Dew-Point Instrument Minimized the Effect of Contamination on a Mirror

Abstract

Contamination on a mirror is one of the most difficult problems in measuring dew or frost-point by a dew-point instrument with an optical or radioactive dew-sensing system. As a result of experiment, it became evident that the lights diffusely scattered I_n from the deposit on the mirror were sensitive to dew or frost and insensitive to contamination of monoethanolamine or the fine particles of molecular sieve, as far as the angles of incidence were greater than 45° and that of reflection normal to mirror surface.
On the basis of those experimental results, the light diffusely scattered normally to mirror surface was adopted as the reference to a balanced photocell bridge which is the detector of dew or frost on the mirror.
If dew or frost deposit grows largely on the mirror, the amount of light specularly reflected I_s will decrease and I_n increase, so the output of the bridge will increase. The output amplified regulates rf induction heating to raise the temperature of the mirror and diminish dew or frost. On the contrary, if dew or frost deposit diminishs, rf induction heating will decrease and the deposit will grow. When bare areas on the mirror around individual droplets or crystalline particles are stained with contamination, the amount of light I_s will decrease, and then mirror temperature will rise to diminish the deposit. The amount of light I_n, however, will decrease, for dew or frost decrease in volume, so mirror temperature will be down and dew or frost will be kept constant. Adopting this method, the interval to be able to measure dew or frost-point accurately was prolonged about 3∼5 times.