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Methods of Measuring Humidity and Testing Hygrometers

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Methods of Measuring Humidity and Testing Hygrometers NBS CIRCULAR Methods of Measuring Humidity and Testing Hygrometers UNITED STATES DEPARTMENT DF COMMERCE NATIONAL BUREAU OF STANDAROS PERIODICALS OF THE NATIONAL BUREAU OF STANDARDS As the principal agency of the Federal Government for fundamental research in physics, chemistry, mathematics, and engineering, the National Bureau of Standards conducts projects in fourteen fields: electricity, optics, metrology, heat and power, atomic and radiation physics, chemistry, mechanics, organic and fibrous materials, metallurgy, mineral products, building technology, applied mathematics, electronics, and radio propagation. The Bureau has custody of the national standards of measurement, and conducts research leading to the improvement of scientific and engineering standards and of tech- niques and methods of measurement. Testing methods and instruments are developed; physical constants and properties of materials are determined; and technical processes are investigated. Journal of Research Internationally known as a leading scientific periodical, the Journal presents research papers by author- ities in the specialized fields of physics, mathematics, chemistry, and engineering. Complete details of the work are presented, including laboratory data, experimental procedures, and theoretical and mathe- matical analyses. Each of the monthly issues averages 85 two-column pages; illustrated. Annual subscription: domestic, $5.50; foreign, $6.75. Technical News Bulletin Summaries of current research at the National Bureau of Standards are published each month in the Technical News Bulletin. The articles are brief, with emphasis on the results of research, chosen on the basis of their scientific or technologic importance. Lists of all Bureau publications during the preceding month are given, including Research Papers, Handbooks, Applied Mathematics Series, Building Materials and Structures Reports, Miscellaneous Publications, and Circulars. Each issue contains 12 or more two-column pages; illustrated. Annual subscription: domestic, $1.00; foreign, $1.35. Basic Radio Propagation Predictions The Predictions provide the information necessary for calculating the best frequencies for communication between any two points in the world at any time during the given month. The data are important to all users of long-range radio communications and navigation, including broadcasting, airline, steamship, and wireless services, as well as to investigators of radio propagation and ionosphere. Each issue, covering a period of one month, is released three months in advance and contains 16 large pages, including pertinent charts, drawings, and tables. Annual subscription: domestic, $1.00; foreign, $1.25. Order all publications from the Superintendent of Documents U. S. Government Printing Office, Washington 25, D. C. UNri’ED STATES DEPARTMEN'l’ OF COMMERCE • Charles Sawyer, Secretary NATIONAL BUREAU OF STANDARDS • E. U. Condon, Director Methods of Measuring Humidity and Testing Hygrometers Arnold Wexler and W. G. Brombacher National Bureau of Standards Circular 512 Issued September 28, 1951 For sale by the Superintendent of Documents, U. S. Government Printing Office, Washington 25, D. C. Price 15 cents Contents J’iige ]. Introduction 1 2. Detinitions 1 3. Classilication of instruments 2 4. Description of insti-uinents 2 4.1. Dry- and wet-hull) ]isyehroineter 2 4.2. Mechanical hygioinelei- 5 4.3. Dc'wpoini indicators aiul recorders G 4.4. Electrical hygi’onu'tc'i'S 8 4.5. (iravimctric hvgi'oinctry 9 a. Change in weight, of dryiiig agent. 9 h. Change in weight of ahsorhing matc'rial 9 4.0. Thermal conduct ivit_v 9 4.7. Spcctiosco])ic method 10 4.5. Index of rclraction 10 4.9. Mcasurcunent of ])rcssure or volume 10 4.10. Thermal I'ise 10 4.1 1 . Alohility of ions 10 4.12. Dicdeetrie constant 11 4.13. Critical lh)w 11 4.14. Diffusion hygromctc'r 11 4.15. Chemical methods 11 5. dVst methods 12 5.1. Basic methods 12 5.2. Secondary methods 13 5.3. Control methods 14 5.4. Comparisons wdth standaids 14 G. Selected references 14 C.l. Books and pamphlets 14 0.2. Articles 15 Methods of Measuring Humidity and Testin Hygrometers A Review and Bibliography Arnold Wexler and W. G. Brombacher This paper is a review of metho(is for tiie measurement of the water-vapor content of air and other gases and for tiie production and controi of atmospiieres of known iinmidity for iiygronicter testing and caiihration and for geiun-ai researcii. Among tiie iiygroinetric teciiniqnes discussed are psyclirometry, uieciianical iiygronietry, dewpoint measurement, eiectric hygrometry, gravimetric hygrometry, thermai comiuctivity, imiex of refraction, pressure ami volume measurements, and dielectric constant. Descriptions are given of suitalile equipment for estaldisliing desired luimidities over a wide range of temperature. A list of 157 selected references pertaining to iiygronietry is included. 1. Introduction The conditions under which the relative or ab- terials, particularly paper, wood, and grain, is solute hninidity of gases are measured, recorded, often required during some stage of processing or or controlled vary greatly, and as a result, many storage. The equipment, known as moisture methods of humidity measurement have been de- meters, used for this purpose is not discussed in veloped or proposed. However, for some applica- this Circular. tions there is at present no satisfactory method In many cases a substantial gaseous equilibrium available for such measurements, and the avail- exists between the solid and the surrounding at- able information on methods and instruments for mosphere. A satisfactory determination of the measuring humidity is quite scattered. This Cir- moistiu’e content of the solid can he made hy the cular, with the appended inferences, is intended relatively easy measurement of the humidity of to answer inquiries on humidity measurement in the atmosphere, once the lelation between the two a more comprehensive manner than may be done has been estaldished. The same is true, of course, by corres])ondence. for liquids of definite composition, except that Knowledge of the moisture content of solid ma- the water content may vary. 2. Definitions Relative humidity is the ratio, usually expressed tioning, is the Aveight of Avater Auipor in air per in percent, of the pressure of water vapor in the unit Aveight of the dry air. gas to saturation pressure of water vapor at the Devxpomt is the temperature to Avhich Avater I temperature of the gas. In engineering, relative vapor must be reduced in order to obtain a relative I hninidity is sometimes defined as the ratio of the humidity of 100 percent that is, to obtain satura- in tion vapor pressure. I weights per unit volume of the water vaiior the gas mixture and the saturated vapor at the tem- Instruments from Avhich the relative humidity I perature of the gas mixture. The two definitions is determined are called either psychrometers or are equivalent for all practical pur]ioses. hygrometers. Genei'ally, the dry- and Avet-biilb I Ahsolute Iniviidity of a gc/.s mixture may be de- instrument (described later) is called a psychrom- fined as the pressure of water vapor, or as the eter, and direct indicators of relative humidity weight per unit volume of Avater vapor. are called hygrometers. Specific humidity^ a term of use in air condi- Ilygrof/raphs are recorders of relative humidity. 1 3. Classification of Instruments Hiiinidity-ineasuring iustriiinents and methods of the variation in thermal conductivity of a gas may be divided into the following classes for con- sample with water vai)or. venience. No exhaustive search of the literature /Spectroscopic method. Measui'ement of the was made to insure a complete, listing of all pro- change in intensity of selected absorbing spectral posed instruments, so that it is anticipated that lines with change in humidity of the gas under some omissions exist. test. Ih'ij- and mef-hidh. Measurements depending Index of refraction. Measurement of change on the change in temperature due to rate of evap- i)i index of refraction of a moisture absoibing oi-ation from wet surfaces, an example of which liquid with change in ambient humidity. is the dry- and wet-bulh or sling psychrometer. JS'cssure or eolnme. Measui'ement of the ])res- Me(diunlcal hygrometers. IVIeasui-ement of a sure or volume of the water vapor in a gas sample. change in a dimension of an absorbing material, Thermal rise. Measurement of the tempera- such as human hair or goldbeater’s skin. ture I’ise of a material exposed to water vapor. Dearpo'nit 'rnd/cators and recordei‘S. Instru- Mohility of ions. Measurement of the cliange in ments which indicate or I'ecord the temperature mobility of ions due to the presence of water at which dew fiom the gas (air and water vapor) vai)or. mixture under test will just form. A common Dielectric constan t. Measurement of the dielec- type em]doys a mirror for observing the forma- tric constant of a gas with change in water-vapor tion of tlie dew. content. Electrical hygrometer. jMeasurement of the Critical floni. Measurement of the variation of electrical resistance of a film of moisture absorb- pressure drop aci'oss two combinations of nozzles, ing material exposed to the gas under test. operating at ci'itical tlow, with a desiccant be- G ravimetric hygrometer. Measni-ement of tween one pair of nozzles. weight of moisture absorbed by a moisture absorb- Diffnsion. hygrometer. Measurement of effects ing matei'ial. due to dilfiision of gases with and without water (a) Increase in weight of a drying agent as gas vapor. with a constant moisture content is passed Chemical methods. Procedures involving through it. chemical I'eactions or phenomena. (h) Change in weight of an absorbing material Foi‘ sources of su])ply for the instruments in- with change in moisture content of the gas under dicat(‘d as commei'cially available, consult Thomas test. Register of American Manufac hirers or The Thermal condvctrvlty method. Measurement I nst ruments Index. 4. Description of Instruments 4.1. Dry- and Wet-Bulb Psychrometer and the actual ]>ressure of the water vapor must be known.
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