(SO2) monitors are available. However, data from such measurements would be of limited utility and do not justify the expense or the use of limited space.

HYDROCARBON DETECTORS

Photoioniziation detectors respond to a large number of organic compounds but are non-specific and have limited sensitivity. In general they are not sensitive enough to detect many of the hydrocarbons at levels that are of potential concern in this setting. Furthermore, these instruments respond with different sensitivity to different compounds, and they are plagued by many of the problems that affect volatile organic compounds (VOC) measurements including drift, surface losses, and marked interference from water vapor. Primarily because of their non-specific response, they are probably of limited additional value unless a measure of total hydrocarbon concentration would be useful.

Catalytic hydrocarbon detectors measure carbon dioxide (CO2) before and after air is passed over a hydrocarbon oxidation catalyst, which provides a measure of oxidizable carbon species. Such systems must be designed to scrub the majority of CO2 from the airstream prior to before/after measurement or signal will be swamped by atmospheric CO2; the same is true of methane and propane. Even taking such precautions, signals will probably still be dominated by ethanol from both breath and beverage service and by acetone from breath. Against this large and varying background, it would be quite difficult to discern changes in CO2 resulting from a condition such as leaked fluid in the bypass air. Consequently, this approach does not appear to be feasible.

PARTICLE DETECTORS

In addition to the light scattering instrument discussed in Chapter 7, there are direct-reading particle methods based on the behavior of electrically charged particles (Hinds 1999). Instruments using electrical charge include commercial smoke detectors as well as more technically sophisticated electrical aerosol analyzers. Smoke detectors employ an ionizing radiation source to generate electric charge on particles, and the resulting change in electric current is used to sense the presence of particles in air. These devices respond within seconds to relatively high concentrations of fine particles (e.g., combustion aerosols), but may not be suitable for continuous monitoring of lower levels



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