Method of cleaning the reflector mirror in an optical dew point meter and an optical dew point meter equipped with a cleaning device

An optical dew point meter equipped with a cleaning device, said meter comprising:

• (i) a reflector mirror whose temperature is variable;
• (ii) a gas inlet or a gas blowing nozzle for supplying a sample gas into a compartment including said reflector mirror in such a way that said sample gas will contact said reflector mirror;
• (iii) a means for protecting condensed rays of light to that part of the reflector mirror where dew or frost is to form;
• (iv) a means for detecting the change in the intensity of scattered light or reflected light due to the dew or frost formed on the reflector mirror;
• (v) a means for sensing that the capability of detecting a normal change in the intensity of reflected light or scattered light has decreased on account of the fouling of the reflector mirror;
• (vi) a means for causing CO.sub.2 gas of five-nine grade purity, purified CO.sub.2 or purified CO.sub.2 containing gas to contact the reflector mirror either automatically or manually, said CO2 or CO2 containing gas having been preliminarily purified by removing water, oils and particles;
• (vii) a means for detecting the temperature at which the change in the intensity of scattered light or reflected light occurs, and
• (viii) means for controlling the temperature of said reflector mirror down to at least -78.5.degree. C.

An optical dew point meter equipped with a cleaning device, said meter comprising;

• (i) a reflector mirror;
• (ii) means for controlling the temperature of said mirror down to at least -78.5.degree. C.;
• (iii) a gas inlet or a gas blowing nozzle for supplying a sample gas into a compartment including said reflector mirror in such a way that said sample gas will contact said reflector mirror;
• (iv) a means for projecting condensed rays of light to that part of the reflector mirror where dew or frost is to form;
• (v) a means for detecting the change in the intensity of scattered light or reflected light or reflected light due to the dew or frost formed on the reflector mirror;
• (vi) a means causing liquefied CO.sub.2 gas of five-nine grade purity, CO.sub.2 or CO.sub.2 containing gas to contact the reflector mirror either automatically or manually, said CO.sub.2 or CO.sub.2 containing gas having been preliminarily purified for removing water, oils and particles;
• and
• (vii) a means for detecting the temperature at which the change in the intensity of scattered light or reflected light occurs.