I've checked mine against the litmus paper test strips, and it seems real close (at least close enough for me !), but I've never had a 'professional" test done. I got an N-P-K, & ph test kit at the garden center(about $18, if I remember right), and test with that once, or maybe twice a year, and use the "electrical" ph tester about every month in the areas I'm trying to amend ( SIBs, blueberries, etc.)