It depends on what the process is that needs to be tested. If I am testing a buffer solution addative for intravenous amphotericin B infusion I would need an accurate and verified pH meter. In the garden the strips tell the story with plenty of accuracy. There is also redundancy testing. If the solution being made is the same formula as yesterday and the test is the same that is verification. When the formula changes and then the pH correction needed changes that is also validation. From reading this board for several years now, it is very frequent that when someone has a pH problem it can be traced back to a pH pen that has either not in calibration or has just drifted away from reality. Strips are easy.