That has the purple look of high FC.Here’s a pic of the sample I took today. I do believe the meter that was used as we use it and calibrate it regularly, as we measure boiler water chemistry with it all the time. And our boiler water has to be as pure and as clean as possible to make sure we send high quality steam to our steam turbines. I’m going to add some more acid ASAP.
FALSE READINGS: high levels of chlorine (usually > 10 ppm) will quickly and completely convert phenol red into another pH indicator (chlorphenol red). This new indicator is a dark purple when the water's pH is above 6.6. Unfortunately, some pool operators mistake the purple color for dark red and think the pool water is very alkaline and wrongly add acid to the pool.
When a sanitizer level is not extreme, only some of the phenol red may convert to chlorphenol red. However, purple + orange (for example, pH 7.4) = red. This error is more subtle as no purple color is observed and the operator does not suspect that a false high pH reading has been produced. Some operators neutralize the sanitizer first by adding a drop of chlorine neutralizer (i.e. sodium thiosulfate). However, thiosulfate solutions have a high pH and, if heavily used, may cause a false higher sample pH.