16SpecificationsThe oxygen partial pressure (pO 2 ) is then linked to the corresponding phase-shift measurement( to build the sensor calibration curve (shown right in Figure 3 below). This curve is describedby the Stern-Volmer equation (shown left in Figure 3 below) where K sv is the indicatorquenching constant (in mbar -1) representing the quenching efficiency of the oxygen and thus thesensor sensitivity, f 0 is a constant and 0 is the phase-shift at zero oxygen representing theunquenched fluorescence decay time of the dye. The calibration curve thus relies on twoparameters: the phase-shift at zero oxygen and the luminescent spot sensitivity, K sv. Thedissolved oxygen concentration is then calculated with Henry’s law using the water solubilitycurve as a function of the temperature.2.3 Hardware descriptionThe instrument hardware is made of one main board, and one measurement board for themeasurement channel (= the sensor).The main board includes the controls for power, display, the touch screen, the barometricsensor, the alarms, and communication ports. The measurement board performs measurementsand executes commands from the main board. It holds the "Analog output" and "Relays" thatsend information to external systems.A hardware watchdog is activated at program start up, to check that the system is not frozen(i.e. infinite loop, system crash, etc.). If the watchdog is not refreshed by the software everyminute, the measurement display, the relays and the analog output are frozen up to 2 minutes.Then the reset shuts down the instrument for 10 seconds and the start-up procedure isperformed. At the same time all the hardware (sensor, measurement board) are reset.Figure 3 Stern-Volmer equation and calibration curve
