User Manual PolyGard
®
2 Sensors 2023-11 Page 8 | 40
B: Electrochemical Sensor Element for Toxic Gases and Oxygen
The integrated sensor is a sealed electrochemical cell with 3 electrodes, sensing, reference and coun-
ter or with 2 electrodes, sensing and counter. The ambient air to be monitored diffuses through the
opening on the sensor head into the measuring chamber of the sensor. The chemical process of the
measurement is based on a redox reaction consisting of oxidation and reduction. The electrolyte in
the sensor is responsible for the transport and exchange of ions between the electrodes. Electrolyte,
cathode and anode material are designed to allow a redox reaction in the cell. The reactions at the
anode and cathode result in a current signal (nA) between the measuring and counter electrodes.
This signal is linear to the concentration of the gas present. The current is subsequently amplified,
evaluated and converted into a linear output signal. The electrochemical process leads to a loss of
sensitivity over time. Therefore, calibration is necessary at regular intervals.
The oxygen sensor works according to the principle of a galvanic fuel cell. Due to oxidation, an elec-
tric current proportional to the oxygen partial pressure flows. The electrolyte is partially consumed
by the electrochemical process. This limits the service life of the sensor.
Caution:
There is a small quantity of corrosive liquid in the sensor. If, in the event of damage, per-
sons or objects should come into contact with the liquid, you have to clean the affected
areas as fast and carefully as possible with clean water. Out-of-use sensors must be dis-
posed of in the same way as batteries.
C: Semiconductor Sensor Element for Freon Gases, Ethylene and Ammonia
Semiconductor gas sensors (metal oxide sensors) are electrical conductivity sensors. The resistance
of its sensitive layer changes upon contact with the gas to be detected. The gas then reacts with the
sensor surface. This reaction is completely reversible in the ideal case. Due to their chemical prop-
erties, metal oxide gas sensors are suitable for a wide range of applications and the detection of all
reactive gases. Depending on the materials used and the gases to be detected, common operating
temperatures in the semiconductor sensor are between 300 °C and 900 °C. The signal is double-
logarithmic to the gas concentration – not linear.
Oxidation processes lead by-and-by to an unwanted influence on the alteration of the conductivity.
Therefore, regular calibrations are necessary. If a semiconductor sensor has been exposed to a high
gas concentration, it should be replaced.
Semiconductor sensors are not linear on principle. This means that the sensor would only measure
accurately on the calibrated point and not over the entire measuring range. Therefore, a linearization
is included in our software to improve this behaviour. But since the sensor resistance of a semicon-
ductor elements can be very different, it must be normalized. This can be done via the potentiometer
on the PCB. Thus, the different output signals can be individually adjusted to a normalised value.
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