MULTIPOINT FIBEROPTIC HUMIDITY MONITOR

Spectral Sciences, Inc. is developing a multipoint humidity monitor for drying application in the pulp and paper industry. The humidity is measured by monitoring near-infrared optical absorption in the process stream atmosphere. An array of sensor heads distributed throughout the plant is linked by fiberoptic cables to a single source/readout device which can be located in a control room hundreds of meters away. The sensor heads and cables are modular, allowing for rapid installation and reconfiguration of a distributed sensor system in process control applications. The method used for quantifying the optical absorption provides substantial stability, accuracy, and signal processing advantages over standard wavelength scanning or bandpass techniques. The laser diode's output wavelength is actively locked to the center of a water absorption line, thereby providing a constant monitor of water vapor concentration which is insensitive to all other species in the process stream. Wavelength modulation and second harmonic detection techniques are used to quantify the water vapor concentration. A simultaneous temperature measurement then permits an output of the absolute humidity or humidity ratio for process monitoring or control.