Other considerations to proper use of a pH meter:
- Always clean the probe with distilled or RO water between transfers so that calibration solution, storage solution and/or wort solution are not contaminating each other. It's best to have a squirt bottle of distilled water handy for these transitions.
- Always try to take measurements of wort at room temperature. The ion selective glass membrane (the business end of the pH meter) is a delicate thing. If you're working with a non-analytical grade pH meter, it's even more important to take good care as the glass membrane, seals, connections, etc are typically not as robust.
- Get a good storage solution and always keep your probe submerged in it when not in use. pH 4 solution with excess KCl dissolved in it makes a good storage solution.
- pH measurments being logarithmic, it's best practice to recalibrate before each brewing session even if you brew as often as once per week. pH = 5 vs pH = 6 is a tenfold difference in acid concentration.