Optical sensor for the determination of moisture

Cobalt(II) chloride in gelatin is cast as films on the surface of 600-μm optical fibers. The absorption of these films at 680 nm is measured through the fiber by internal reflection spectroscopy to determine the relative humidity of air between 40% and 80%. The spectrum of cobalt chloride on the fiber is similar to a transmission spectrum rather than an attenuated totla reflection (a.t.r.) spectrum because of the refractive index of the film, which is slightly greater than that of the fiber. Consequently, the spectrum is less sensitive to measurements near the critical angle, and to refractive index changes than is the case with an a.t.r. spectrum. The uptake of water by cobalt chloride at different humidities suggests that the spectral change is due to the formation of hydrates having molecules of bound water similar to those shown by the free salt.