Abstract: A method of determining the dew point or frost point of a gas containing a very small amount of water uses an optical dew point meter having a reflector mirror the temperature of which can be varied from room temperature to any point of -80.degree. C. or below, a device for contacting the reflector mirror with the gas to be measured, a light source for irradiating the reflector mirror with focused rays of light, and a detector for detecting the change in scattered light and/or reflected light due to the dew and/or frost condensed on the reflector mirror. The method gradually reduces the temperature of the reflector mirror, either before or while the reflector mirror and the gas contact, thereby condensing dew and/or frost on the reflector mirror. Following formation of dew and/or frost on the reflector mirror, the intensity of scattered light or reflected light is adjusted from a maximum value to a constant level, at a controlled temperature.

Abstract: The invention relates to an apparatus for determining the frost point of a gas containing a very small amount of water. The apparatus comprises (I) compartment A made of a good heat conductor; (II) an inlet for the gas to be measured, which inlet is provided on compartment A; and (III) a heater for adjusting the temperature in compartment A, and a temperature sensor. The apparatus further comprises (IV) compartment B which is adjacent to compartment A. At least part of compartment B is made of a poor heat conductor. Further, the apparatus comprises (V) a hole at the interface between compartments A and B and (VI) a reflector mirror having a temperature sensor. The reflector mirror is provided on compartment B in such a way as to cover the hole is connected thermally to a freeze source in such a way that the temperature of the reflector mirror can be varied from room temperature to at least -80.degree. C.

Abstract: A method of determining the dew point of a gas containing a very small amount of water using (1) a reflector mirror the temperature of which can be varied from room temperature down to at least -80.degree. C.; (2) a source of condensed rays of light for irradiating the reflector mirror; and (3) a light recovering device that detects changes in scattered light due to condensation on the reflector mirror by exposing the mirror to the gas. The method comprises cooling the mirror and detecting the temperature at which the scattered light has a maximum intensity; slowly raising the temperature and detecting the temperature at which the scattered light has a minimum intensity and repeating the cycle until the temperature at which the intensity is a maximum and the temperature at which the intensity is a minimum are almost the same.

Abstract: A method of cleaning a reflector mirror in an optical dew point meter that measures the dew point or frost point of a sample gas by a process including the steps of bringing the sample gas into contact with the reflector mirror whose temperature is variable, projecting condensed rays of light or laser light onto said reflector mirror, and detecting the change in the intensity of scattered light and/or reflected light on account of the dew or frost formed on said reflector mirror, characterized by bringing a CO.sub.2 gas or a CO.sub.2 containing gas into contact with the reflector while the latter is in a cooled state, thereby condensing CO.sub.2 on the surface of the reflector mirror, and the subsequent step of heating the reflector mirror to vaporize the CO.sub.2 is dischosed.

Abstract: An improved method of and an apparatus for measuring a very low water content in a non-condensible gas which has a dew point in the range of from ordinary temperature to -80.degree. C. or less. The gas to be measured is pre-cooled to a temperature slightly higher than that of the dew point, and is then blown against a reflecting mirror whose surface is cooled down to the dew point temperature of the gas. A condensed light having high luminance is projected toward the reflecting mirror. When the gas is blown against the mirror and cooled down to the temperature of the dew point, thereof a dew or frost is formed on the mirror, and the amount of scattered light reflected from the mirror is increased by the dew or frost. This increase in scattered light is detected by a photodetector. A silicon wafer which is polished to have a surface precision of 1/4 or less that of the wavelength of the condensed light is employed as the mirror to enable measurement of dew points of -80.degree. C. or less.