda to raise the TA.

I ran the pump all night & tested again first thing in the AM on 4/4. I got:

FC = 3
TC = 5
CC = 2
pH = 8.0
TA = 70

I ran out for more bleach and came home with 10%. I added 158 oz. of the 10% plus 51 oz. of 14% muriatic acid and kept the pumps on.

Last night I tested again and got:

FC = 25
TC = 25
CC = 0
pH = 7.4
TA = 70
CYA = 60
CH = 40 (!)
Temp: 74

I didn't touch the CH because it was such an erroneous reading from the 400-ish I usually get and figured I would wait and run it again tonight. Tonight's #'s are:

FC = 3 (after diluting. Had been off the charts, beyond 8. How could it only be 3 then??)
TC = 5 (using the diluted specimen)
CC = 2 (same specimen)
pH = 8.0 (probably higher)
TA = 70
CYA = 50
CH = 450
Temp = 74
Appearance: remains crystal clear

I am very confused now :? . If anyone could explain why these numbers are so baffling to me and what to do about it before I make more of a mess, I would be very appreciative. Thanks in advance.

 

[JasonLion]

Which chlorine test are you using (DPD, FAS-DPD, test strips, etc)?

Also when you say "diluting", what exactly are you doing?

 

[CheyDee]

I am using DPD.

In the yellow box (chlorine instruction section) under the cover, it says:

If color is off-scale: Repeat test using 4.5 ml sample diluted to 9 ml mark wtih tap water. Multiply reading by 2 to obtain approximate sanitizer level.

That is what I did.

 

[JasonLion]

The first thing to understand is that the tests have a fair bit of variability, especially when you are using dilution with the DPD test. Testing CYA a couple of times and getting results that are +-10 or +-20 of each other is normal. TA and CH tests are normally within 10 across repeated attempts, but the CH test can be a little tricky to read correctly at high FC levels.

Adding 158 oz of 10% bleach/liquid chlorine would not have raised FC up to 25, nor is the DPD test capable of measuring FC levels around there. That much bleach would have raised FC by around 12. As the FC level gets higher the DPD test gets less precise and when you use dilution the precision gets dramatically worse.

Why are you adding so much chlorine? Normal chlorine levels with CYA at 50 are between 4 and 8. At those levels the DPD test is usable, but with the amounts of bleach you are adding the DPD test becomes problematic.

 

[CheyDee]

I tested the CYA twice and got 50 and 60.

Since the CC went to 2, I thought I should shock it - thus bringing the FC up to 20. (Since I had two CYA #'s, I went with the higher one to make sure I killed whatever might be growing.) Instead of bringing it up to 20, the next test showed the FC to be 25.

So then, all the test results are inaccurate at this point? Should I just treat the pH tonight then, (it was a very strong 8.0), or is that also inaccurate now?

 

[JasonLion]

Alright, it is making more sense now. Assuming the FC level is very high the PH test is not reliable. If you have been adding acid at regular intervals all along then you probably need more at about the same interval. Just keep in mind that you can't depend on the PH test until FC is below 15.

Shock level with CYA of 60 is around 18. It only takes 237 oz of 10% to raise FC by 18 in 10,600 gallons and less if your FC isn't zero to begin with. You added 328 oz and brought FC up to around 25, at which point the DPD test should have read either zero or off the scale high. From that point the FC level has probably stayed off the scale high and the reading errors have continued.

You are getting apparently normal, but incorrect, readings from the DPD chlorine test when it should be reading off the scale high or bleaching out and reading zero. I am not quite sure why that would happen, but regardless the answer is to get a FAS-DPD test kit which will read FC and CC reliably up to 50 or higher.

The remaining test behavior is more or less normal. With more practice you probably won't have the large CH test error you had and the remaining differences are just the normal variations in test results that I would expect.

 

[CheyDee]

Thank you so much, Jason. I appreciate you!