Oxygen Sensor Principles of Operation

Materials: Membranes sealed to a plastic body encapsulate anode, cathode and a base electrolyte. Wires conduct outputs from anode (-) and cathode (+) via an external circuit typically a PCB. The PCB consists of various electrical connectors, a resistor-thermistor temperature compensation network and is attached to the rear of the sensor.

Operation: The galvanic fuel cell sensor is actually an electrochemical transducer which generates a current (µA) signal output that is both proportional and linear to the partial pressure of oxygen in the sample gas. Oxygen diffuses through the front sensing membrane and reaches the cathode where it is reduced by electrons furnished by the simultaneous oxidation of the anode. The flow of electrons from anode to cathode via the external circuit results in a measurable current proportional to the partial pressure of oxygen (PO2). The sensor has an inherent absolute zero, therefore, no oxygen no signal output.

Life: In theory, sensor life is limited by the amount of anode material and signal output. A higher signal output yields a shorter life because the anode is being consumed at a faster rate. In reality, however, the Expected Life specification considers the signal output range. In general, sensor life is inversely proportional to changes from the specified parameters: oxygen concentration (air 20.9%) and pressure (1 atm), and, exponentially (2.5% per °C) for temperature (25°C/77°F).

Signal Output: A higher or lower signal output within the specified output range offers no performance advantage. The PCB network converts the signal output from current (µA) to (mV) signal output. Signal output can be influenced (and compensated) by several factors such oxygen concentration, temperature and pressure. However the design of sensors for low level measurements involves a delicate balance between a higher signal output that improves stability by reducing the influence of temperature, and, life.

Temperature: Influences the signal output at the rate of 2.54% per °C. Ambient (gradual) changes in temperature can be compensated within the +2% accuracy specification by processing the signal output through an appropriate resistor-thermistor temperature compensation network. Step (rapid) changes should be avoided or allow at least 15 minutes for the signal output and temperature compensation network to equalize. The effect depends on the temperature change. In some applications, electronic compensation is used to eliminate the effect of temperature.

Pressure: Influences signal output on a proportional basis. Tests show sensors are accurate at any constant pressure up to 30 atm provided the sensor is pressurized equally front and rear. A pressurized sensor must decompressed gradually (similar to a human).

Operation: Altitude: 200 ft. produces an error of 0.3% and do not have a significant effect on the signal output. Recalibrate when altitude changes by more than 500 ft.

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Humidity: Water vapor according to Dalton’s Law of Partial Pressure exerts its own partial pressure when added to a gas stream, thereby, reducing the partial pressure of oxygen and the reading displayed. Conversion charts are available for air calibration which define the effect of humidity on the oxygen level.

Carbon Dioxide (CO2): An acid gas that reacts with the sensor’s base electrolyte. The effect on the sensor varies with exposure time. Exposure to CO2 for 15-20 minutes followed by flushing with air has virtually no effect on the sensor. Repeated exposure of 3-4 hours can result in a temporary loss of signal output. Continuous exposure has a dramatic effect on sensor life. For example a sensor with a normal 12 month life in air at 25°C/77°F and 1 atm that is continuously exposed to 5-6% CO2 will expire in 3-4 weeks.

Load: The sensor does not tolerate reverse current flowing into the sensor. No load is recommended, but 10K Ohm is the maximum permissible. Exceeding a load of 10K Ohm produces an error in linearity.

Calibration: Follow the recommendations included in the manufacturer’s Owner’s Manual. Perform at or near operating conditions, e.g. if measuring dry compressed gas, calibrate with same or if calibrating in air use a conversion chart which defines the effect the humidity (above) and temperature on the oxygen level. Do not calibrate with air when intending to measure above 30% oxygen, calibrate with 100% oxygen.