I did use kalk and it would boost the PH a bit. I switched to dosing now though, but I am using soda ash for the alk. This reading is across two different PH probes, including from my KH Monitor, and from API test kit, for what its worth. The probes are calibrated.If it’s not broke don’t fix it. Do you use kalkwasser? Have you calibrated your PH probe lately?