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5 <br /> detectable species, however, it is expensive and requires constant maintenance and frequent lamp <br /> replacement For many applications, the 10 2-eV lamp can be used It offers relatively high <br /> radiation levels without frequent lamp replacement, and will detect many main species One notable <br /> exception are the chlorinated aliphatics <br /> Table at the end of this attachment, shows the ionization potentials for species of various <br /> chemical compounds <br /> 4.1.3 Limitations <br /> Although the HNU ultraviolet photoionization unit is an excellent field instrument for initial <br /> air monitoring, there are very important limitations <br /> I • The response to a gas or vapor may radically change depending on the choice of <br /> calibration gas and when the gas or vapor is mixed with other materials As an example, <br /> a HNU calibrated to ammonia (NH,) and analyzing an atmosphere containing 100 ppm <br /> NH, would indicate 100 on the meter Likewise, a unit calibrated to benzene would <br /> record 100 in an atmosphere containing 100 ppm benzene However, in an atmosphere <br /> containing 100 ppm of each, the unit could indicate considerably less or more than 200 <br /> ppm, depending on how it was calibrated <br /> • Radio frequency interference from pulsed Direct Current or Alternating Current power <br /> lines, transformers, high voltage equipment, and radio wave transmission may produce an <br /> error in response <br /> • The lamp window must be periodically cleaned to ensure ionization of the air within the <br /> containment <br /> • Although the HNU measures concentration in the range from 1 to 2,00(}ppm, the <br /> I response is not linear over this entire range For example, the response to benzene is <br /> linear from about 0 600 parts per million This means the HNU reads a true <br /> concentration of benzene only between 0 and 600 Greater concentrations are "read" at a <br /> lower level than the true value <br /> • The HNU is sensitive to many organic and inorganic vapors and gases and cannot be used <br /> I as a quantitative measuring instrument in unknown situations It measures total organic <br /> vapors and gases in ppm It is strictly qualitative, except when the nature of the <br /> contamination is known, and the instrument has been calibrated to, or a calibration curve <br /> has been generated for, the contaminant being measured <br /> • High humidity (90 percent and above) will affect the sensitivity <br /> • The HNU will not respond to methane (CH,) <br /> • Water vapor in air interferes with the HNU response <br /> 0 - 9 <br />