Manufacturers of sensors will have a recommended re-calibration period; for Tek Troniks temperature and humidity probes we recommend annual re-calibration. However, the manufacturers recommended period cannot override any legislative, performance, contractual or audit defined calibration requirement. If you're using probes for critical applications more frequent calibration would be the norm. If the probe is damaged, dropped or is generally mishandled then a calibration to check performance should be completed. Periodic checking of the probe against a known source to sense check performance should also be completed. If you have any concerns over performance of the probe then calibration should be carried out.