ARCHIVED REPORTS_XR0011603 - Laserfiche WebLink

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' As the temperature of the filament increases so does its resistance This change to resistance causes an imbalance in the Wheatstone Bridge This is measured as the ratio of combustible vapor present compared to the total required to reach the LEL For example, if the meter reads 0 5 (or 50, depending upon readout), this means that 50 percent of the concentration of combustible gas needed ' to reach an unstable flammable or combustible situation is present If the LEL for the gas is 5 percent then the meter indicates that 2 5 percent concentration is present Thus the typical meter ' readout indicates concentration up to the LEL of the gas If a concentration greater than LEL and lower than the UEL is present, then the meter needle will stay beyond the 1 0 (100 percent) Ievel on the meter This indicates that the ambient atmosphere is readily combustible When the atmosphere has a gas concentration above the UEL the meter needle will rise above the 1 0 (100 percent) mark and then return to zero This occurs because the ' gas mixture in the combustion cell is too rich to burn This permits the filament to conduct a current dust as if the atmosphere contained no combustibles at all ' 3.2 LIMITATIONS As with any instrument based on an electrochemical reaction, alI CGI's have several ' limitations • The reaction is temperature dependent Therefore, the measurement is only as accurate as the incremental difference 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 pentane, but not all combustible gases and vapors will give the ' same response as the calibration gas Because of the variation in the relative response of the flammable substance in the atmosphere to the calibration case (e g , methane), the instrument may not give an accurate indication of the flammable hazard -- the reading ' (percent LEL) may be higher or lower than the actual concentration 0 There is no differentiation between petroleum vapors and combustible gases unless a ' charcoal pre-Filter is employed • The unit is intended for use only in normal atmospheres, not ones that are oxygen enriched or deficient Oxygen concentrations that are less than or greater than normal ' may cause erroneous readings • Leaded gasoline vapors, halogens, sulfur compounds, and selenium compounds will foul ' the filament, thereby decreasing its sensitivity Compounds containing silicone will destroy the platinum filament 0 - 5 1