I've read that PH readings can be affected during the shock process and high FC numbers. How high does the FC need to be to start affecting the readings? Also what other readings can be thrown off due to other high levels?
Calcium, Magnesium, Total Hardness Metal ions may cause interference; to prevent, add titrant containing EDTA to sample before buffer and indicator, then test as normal making sure to count drops of titrant added initially in total required to reach endpoint. If interference still occurs, dilute sample with DI water as necessary and retest.
Chlorine Chlorine levels > approx. 25 ppm may bleach out indicator or cause sample to develop a brown color; to prevent, add more DPD powder or dilute sample with DI water as necessary and retest. Other halogens and oxidized manganese may cause positive interference.
pH Iron > 10 ppm may cause negative interference.
pH Sanitizer levels > approx. 10 ppm may cause a blue-purple color resulting in false high readings. Wait for sanitizer level to decrease to normal levels and retest to assure an accurate reading.
If you have over 10 ppm FC, and your FC is not higher than shock level for your CYA, then the pH interference might be delayed a little bit, which might allow you to get an accurate pH reading, even at FC>10 ppm (If the sample is matched quickly). However, this is not certain, and the pH test cannot be deemed reliable at FC>10 ppm.