milwaukee systems ph meter. nicely accurate and you can calibrate it with calibration solutions. Or just invest in a real pH meter that they use in scientific settings. Our lab has one, a little big, but accurate to a thousandth of a degree. That having been said, it's rather pricey. I bought mine for $140.
Yeah get a Hanna or Milwaukee PH tester. Pens are garbage. I ahve 8 of these and all work great. I tried 5 pens all read different even after all were calibrated in the same solution with in mins of each other.
"source water's pH" source water PH varies so much... Mine chages with the season and with the plant it runs through. Agency says my ph is 8.2 but my tap is 6.8 right now.... then the RO comes out 5.7. But 2 months it will change