Check your pH once the chlorine level is below about 10 or 15 ppm. It should be in the ballpark if you have a decent test kit. the 'cheapo' ones are usually worse about the interferance but the better kits by Taylor, LaMotte, and the kit sold on the poolsolutions website work pretty well up to about 15 ppm chlorine. Even the cheap walmart 6 way kit works at chlorine levels up to a bit over 10 ppm but if your chlorine is really high even these kits will show a PH of 8.2 or close to it when the pH is actually lower.