A fast response, open path, infrared hygrometer, using a semiconductor source

Abstract

Investigations were carried out as to the feasibility of using a semiconductor source in the design of a new rapid response, open-path hygrometer. A single-beam instrument was constructed employing an infrared light emitting diode (LED) as a source instead of the usual high energy, wideband filament. The spectral emission envelope encompassed the 1.87 μm water absorption band. Electronic modulation and thermoelectrical cooling of the diode eliminated the conventional chopper wheel and stabilized the peak wavelength emission. Path length was 200 mm. Over a water vapour concentration range of 0–16 g m−3, absorption varied by 2% in a linear fashion. At 10 Hz, the noise level was 0.1 g m−3 rms. Hygrometer resolution and stability are constrained by the detector noise level, the small source emission in the absorption band and low frequency drift in the optical filter. Despite these problems, the new instrument showed comparable performance characteristics to a commercial Lyman-α hygrometer. Latent heat fluxes measured with both instruments and a Kaijo-Denki, 3-D sonic anemometer agreed to within 4% over a range 0–350 W m−2. Further improvements in performance can be anticipated with advances in detector and LED technology.