pH-Phenol Red.
Photometric test for pH in pool and spa water temperature (27⁰C to 39⁰C) using Phenol Red
Colour Change
Yellow to Red
6.5 - 8.4 pH units
Technical Resource:
pH-Phenol Red Test Method, Technical Information

High levels of chlorine or other disinfectants will cause an intense purple coloration. This will mean the result is not valid

Ionic strength, temperature and other water factors may have an effect on pH readings. This test has been calibrated for conditions most likely to be encountered in spas and swimming pools from 27°C to 39°C

Fill to the 10 mL line with sample.
Use this to "blank" the instrument. Place in cell holder and press Blank.
Prepare sample by adding a Phenol Red tablet and crush and stir.
Place the tube in the cell holder and press Measure to obtain the pH value.