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an instrument for measuring atmospheric pressure by the boiling point of a liquid. A liquid begins to boil when its vapor pressure equals the external pressure; after measuring the temperature of the vapor of the boiling liquid, the atmospheric pressure is found from special tables.
A hypsometer consists of a special thermometer, from which the temperature may be read to 0.01°P, and a boiler that consists of a metal vessel containing distilled water and an extensible tube with double walls. The thermometer is put inside the tube and is bathed in the vapor of the boiling water. Hypsometers are made in which the thermometer scale divisions are units of pressure (mm of mercury, or millibars).
A hypsometer in which the liquid boils without artificial heating is used to measure pressure in the free atmosphere. The liquid used (Freon, carbon disulfide, and others) has a boiling point below the temperature of the surrounding atmosphere. Such a hypsometer usually consists of a Dewar flask containing the liquid and a miniature resistance thermometer.
A hypsometer is superior to an aneroid barometer, since it is free from errors caused by the elastic properties of the box’s membrane and by temperature; it has no mechanical transmission. However, because of its complexity it is used in radiosondes and under expedition conditions only when an aneroid barometer cannot offer the necessary accuracy of measurement.
REFERENCESSternzat, M. S. Meteorologicheskie pribory i nabliudeniia. Leningrad, 1968. Chapter 4.
Nepomniashchii, S. I. “Gipsometr dlia radiozondov.” Tr. Nau-chno-issledovatel’skogo in-ta gidrometeorologicheskogo priboro-stroeniia,1966, ISSUE 16, P. 25.
S. I. NEPOMNIASHCHII