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' Although several instruments can measure an oxygen-enriched atmosphere (Oz greater than <br /> 20 8 percent by volume), no testing or other work should ever be performed under such conditions <br /> because a spark, arc or flame could lead to fire or explosion <br /> ' 2.3 CALIBRATION <br /> ' Oxygen survey meters are calibrated electrically to read 20 8 percent oxygen when the meter <br /> is in normal ambient air at standard atmospheric pressure The oxygen reading function button is <br /> ' depressed, and the instrument is allowed to stabilize The span control knob is adjusted to set the <br /> display reading to 20 8 percent Table at the end of this attachment shows the correct calibration <br /> ' volume percent of oxygen at various altitudes <br /> ' 3.0 EXPLOSIVE ATMOSPHERES <br /> ' The CGI is one of the first instruments to be used to survey a site Typically, CGI readings <br /> are taken concurrently with 02 level readings It measures the concentration of a flammable vapor or <br /> gas in air, indicating the results as a percentage of the LEL of the calibration gas, usually methane or <br /> pentane in air <br /> ' The LEL of a combustible gas or vapor is the lowest concentration by volume in air that will <br /> explode, ignite, or burn when there is an ignition source The upper explosive limit (UEL) is the <br /> maximum concentration Above the UEL, there is insufficient oxygen to support combustion so <br /> ignition is impossible Below the LEL, there is insufficient fuel to support ignition <br /> 3.1 THEORY <br /> Most combustible gas indicators operate on the "hot wire" principle In the combustion <br /> ' chamber is a platinum filament that is heated The platinum filament is an integral part of a balanced <br /> resistor circuit called a Wheatstone Bridge The hot filament combusts the gas on the immediate <br /> ' surface of the element, thus raising the temperature of the filament <br /> O - 4 <br /> 1 <br />