ASTM E800 Guide for Measurement of Gases Present or Generated During Fires
11. Analytical Methods for Water Vapor Analysis
11.1 General Comments:
11.1.1 Water vapor is often desirable to measure in combustion systems when it is necessary to make chemical stoichiometry calculations, compute elemental compositions to be ascribed to pyrolysates, or to correct readings from other gas analyzers where water vapor had to be removed.
11.1.2 The highest fraction of water vapor present can often be computed from reaction stoichiometry; readings of 10 % or greater can be expected. The sample lines must be heated and the measuring instrument so designed that water is not condensed in the lines nor inside the instrument.
11.2 Apparatus and Procedures:
11.2.1 Infrared Techniques - Water vapor is a very strong absorber in the infrared and has numerous spectral lines and bands. It can be readily measured by conventional nondispersive infrared techniques, or monitored as part of spectral scanning. The requirement that condensation must not occur limits the use of those instrument designs where sufficiently high case-heater temperatures are not available. Due to the strong-absorber nature of water vapor and the fact that much higher concentrations of water vapor are encountered than of most other species, the discrimination against interfering species tends to be good for even simple optics designs.
11.2.2 Other Techniques - Commercial instruments are available for water-vapor measurements which are based on numerous other principles including semiconductor surface effects, chilled-mirror dew point, hygroscopic length change, and others. These are generally designed for the measurement of water-vapor levels in relatively clean environments and are normally not appropriate for use in fire-gas environments due to the highly corrosive and reactive nature of most combustion streams.
