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Direct measurement of vapour pressure and its fluctuations using fine thermocouples
Author(s) -
Polavarapu R. J.
Publication year - 1972
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49709841517
Subject(s) - thermocouple , thermistor , wet bulb temperature , materials science , voltage , amplifier , psychrometrics , hygrometer , temperature measurement , pressure measurement , analytical chemistry (journal) , mechanics , electrical engineering , thermodynamics , composite material , optoelectronics , chemistry , physics , humidity , cmos , chromatography , engineering
An instrument was developed to measure directly vapour pressure and temperature and their fluctuations compared to a moving average sampling line of 10 min. Copper‐constantan thermocouples of 75 μm in diameter are used for the measurement of fast fluctuations in dry‐ and wet‐bulb temperatures and a thermistor (100 kω at 25°C) for the measurement of mean wet‐bulb temperature. By applying the thermocouple voltages to a specially designed operational amplifier circuit, the vapour pressure is obtained as output voltage of the operational amplifier. The output voltage varies linearly with vapour pressure in the range 5‐40 mb. The values of the input and feed back resistances were theoretically determined by relating the output voltage with the Regnault's psychrometric equation. Field measurements showed that the power spectra of temperature and water vapour pressure followed −5/3 law in the frequency range 0.2 – 2.0 Hz.