Abstract
Mirror contamination is one of the most difficult problems in measuring dew or frost-pointcontinuously by a dew-point instrument which detects dew or frost by specularly reflected light. In a recent paper, as a method of minimizing the effect of mirror contamination, the light diffusely scattered normally to mirror surface was adopted for reference of a balanced photocell bridge which was the detector of dew or frost. In order to obtain more effective results, near infrared rays are employed as specularly reflected light. The reason for it is that the intensity of specularly reflected light from a rough surface is related to σ/λ (Porteus formula), where λ is the wavelength of the light and σ is the r.m.s. roughness of the surface.
That is, in the case of near infrared rays, the specularly reflected light is strongly scattered and decreased in intensity by islands of frost (σ>λ), but scarcely decreased by monoethanolamine (M.E.A.) deposit (σ<λ). The r.m.s. roughness of M.E.A. deposit increased inproportion to t1/2 under the usual measuring condition.
By those experimental results and Porteus formula, the relation that the operating interval allowing a continuous frost-point measurement increases in proportion to λ2 is derived. The new type dew-point instrument is constructed using together the infrared specularly reflected light and the diffusely scattered one, and the interval of measuring frost-point is prolonged about 10 times or more.
