Bacharach MGS 400 User Manual

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Bacharach MGS 400 User Manual Manual pdf 45 page image

1100-2294 Rev 1 44It may be necessary to check the calibration if the sensor has been exposed for a long time to ahigh concentration of flammable gases, vapors, or the above-mentioned contaminatingsubstances.The nature of catalytic bead sensor technology means that sensor drift may typically be up to±5% LEL per month. Instruments using these sensors should be zeroed regularly following theinstructions in section 5 of this manual.6.1.3 Semiconductor SensorsSemiconductor or metallic oxide sensors (MOSs) are among the most versatile of all broad-rangesensors. They can be used to detect a variety of gases and vapors in low ppm or even combustibleranges. The sensor is made up of a mixture of metallic oxides. They are heated to a temperaturebetween 150º and 300º C depending on the gas(es) to be detected. The temperature of operationas well as the “recipe” of mixed oxides determines the sensor selectivity to various toxic gases,vapors, and refrigerants. Electrical conductivity greatly increases as soon as a diffusion processallows the gas or vapor molecules to come in contact with the sensor surface. Water vapor, highambient humidity, temperature fluctuations, and low oxygen levels can result in higher readings.IMPORTANT: Certain substances in the environment to be monitored mayimpair the sensitivity of the sensors: Materials containing silicone or silicone rubber/putty Corrosive gases such as hydrogen sulfide, sulfur oxide, chlorine, hydrogenchloride, etc. Alkaline metals, salt water spray.6.1.4 Infrared SensorsThe infrared (IR) gas sensor is designed to measure the concentration of combustible gases andvapors in the ambient air. The sensor principle is based on the concentration-dependentabsorption of infrared radiation in measured gases.The monitored ambient air diffuses through a sintered metal material into the enclosure of anoptical “bench”. The broadband light emitted by an IR source passes through the gas in the opticalbench and is reflected by the walls from where it is directed towards a dual-element detector. Onechannel of the detector measures the gas-dependent light transmission, while the other channelis used as a reference. The ratio between measurement and reference signal is used to determinethe gas concentration. Internal electronics and software calculate the concentration and producean output signal.