2.3 Gas Detection
43
2.3.2 Electrochemical Sensors
This is the most common type of sensor (MSA 2007). Electrochemical sensors are
actually small reactors in which, when certain gases are present, a chemical reaction that releases electrons takes place (Kumar et al. 2013). To be more specific, the
gases usually penetrate a hydrophobic membrane and come into contact with an electrode embedded in an electrolyte. This causes oxidation and reduction reactions that
discharge electrons (Fig. 2.4). The electrons that are generated are measured using
electronic equipment that establishes the relationship between the current generated
and the concentration.
A
CO
CO 2
e
H 2 O
O 2
O
H
+
Fig. 2.4 Scheme of a CO meter
2.3.3 Catalytic Sensors
Catalytic sensors are based on oxidizing flammable gases or vapours and measuring
the heat released from the reaction. The most common sensors consist of two platinum pellistors that have an electrical connection between them in the form of a
Wheatstone bridge (Fig. 2.5). The coil of one of the pellistors is treated with a catalyst for the reaction, and the other is treated with a substance that inhibits the reaction
(Kumar et al. 2013).
8 An electric current passes this structure to promote the oxidation reaction of the gas that is being measured at approximately 450 °C. When the
catalytic reaction takes place, the temperature of this coil rises, increasing its resistance and unbalancing the bridge (MSA 2007). The bridge current is approximately
proportional to the gas concentration at the Lower Explosive Limit (LEL).
8 Pellistors are miniature calorimeters used to measure the heat released during the burning of a gas
or combustible vapour.
43
2.3.2 Electrochemical Sensors
This is the most common type of sensor (MSA 2007). Electrochemical sensors are
actually small reactors in which, when certain gases are present, a chemical reaction that releases electrons takes place (Kumar et al. 2013). To be more specific, the
gases usually penetrate a hydrophobic membrane and come into contact with an electrode embedded in an electrolyte. This causes oxidation and reduction reactions that
discharge electrons (Fig. 2.4). The electrons that are generated are measured using
electronic equipment that establishes the relationship between the current generated
and the concentration.
A
CO
CO 2
e
H 2 O
O 2
O
H
+
Fig. 2.4 Scheme of a CO meter
2.3.3 Catalytic Sensors
Catalytic sensors are based on oxidizing flammable gases or vapours and measuring
the heat released from the reaction. The most common sensors consist of two platinum pellistors that have an electrical connection between them in the form of a
Wheatstone bridge (Fig. 2.5). The coil of one of the pellistors is treated with a catalyst for the reaction, and the other is treated with a substance that inhibits the reaction
(Kumar et al. 2013).
8 An electric current passes this structure to promote the oxidation reaction of the gas that is being measured at approximately 450 °C. When the
catalytic reaction takes place, the temperature of this coil rises, increasing its resistance and unbalancing the bridge (MSA 2007). The bridge current is approximately
proportional to the gas concentration at the Lower Explosive Limit (LEL).
8 Pellistors are miniature calorimeters used to measure the heat released during the burning of a gas
or combustible vapour.