Direct-Reading Instruments for Gases, Vapors, and Particulates

Direct-Reading Instruments for Gases, Vapors, and Particulates

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Direct-Reading Instruments for Gases, Vapors, and Particulates Indiana State University IH 315 L
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Direct-Reading Instruments for Gases, Vapors, and Particulates In the workplace setting, the safety professional is responsible to monitor the air for hazardous and or combustible gases, vapors, and particulates that are in the air. It is important to closely monitor the levels of combustible gases and vapors because at certain levels and with the right spark, an explosion could occur. In order to monitor the levels and to prevent the levels of these gases and vapors from reaching an explosive level, direct-reading instruments are commercially available. The operator must be aware of the calibration, use, and limitations of the instrument being used. Direct-reading instruments are based on one of two principles: the change in resistance of a conductor subjected to heat released by gas combustion, or the change in electrical conductivity of a metallic oxide semiconductor in the presence of a combustible gas. Both of these types require calibration, and must be analyzed and interpreted correctly. Combustible gas monitors are based on three different types of detectors. They are the catalytic combustible gas sensor, the metal oxide semiconductor (MOS), and the thermal conductivity detector. There are several different types of direct-reading instruments available commercially and many are multi-gas monitors. Some have sensors that allow for percent oxygen and parts per million (ppm) levels of toxic gases such as carbon monoxide or hydrogen sulfide. In the catalytic combustible gas detector, heat is released when a combustible gas or vapor, in contact with the detector, is burned or oxidized. In simple versions of these monitors, the detector element is a heated coil of platinum wire that forms one arm of a Wheatstone bridge circuit. The heat released by the burning causes a change in the electrical resistance of the detector filament that proportional to the combustible gas concentration. The change in resistance produces an imbalance in the bridge circuit that can be measured electrically and is translated into combustible gas concentration. More recent versions of the sensors for catalytic combustion devices have a matched-pair of alumina coated filaments. The sensing filament forms one leg of a Wheatstone bridge and burns the combustible gas. The compensating filament forms a second leg of the Wheatstone bridge but does not burn the combustible gas. In all other respects the two filaments behave similarly. This improved catalytic combustible gas sensor allows for significantly improved zero and span stability. A combustible gas monitor that uses this type of sensor is shown in Figure 17-2. Figure 17-2. A multi-gas monitor that uses catalytic
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Direct-Reading Instruments for Gases, Vapors, and Particulates

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