I'm not sure if they are the same ones I used in the past (without the phone) but some pH ones I tried once had the graduations too far apart to be of use. They went like 6.2, 6.8, 7.4, 7.8. We need to know more specific than that. 6.8, 7, 7.2, 7.4, 7.6, 7.8 etc
I would make sure you aren't running into that before you spend the money.