FUNCTIONAL MODEL OF A TOXIC GAS MONITORING SYSTEM

A basic gas detection system can be broken down into several distinct functional components as shown in Fig. 1  

Gas Detection Principles. Some of the more common gas detection principles used in toxie gas monitoring include electroehemistry, electro-optical detection, solid state detection, mass spectrometry, moleeular (or flame) emission speetrometry,irrfrared speetrophotometry, ionization teeh-niques, and thermal conduetivity. Brief deseriptions of several different gas detection technologies are deseribed below. 

Solid State. A solid-state sensor consists of two electrodes imbedded into a metal oxide material as shown in Fig. 4. Operating at a constant power, the sensor has a fixed resistance between the two electrodes. When the target gas is present, it causes a change in resistance of the solid-state material proportional to the target gas concentration. The effects of interferent gases and chemical vapors are minimized through the selection of the metal-oxide sensor material and sensor fabrication techniques. A 

Infrared (IR) Spectrophotometry. As shown in Fig. 7, spectropho-tometric IR sensors introduce an air sample into the gas cell of an infrared spectrometer. Inside the gas cell is a system of lenses and mirrors that directs a beam of monochromatic infrared light in a preselected path through the sample. The amount of energy absorbed by the sample is measured. The same sample is examined at other additional wavelengths. This multiwavelength, multicomponent absorbance data is analyzed by a built-inmicroprocessor to determine the concentration of the compound. 

Catalytic Reaction. A bead or wire is coated with a catalytic material so that it reacts with atarget gas. As the reaction on the catalyzed surface takes place, the bead or wire heats up, and changes its resistance. This resistance change can be proportionally related to the target gas concentration. An example of a catalytic bead detector is a sensor that consists oftwo beads placed in a wheatstone bridge circuit. One of the beads acts as a 

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