I usually yellow pH meter that was in video did not care for the quality of it after 2 months and have purchased a newer one and had not recalibrated it in over a week now I were calibrate it every week
Yeah it is amazing how quickly a PH pen can give bad readings after a few test. In the beginning I would calibrate once every 3 or 4 months then I started calibrating once every month now I calibrate every other week.