I use these from amazon. if you buy both the things its like 20% off or something, the ph meter is fairly accurate, you definitely need to calibrate it a few times though. ppm meter is pretty cool, stable and measures water temp for DWC https://www.amazon.com/gp/product/B017J5GQHY/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1