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' In most circumstances, using the HNU at the lowest setting (span pot 9 8) provides adequate <br /> data to determine the proper health and safety protocols for on-site workers Unfortunately, several <br /> chemicals - for example, acrolem - exhibit medium to low sensitivity (0 to 5), while their <br /> toxicological effects place their threshold limit value (TLV) at a very low level If these chemicals <br /> ' are indicated by the HNU set to 9 8, for example, improper personal protective equipment could be <br /> chosen <br /> Consider this scenario <br /> ' The air in an unknown hazardous environment must be sampled Response personnel survey <br /> ' the site with an HNU that indicates 2 0 ppm (instrument span set to 9 8) Later, the air contaminant <br /> is found to be acrolein with a TLV of 0 1 ppm (100 ppb) and an immediately dangerous to life or <br /> ' health (IDLH) level of 5 ppm Since acrolein has a low relative sensitivity, its concentration probably <br /> was in excess of 5 ppm, the IDLH value <br /> ' Thus total reliance to the HNU data without regard for the chemical makeup of the sample <br /> can be a problem, and more chemical-specific monitoring may be required for some hazardous waste <br /> sites <br /> ' 4.1.4 Calibration <br /> ' HNU calibration gas consists of approximately 100 ppm isobutylene in an air matrix that has <br /> been referenced to benzene The meter reading that should be obtained during calibration is marked <br /> ' on the gas canister Iabel Most calibration gas canisters supplied by HNU have a label stating a <br /> particular ppm to be read with a span setting of 9 8 This is the recommended span setting for the <br /> ' 10 2 eV lamp only, the most commonly used lamp The span setting for the 9 5 eV lamp should be <br /> 1 0, and the span setting for the 11 7 eV lamp should be 5 0 <br /> 1 <br /> Any calibration gas used should have the same matrix gas as the atmosphere in which the <br /> ' HNU will be used Use of any other gas will significantly affect the readings obtained, making <br /> readings appear far higher or lower than they actually are If the monitor is to be used in air, <br /> ' nitrogen or any gas other than air should never be used as the matrix gas in calibration standards <br /> 0 - 11 <br />