Question
I have a (rapitest ph METER). How do I check to see if it is within tolerance of being calibraded correctly

Answer
Hi, I'd pick up an inexpensive liquid or strip test kit from a local garden center and compare the results.  However, I have the same meter (over 15 years old) and when I've checked it, it's been within 1/4 of a point.  I'm very comfortable with their accuracy.  Jim