THE DEVELOPMENT OF A SENSOR AND AN ANALYZER FOR ON-LINE AN ULTRA-LOW DEW (FROST) POINT HYGROMETER WITH WIDE DYNAMIC

THE ABILITY OF A PIEZOELECTRIC CRYSTAL TO DETECT EXTREMELY SMALL AMOUNTS OF MASS ON ITS SURFACE ACCURATELY IS USED AS A MEANS OF DETECTING THE ONSET OF CONDENSATION. WHEREAS THE CONVENTIONAL COOLED MIRROR DEWPOINT HYGROMETER REQUIRES AT LEAST 10(-5) G/CM(2) OF CONDENSATE COVERAGE FOR DETECTION BY OPTICAL REFLECTANCE OF CONDENSATION, THE PIEZOELECTRIC CRYSTAL REQUIRES ONLY 10(-7) G/CM(2) TO EFFECT DETECTION BY VIRTUE OF A SIGNIFICANT CHANGE IN ITS RESONANT FREQUENCY. WHILE A PIEZOELECTRIC CRYSTAL IS ALSO SENSITIVE TO TEMPERATURE CHANGE, IT IS SHOWN THAT THE FREQUENCY SHIFT DUE TO COOLING OF THE CRYSTAL IS TWO ORDERS OF MAGNITUDE SMALLER THAN THAT CAUSED BY CONDENSATIONAT VAPOR PRESSURE AS LOW AS 10(-3) TORR. THE DEWPOINT WILL BE DETERMINED BY DETECTING THE ABRUPT CHANGE IN THE RATE OF FREQUENCY CHANGE AS A CRYSTAL IS COOLED, USING THE FREQUENCY AT THE POINT OF THE ABRUPT CHANGE TO RELATE TO THESATURATION TEMPERATURE THROUGH THE KNOWN FREQUENCY-TEMPERATURE RELATIONSHIP OF THE CRYSTAL. THUS THE MEASUREMENT OF A SINGLE PARAMETER, THE FREQUENCY, INSTEAD OF TEMPERATURE AND REFLECTANCE, DETERMINES THE DEWPOINT. IT SHOULD BE POSSIBLE TO DETECT WATER VAPOR PRESSURE AS LOW AS 10(-4) TORR.