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e • Readings must be maintained at this level or higher for a minimum of 10 minutes to justify an upgrade or downgrade in PPE. • If air concentrations in the breathing zone exceed background level, personnel will upgrade to Level C PPE, don respirators with HEPA filters, and periodically (each 10 minutes) check for airborne concentrations within the exclusion area. If airborne concentration of toxic vapors exceeds 500 ppm, personnel will leave the site. • The combustible gas indicator (CGI) in combination with OZ level readings will be used to measure the concentration of a flammable vapor or gases in air, underlying the results as a percentage of the lower explosive limit (LEL) of the calibration gas which is usually methane or pentane in air. The LEL of a combustible gas or vapor is the lowest concentration by volume in air that will explode, ignite, or burn when there is an ignition source. The upper explosive limit (UEL) is the maximum concentration. ' Above the UEL, there is insufficient oxygen to support combustion thereby making ignition impossible. Below the LEL there is insufficient fuel to support ignition. The limits of the instruments are the following: • The reaction is temperature dependent. Therefore, the measurement is only as accurate as the incremental differences between calibrations and ambient (sampling) temperatures. • Sensitivity is a function of physical and chemical properties of the calibration gas versus those of the unknown contaminant. Most combustible gas indicators are calibrated to read accurately for methane or propane, but not all combustible gases and vapors will give the same response as the calibration gas. • There is no differentiation between petroleum vapors and combustible gases unless a charcoal pre -filter is employed. • Leaded gasoline vapors, halogens, sulfur compounds, and selenium compounds will fill the filament, thereby decreasing its sensitivity. • CGIs should be calibrated at the altitude that the testing will be performed using a calibration gas, furnished by the manufacturer. The instrument is "warmed up" and ' the zero control is adjusted to reach 0 percent LEL on the meter. The calibration gas is introduced into the sensor opening following the manufacturer's instructions. The span control for the meter reading is adjusted, if necessary, to indicate the correct percent LEL for the calibration gas used. I Information and guidance on use of field monitoring equipment is found in Attachment 8. 1 A-13 i�