Continuous Emission Monitoring. James A. Jahnke
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a similar method; both methods are a breakthrough in trace gas analysis (Figure 4‐15).
Instruments using these methods are able to measure gases at part per trillion levels. The CRDS and ICOS techniques are discussed in Chapter 5.
Optical Components
In most electro‐optical analyzers, other components are used to direct and focus light. The following components can be found in gas monitoring instrumentation:
Neutral density filters
Lenses
Mirrors (concave and triple)
Beam splitters (half‐silvered mirrors)
Optical light fibers
Rotating shutters/window slits/irises
Lenses, slits, and diaphragms are used to focus light on through the system. Plain glass windows are used to separate the primary optical system from flue gases. Optical light fibers can be used to transmit light from the stack to a spectrometer or can route light in a spectrometer to simplify otherwise complex lens or mirror arrangements. Rotating shutters are used to reflect or block light to create oscillating signals.
Figure 4‐16 Constructing a spectrophotometer.
A half‐silvered mirror is a partially silvered mirror which both reflects and transmits light – essentially enabling two functions out of one component in an optical design. Half‐silvered mirrors may be more familiar in their application in mirrored sunglasses, or the “transparent mirror” seen in movies or amusement centers. Half‐silvered mirrors can be found in most double‐pass opacity monitors and in‐situ gas analyzers.
Constructing a Spectrophotometer
The components discussed above are used in various combinations in the construction of pollutant gas monitors. Putting them together, one can construct a spectrophotometer for gas measurements (Figure 4‐16), or a particulate monitor for light scattering or light transmission measurements. Different combinations of these components will be examined in the following chapters that discuss the many unique systems commercially available today.
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