34 • MRLDS-450 Refrigerant Gas Detection I&O Manual
026-1316 Rev 0
6
Additional Information
6.1. Sensor Principle
6.1.1. Electrochemical Sensors
Electrochemical sensors measure the partial pressure of gases under atmospheric
conditions. The monitored ambient air diffuses through a membrane into the liquid
electrolyte in the sensor. The electrolyte contains a measuring electrode, a
counter-electrode, and a reference electrode. An electronic “potentiostat” circuit
ensures a constant electrical voltage between measuring electrode and reference
electrode. Voltage, electrolyte, and electrode material are selected to suit the gas
being monitored so that it is transformed electrochemically on the measuring
electrode and a current flows through the sensor. This current is proportional
to the gas concentration. At the same time, oxygen from the ambient air reacts at
the counter electrode electrochemically. The current flowing through the sensor is
amplified electronically, digitized and corrected for several parameters (For
example, the ambient temperature).
6.1.2. Catalytic Bead Sensors
A catalytic bead sensor measures the partial pressure of combustible gases and
vapors in ambient air. It uses the heat-of-combustion principle.
The monitored air diffuses through the sintered metal disc into the sensor. The
mixture of combustible gases, vapors, and air are catalytically combusted at a heated
detector element (called a pellistor). The oxygen content in the air must be greater
than 12 Vol%. Due to the resulting heat-of-combustion, the temperature of the
detector element rises. This increase in temperature causes a change of resistance in
the detector element, which is proportional to the concentration of the mixture of
combustible gases and vapors in the monitored air. In addition to the catalytically
active detector element, there is a compensator element. Both elements are parts of
a Wheatstone bridge. Thus environmental effects like changes in ambient
temperature or humidity are almost entirely compensated.
IMPORTANT: Certain substances in the atmosphere to be monitored may
impair the sensitivity of the sensors. Such substances include, but are not limited to:
1. Polymerizing substances such as acrylonitrile, butadiene, and styrene.
2. Corrosive compounds such as halogenated hydrocarbons (releasing halogens such as
bromine, chlorine or fluorine when oxidized) and halogen hydride acids as well as acidic gaseous
compounds such as sulfur dioxide and nitrogen oxides, Catalyst poisons such as sulfurous and
phosphorous compounds, silicon compounds (especially silicones), and metal-organic vapors.