An evaluated compilation of equilibrium relative humidities in air versus temperature from pure phase to approximately 105 pascal (1 atm) in pressure is presented for 28 binary saturated aqueous solutions. The relative humidities of the solutions range from about 3 to 98 percent. Using a data base from 21 separate investigations comprising 1106 individual measurements, fits were made by the method of least squares to regular polynomial equations with two through four coefficients. Equations and tables are presented along with the estimated uncertainties in the correlated results. Research, hygrometer calibration, testing and material conditioning often require the accurate control of humidity in a working space. The common methods of controlling the humidity accurately use either a humidity generator (1A)1 or the equilibration of a closed space with a chemical system (IB) which produces the desired equilibrium vapor pressure. Humidity generators tend to be expensive and complex whereas equilibration with chemical systems that provide fixed points is a relatively inexpensive and simple method of humidity control. Among the chemical systems used for this purpose are aqueous sulphuric acid solutions, glycerine and water solutions and single and binary salt solutions. Each such solution offers a degree of humidity adjustment that can be achieved by changing its concentration. On the other hand, special problems are associated with the use of solu- tions because their concentrations must be measured and controlled. Not only must the concentration of the solution be determined initially but the presence of any humidity sources or sinks in the controlled space and even the initial equilibra- tion process of the space can alter the solution concentration. An especially useful method of humidity control by chemi- cal system involves the use of binary saturated aqueous solutions (primarily of single salts) in which the solute is highly non-volatile. At any temperature, the concentration of a saturated solu- tion is fixed and does not have to be determined. By provid- ing excess solute, the solution will remain saturated even in the presence of modest sources or sinks. Where the solute is a solid in the pure phase, it is easy to determine that there is indeed saturation. Due to the ease of its use, this is a popular method of humidity control. Since a given saturated salt solution provides only one relative humidity (RH) at any desired temperature, a different relative humidity must be achieved by selecting another appropriate salt. Though much data on saturated salt solu-

/pdf/humidity-fixed-points-of-binary-saturated-aqueous-solutions-4xfomjtsbm.pdf

Humidity fixed points of binary saturated aqueous solutions

Publisher Summary This chapter is concerned with the quantitative data that are available at present, and with propounding the view that the water requirements of microorganisms are best considered in terms of the activity of the water in the immediate environment of the organisms. Whether or not the substrate is liquid or solid, all microorganisms grow only in aqueous solutions. For intermediate conditions, growth of some organisms may occur in very dilute solutions such as tap water or in highly concentrated solutions. Therefore, the chapter considers some of the simple properties of aqueous solutions insofar as these bear on the availability of water to microorganisms. It presents methods for study of water requirements. It considers the effects of the status of the water in the solution or substrate on the growth of molds, yeasts, and bacteria and to what extent these findings are applicable to all microorganisms. The rate of mold growth on foods has also been discussed. The chapter discusses factors affecting water requirements such as nutrition, temperature, oxygen, pH, and adaptation. In discussing some applications in food preservation, fresh foods, dried foods, concentrated foods, frozen foods, and canned foods have been considered.

Water Relations of Food Spoilage Microorganisms

The relative humidities maintained over saturated and unsaturated salt solutions and of some salt hydrate mixtures have been collated from the literature over a wide range of temperature. Results are presented for 38 saturated salt solutions, and details listed for salts which are recommended as suitable for humidity control. The control of humidity in the laboratory by the use of salt solutions is critically discussed.

Humidity control in the laboratory using salt solutions—a review

Composition and temperature dependence of the deliquescence properties of hygroscopic aerosols

Second-generation inorganic aerosol model

The relative humidity-temperature relationships have been determined in air in equilibrium with saturated salt solutions of lithium chloride. LiCl. H 2 O; magnesium chloride, MgCl 2 . 6H 2 O; sodium dichromate, Na 2 Cr 2 O 7 .2H 2 O; magnesium nitrate, Mg(NO 3 ) 2 . 6H 2 O; sodium chloride, NaCl; ammonium sulfate, (NH 4 ) 2 SO 4 ; potassium nitrate, KNO 3 ; and potassium sulfate, K 2 SO 4 , over a temperature range of 0° to 50°C, using the dewpoint method. The relative humidity is a continuous function of temperature, and, except for sodium chloride, is monotonic. The curve for sodium chloride increases from 74. 9 percent relative humidity at 0°C to a maximum of 75.6 percent at 30°C and then gradually decreases to 74.7 percent. The maximum change in relative humidity with temperature, about 15 percent relative humidity as the temperature increases from 0° to 50°C, occurs with saturated salt solutions of sodium dichromate and magnesium nitrate.

https://nvlpubs.nist.gov/nistpubs/jres/53/jresv53n1p19_A1b.pdf

Relative humidity-temperature relationships of some saturated salt solutions in the temperature range 0 degree to 50 degrees C

An electric hygrometer element which responds rapidly to changes in humidity is described. The relative humidity indication of the element passes through 90 per cent of a total change in indication in 3 seconds at −20°C and in 8 seconds at −40°C.

The element consists of a thin film of barium fluoride deposited over metallic film electrodes on a glass substrate. The electrical impedance of the element varies about 3 orders of magnitude with relative humidity throughout the range of 20 per cent to 100 per cent relative humidity at a given temperature. It can be fabricated reproducibly to conform to mean calibration curves.

The results of a laboratory study of performance characteristics indicate that the element responds to changes in relative humidity over a wide range of humidities. In the range studied, no upper or lower limits to the operation of the element were found, Hysteresis effects are small. Exposure to high humidity, water spray, or water immersion had no permanent harmful effect on the functioning of the element. Polarization effects are avoided by using the element only in a-c circuits. Aging effects resulting in shifts in calibration with time require further study.

A barium fluoride film hygrometer element

The performance characteristics of a fas t responding electric hygromete r of high sen it ivity are presented in some detail. The hygrom eter is made by evaporat ing, under vacuum, potassium m etaph osphate t o obtain a thin film of material (3 microinches thick) on a specially treated glass blank. The logarithm of the r es istance is approximately linear with relative humidity at constant t emperature. The resistan ces vary from 4x10' t o 1013 ohms. For the temperature range of 40 0 t o + 40 0 C, a group of nearly para llel calibration curves is obtained. The manufacturing process is s ufficiently well controlled so tha t hygromete r elements, made in different batches, closely conform t o a m ean calibration curve. The average hysteresis for the hygrometer element over the t emperature range 0 0 to 40 0 C is 2.7 percent relati ve humidity. The polari zation effect for reversed direct current loading (1 minute cycles) is of t he order of 0.3 percent relative humidity at the end of two hours; it is 1.4 to 2.2 percent for in te rmi t tent direct current loading (15 seconds on, 45 seconds off) ; and it is 3.7 to 6.8 perce nt for co ntinuous direct current loading. Elements aged by humidi ty cyclin g appear to be less subj ect t o polarization. F ollowing a discrete chan ge in r elati ve humidity, t he average response time of the clement for a 63 perce nt change in indica tion a t 20 0 C, wi th a n a ir veloc ity of 800 feet per minute para llel to t he long edge of t he element, is 1.9 seconds for a relat ive humidity cha nge from 41. 1 t o 82.3 percent (with respect to water) ; it is 3.4 seco nds for t he sam e relat ive humid ity change in the reverse directi on. Times for 90 percen t change in indi cation a re 5.0 a nd 16.4 seco nds, res pecti vely . At room temperature, t he a verage response time, for a 63 perce nt chan ge in indication, with a veloc ity of 768 feet per minu te, is 0.10 seco nd for a discret e change in rela tive humidity from 33 to 80 percent, and 0.42 second for the same chan ge in the reverse directi on. Cor responding t imes for 90 percent change arc 0.74 a nd 11.2 econds. In comparison wi t h lithium chloride elements currently produced for radiosonde usc, t he response t ime of t he elem ent was found to be less by a t least an order of magnitude.

Arnold Wexler

Papers

Relative humidity-temperature relationships of some saturated salt solutions in the temperature range 0 degree to 50 degrees C

A barium fluoride film hygrometer element

A fast responding electric hygrometer

Divided flow, low-temperature humidity test apparatus

Adsorption electric hygrometer