Manual Version 19 / 01-2019 II Page 14 of 44
SF2 ECCR OPERATION MANUAL
Chapter 3 - Oxygen Sensors
Since the oxygen content in the breathing gas of rebreather systems has to be
measured reliably and permanently, oxygen sensors are used for this. To increase
the safety and accuracy of the O2 measurement, three oxygen sensors are built into
the SF2-CCR. The sensors are located in the head, where they are well protected
and yet easily accessible. The measured values are permanently transmitted to the
controller, which then regulates the setpoint via the solenoid if necessary. If a sensor
becomes defective, it can be voted out by the controller logic.
Nevertheless, two sensors are still available and the dive can be ended.
There are many different types of oxygen sensors, but galvanic oxygen sensors
have become established for use in diving for many reasons. These are built as a
complete unit and are thrown away and replaced when the internal components are
used up.
3.1 Functional principle and structure of a sensor
A galvanic oxygen sensor is a type of fuel cell that converts chemical reaction energy
into electrical energy (like a battery). This means that these sensors do not need an
external energization voltage, but are the power source themselves. Oxygen reacts
with the components of the cell (redox reaction) which creates a current that is
proportional to the prevailing oxygen partial pressure.
A cathode transfers electrons to the oxygen (reduction). The resulting hydroxide ions
(OH-) migrate in an electrolyte to the anode and there release the electrons again
(oxidation of the anode). This means that the anode is chemically changed or used
up. The electrons released in the reaction create an electrical current that is directly
proportional to the oxygen partial pressure. Practically all oxygen sensors measure
the partial pressure and not the percentage of the gas. This also makes sense there
the human body also reacts to the actual oxygen partial pressure and not to the
percentage in the breathing gas. Only if the ambient pressure is 1 bar and does not
vary between calibration and measurement, the percentage display (or fraction) is
equal to the partial pressure.
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