I have been using the GH pH test kit for years - it's never wrong, it's inexpensive, it doesn't need calibration, it doesn't need both 4.0 and 7.0 calibration solutions, and it doesn't need storage solution. I grow in coco so my nute solution is under 6.0, as long as the color is an orangish/yellow I'm good to go. I found that the color chart on the bottle is a bit off so I downloaded a pic of actual vials showing the true colors for 6.5,6.0,5.5, and 5.0. The pic is spot on. Obviously if you're color blind you need a pen. LOL.