How often should I calibrate my pH meter?

Use fresh solution each time, rinse the probe with clean water between steps, and calibrate with pH 7.00 first, then 4.01. Never pour used solution back into the bottle. Keep the probe wet in storage solution, not in distilled water, and never let it dry out.

You can get pH 4.01, pH 7.00 and EC 1413 µS/cm calibration solutions.