# The POSEIDON Ocean Observing System: Technological Development and Challenges
## 2 The POSEIDON System
### System Operations
#### 2.3.2 Calibration Strategy and Methodology  
Monitoring the status of sensors in terms of their operation and accuracy is important to ensure data quality. Sensors are initially calibrated and characterized by their manufacturer who usually offers in house calibration and repair services. In this case, the sensors become unavailable to the system for a considerable period of time. For parameters such as temperature and pressure, there are traceable standards, methods and reference equipment that can be used for sensor calibration experiments. Artificial or natural solutions are used for fluorescence and turbidity optical sensor calibrations, but the lack of certified standards limits the traceability of the results. For dissolved oxygen sensor calibration, the reference data are provided by Winkler analysis [13].  
The large number of the deployed sensors and the need for high-quality data led to developing calibration methods and infrastructure to cover the needs of the POSEIDON system. The calibration procedures followed are compliant with manufacturer recommendations; however, our main focus is on data ranges recorded in the eastern Mediterranean area. Processing and evaluation of calibration data focuses on minimizing sensor drift by recalculating corrected coefficients at regular intervals. In operational mode, the buoys network hosts a total of 20 CTs, and CTDs equipped with biochemical sensors are deployed in several depths for a period of 6 to up to 12 months before they are replaced. In order to minimize the time and expenses, the calibration procedure is designed in such a way as to cover the range of the measured parameters present in Greek seas [14]. For temperature and salinity experiments, all sensors are placed in a 1000 lt thermal isolated tank filled with local seawater. The aim is to produce predefined calibration steps with an achieved homogeneity of the seawater mass inside the tank. During this multistep procedure, discrete gradients are created for each parameter using the heating elements of the tank or crushed (freshwater) ice. The physical properties of the seawater are monitored and high-frequency data and duplicate water samples are collected for later analysis with the high-precision salinometer. (Figure 5).  
The uncertainty of the temperature estimation is based on the Guide to the Expression of Uncertainty in Measurement (GUM) following the recommendations by ISO (International Organisation for Standardisation) and BIPM (Bureau International des Poids et Mesures). The temperature uncertainty budget consists of three components: tank stability, tank homogeneity and reference thermometer accuracy. Tank stability and homogeneity were estimated from the readings of the reference thermometers during the custom bath characterization experiments (type A). Reference thermometer uncertainty was estimated from the specifications of the thermometers (type B). The expanded uncertainty, with a coverage factor k = 2, of the reference temperature sensors is \(\pm\)0.0034 \({}^{\circ}\)C (Table 4).  
For turbidity and chl-a sensors, the calibration procedure is performed with artificial solutions of different concentrations and the experiments validate the offset and the linearity of the sensor. Dissolved oxygen sensors are validated using 0% and 100% saturated solutions and calibration data are collected for different oxygen concentrations depending on predefined salinity and temperature gradients (Figure 6). In the future, we are aiming to adjust even further the calibration range of each sensor to the specific local conditions at each deployment site and expand the calibration experiments to include pH and p C02 sensors.