My water is pretty consistent so I can tell if its off pretty quickly on raw water.
But my issue isn't the probe calibration, it's the water. My water is really clean, with several lab tests between 40 and 50 ppm. But it isn't stable.
It comes out of the tap at 5.9. Let it sit a couple of days and it will drift up about 7.4. Probably a dissolved gas. But now I have to age my water before using it which is a pita. RO won't help with dissolved gasses.
Oh well. I just checked my soil ph and luckily its in a good place. Now I have to up the nutes; since I put in the qb's they are eating a lot more.