This is what I used to use it's all you need stays calibrated pretty good Now I use Advanced Nutreint's ph perfect so I don't need my it's for sale!https://www.amazon.com/Oakton-EcoTestr-Waterproof-Tester-Range/dp/B01MRR61TI/ref=sr_1_2?ie=UTF8&qid=1506718910&sr=8-2&keywords=oakton+ph+meter