Читать книгу Continuous Emission Monitoring - James A. Jahnke - Страница 141

The type of detector used in an analyzer is very dependent on the energy of the light that it is sensing. Because light in the infrared region is relatively weak in the energy it carries, special stratagems are often devised to detect infrared intensity changes. Pneumatic, microphone‐type detectors (Luft detectors) traditionally have been used in infrared analyzers; however, solid‐state detectors, cooled with Peltier coolers, are common today. Sensitivity is often increased by not overly limiting the spectral region of the analyzer, but using a broader band of radiation to obtain more light for the detector. The special methods of gas filter correlation and Fourier‐transform infrared spectroscopy take advantage of this technique.

The most familiar detector in the visible region is the human eye, which is, of course, used as the detector in EPA Reference Method 9 for measuring visible emissions. Phototubes, photomultiplier tubes, photovoltaic cells, and photo‐diode arrays are used in instrumented systems. Photo‐diode arrays are being incorporated increasingly into CEM analyzers (Durham et al. 1990; Saltzman 1990). Diode arrays provide a simple way of measuring multiple wavelengths and are useful for monitoring several gases in one analyzer, rather than using a separate analyzer for each gas.