The calibration laboratory of HCMR has been developed to support the POSEIDON weather forecasting and monitoring system. The lab tests and calibrates sensors for a range of oceanographic parameters focusing on the unique environmental conditions of the Eastern Med sea. The calibration facilities at the HCMR Thalassocosmos complex in Crete include a fully equipped laboratory with a custom made large calibration tank, a number of reference sensors and equipment, and all the necessary apparatus’ for temperature, salinity, chlorophyll-a, turbidity, dissolved oxygen, CO2 and pH sensors calibration. The support team consists of the HCMR technicians and scientists, who prepare the instrumentation, perform field experiments, service and maintain the instruments and assist users during experiments in the calibration facility. The Poseidon calibration lab (PCL) through transnational access activities participation in research projects has proven to be a powerful tool for the calibration of sensors deployed in the wider Mediterranean Sea.