Bacharach MGS 400 User Manual

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Contents

Bacharach MGS 400 User Manual Manual pdf 44 page image

1100-2294 Rev 1 436. Additional Information6.1 Sensor Principle6.1.1 Electrochemical SensorsElectrochemical 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 measuringelectrode and reference electrode. Voltage, electrolyte, and electrode material are selected to suitthe gas being monitored so that it is transformed electrochemically on the measuring electrodeand a current flows through the sensor. This current is proportional to the gas concentration. Atthe same time, oxygen from the ambient air reacts at the counter electrode electrochemically. Thecurrent flowing through the sensor is amplified electronically, digitized and corrected for severalparameters (e.g., the ambient temperature).6.1.2 Catalytic Bead SensorsA catalytic bead sensor measures the partial pressure of combustible gases and vapors inambient air. It uses the heat-of-combustion principle.The monitored air diffuses through the sintered metal disc into the sensor. The mixture ofcombustible 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 theresulting heat-of-combustion, the temperature of the detector element rises. This increase intemperature causes a change of resistance in the detector element, which is proportional to theconcentration of the mixture of combustible gases and vapors in the monitored air. In addition tothe catalytically active detector element, there is a compensator element. Both elements are partsof a Wheatstone bridge. Thus environmental effects like changes in ambient temperature orhumidity are almost entirely compensated.IMPORTANT: Certain substances in the atmosphere to be monitored may impairthe sensitivity of the sensors. Such substances include, but are not limited to: Polymerizing substances such as acrylonitrile, butadiene and styrene. Corrosive compounds such as halogenated hydrocarbons (releasinghalogens such as bromine, chlorine or fluorine when oxidized) andhalogen hydride acids as well as acidic gaseous compounds such assulfur dioxide and nitrogen oxides. Catalyst poisons such as sulfurous and phosphorous compounds,silicon compounds (especially silicones), and metal-organic vapors.