Relative humidity terms

Effective temperature (ET ) is a single number representing those combinations of temperature and humidity which are equivalent in terms of comfort. It is defined as the dry-bulb temperature of the environment at 50% relative humidity. Standard effective temperature loci for normally clothed, sedentary persons are plotted on Eigure 3. The sensation of comfort depends in part upon the wetness of one s skin. Thus, as a person becomes more active the effective temperature lines become more hori2ontal and the influence of relative humidity is more pronounced. 

Protection capabilities of bags can be maximized by maintaining the moisture content of the paper at 6—8% by weight, relative to a shipping environment of 21°C at 60% in relative humidity. When the moisture content of shipping-sack paper drops below 6%, the sacks become excessively brittle however, if short-term storage under such conditions is unavoidable, an attempt should be made to place them in an environment with adequate relative humidity for 24—48 hours prior to their use. 

The humidity term and such derivatives as relative humidity and molal humid volume were developed for the air—water system. Use is generally restricted to that system. These terms have also been used for other vapor—noncondensable gas phases. 

The effect of plasticizers and temperature on the permeabiUty of small molecules in a typical vinyUdene chloride copolymer has been studied thoroughly. The oxygen permeabiUty doubles with the addition of about 1.7 parts per hundred resin (phr) of common plasticizers, or a temperature increase of 8°C (91). The effects of temperature and plasticizer on the permeabiUty are shown in Figure 4. The moisture (water) vapor transmission rate (MVTR or WVTR) doubles with the addition of about 3.5 phr of common plasticizers (92). The dependence of the WVTR on temperature is a Htde more comphcated. WVTR is commonly reported at a constant difference in relative humidity and not at a constant partial pressure difference. WVTR is a mixed term that increases with increasing temperature because both the fundamental permeabiUty and the fundamental partial pressure at constant relative humidity increase. Carbon dioxide permeabiUty doubles with the addition of about 1.8 phr of common plasticizers, or a temperature increase of 7°C (93). 

Moisture measurements are important in the process industries because moisture can foul products, poison reactions, damage equipment, or cause explosions. Moisture measurements include both absolute-moisture methods and relative-humidity methods. The absolute methods are those that provide a primaiy output that can be directly calibrated in terms of dew-point temperature, molar concentration, or weight concentration. Loss of weight on heating is the most familiar of these methods. The relative-humidity methods are those that provide a primaiy output that can be more direc tly calibrated in terms of percentage of saturation of moisture. 

In diying solids it is important to distinguish between hygroscopic and nonhygroscopic materials. If a hygroscopic material is maintained in contact with air at constant temperature and humidity until equilibrium is reached, the material will attain a definite moisture content. This moisture is termed the equilibrium moisture content for the specified conditions. Equilibrium moisture may be adsorbed as a surface film or condensed in the fine capillaries of the solid at reduced pressure, and its concentration will vaiy with the temperature and humidity of the surrounding air. However, at low temperatures, e.g., 15 to 50°C, a plot of equilibrium moisture content versus percent relative humidity is essentially independent of temperature. At zero humidity the equilibrium moisture content of all materials is zero. 

Relative humidity is a term frequently used to represent the quantity of moisture or water vapor present in a mixture although it uses partial pressures in so doing. It is expressed as ... 

Relative humidity is usually considered only in connection with atmospheric air, but since it is unconcerned with the nature of any other components or the total mixture pressure, the term is applicable to vapor content in any problem. The saturated water vapor pressure at a given temperature is always known from steam tables or charts. It is the existing partial vapor pressure which is desired and therefore calculable when the relative humidity is stated. 

Water vapour is essential to the formation of an electrolyte solution which will support the electrochemical corrosion reactions, and its concentration in the atmosphere is usually expressed in terms of the relative humidity (r.h.). 

The proportion of moisture can also be expressed as the ratio of the vapour pressures, and is then termed relative humidity ... 

The specimen may be any shape. Artificial weathering has been defined by ASTM as The exposure of plastics to cyclic laboratory conditions involving changes in temperature, relative humidity, and ultraviolet (UV) radiant energy, with or without direct water spray, in an attempt to produce changes in the material similar to those observed after long-term continuous outdoor exposure. ... 

Coplen, T.B. (2011). Guidelines and recommended terms for expression of stableisotope-ratio and gas-ratio measurement results. Rapid Gommunications in Mass Spectrometry, 25, 2538-2560. DOl 10.1002/rcm.5129 Cormie, A.B., Luz, B., Schwarcz, H.P. (1994a). Relationship between the hydrogen and oxygen isotopes of deer bone and their use in the estimation of relative humidity. Geochimica et Cosmochimica Acta, 58, pp. 3439-49. 

The capacity for aerosols to take on moisture by hygroscopicity gives rise to a kinetic phenomenon of change in particle size as a function of residence time at a particular ambient relative humidity. This phenomenon can best be described in terms of the relationship between saturation ratio and particle size according to the following expression [9] ... 

An example of a vapor pressure profile is shown in Figure 11, where it is assumed that the relative humidity within the chamber is 80%, the critical relative humidity of the solid is 40%, and the thickness of the diffusion layer (8) is 1 cm. From the figure, note that the relative humidity profile is linear and we could have made the simplifying assumption that the convective term is negligible. By ignoring the convective term, Eq. (42) simplifies to... 

The complicating factor is that both equations include the term RH which is the surface relative humidity and is greater than RH0 and is also an unknown. 

Humidity is a general term used to refer to water vapor in the air. The actual amount of water vapor that the air can hold increases with the temperature the higher the temperature, the greater the amount of water vapor that the air can hold. Various measures are used to express humidity absolute humidity and relative humidity are the two most widely used. Absolute humidity is the amount (by weight) of water vapor in the air at a particular temperature and pressure and it is usually measured in grams per cubic meter. 

The official ISO term for specific gravity. Relative Humidity... 

The sorption of water by excipients derived from cellulose and starch has been considered by numerous workers, with at least three thermodynamic states having been identified [82]. Water may be directly and tightly bound at a 1 1 stoichiometry per anhydroglucose unit, unrestricted water having properties almost equivalent to bulk water, or water having properties intermediate between these two extremes. The water sorption characteristics of potato starch and microcrystalline cellulose have been determined, and comparison of these is found in Fig. 11. While starch freely adsorbs water at essentially all relative humidity values, microcrystalline cellulose only does so at elevated humidity values. These trends have been interpreted in terms of the degree of available cellulosic hydroxy groups on the surfaces, and as a function of the amount of amorphous material present [83]. 

Long-term testing Stability studies under the recommended storage condition for the retest period or shelf life proposed (or approved) are performed with several drug substance batches in the stipulate packaging. In the general case, the storage condition is at a temperature of 25°C and 60% relative humidity. 

---