@Burr_nit I have the Blue Lab pH Pen, EC/PPM Pen, & pH Soil Truncheon. I find that I have to calibrate the pH Pen & Truncheon just about every time I do a check, the EC/PPM calibration will last 30 days. I always check the pH Pen & Truncheon against the 7.0 solution just in case it needs calibration, pH changes with solution temperature changes so it's always worth a check IMHO.