Low PH affecting FC test?

scm175

Active member
Jun 28, 2012
29
Bristol, PA
Began a SLAM tonight. I I added muramic acid to bring my PH down to 7.2 and added liquid chlorine to bring my FC up to 20 (CYA is 50). As I was putting everything away I glanced at my bottle of acid and realized it is 31.45% instead of the 15.7% in usually use. Pool Math says that much acid should bring my PH down to 6.1. I tried to test my FC an hour later and I got a reading of 45. I know my FC can't be that high. Can the low PH be affecting the FC test? Am I wasting my time to keep testing FC tonight until I can bring up my PH tomorrow?
 

mgtfp

Bronze Supporter
Mar 5, 2020
1,026
Melbourne, Australia
Pool Size
66000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Astral VX 7T
No, low pH doesn't affect the FC test.

And by now, pH won't be low anymore. While FC is at SLAM level, pH will be higher (increasing FC with liquid chlorine raises pH, using FC to kill stuff decreases pH - net effect of adding and using FC ending up at the same FC as before adding liquid chlorine is pH neutral).

While FC is above 10ppm, the pH test won't work (unless you have an electronic pH-meter).

Nothing you can do about adding to much MA now, but I expect your pH to be above 7 by now due to having such a high FC now and due to aeration, so I wouldn't add any pH-up or something like that. What is your TA and CYA?

What was your FC before you added chlorine, and how much liquid chlorine did you add?

Did you do the FC test with a 10ml or a 25ml sample? Maybe you used a 25ml sample, but accidentally used 0.5ppm per drop (as for the 10ml test) instead of 0.2ppm per drop (as for the 25ml test) to calculate the FC from the number of drops?
 

scm175

Active member
Jun 28, 2012
29
Bristol, PA
Test results before I began the SLAM were

FC 0
PH 7.8
TA 110
CH 70
CYA 50
I added 136oz of #10 liquid chlorine. Just woke up and will test again soon. Pool still looks the same, light green.
 

mgtfp

Bronze Supporter
Mar 5, 2020
1,026
Melbourne, Australia
Pool Size
66000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Astral VX 7T
Yep, that should have increased FC by 20ppm.

How did you do the FC test, what was your sample size, how many drops until it turned clear? You tested after 1 hour, but was the pump running for at least 30min?
 
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