Moisture measurement relative-humidity methods

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. 

Interlayer moisture is one of the important controls for PVB-to-glass adhesion of current formulations (although moisture-insensitive formulations are being developed). The moisture content equiUbrates with the relative humidity to which the interlayer is exposed and thus is variable. Prior to lamination, interlayer moisture content is measured by one of three methods. The most rapid is by air absorption using a spectrophotometric technique to determine a... 

Effect of Relative Humidity and Sorption History. An indirect method for estimating wood moisture content is to measure its equilibrium relative vapor pressure h. This is related to wood moisture content by a sorption isotherm. The percent relative humidity (H) or relative vapor pressure (h) (H = 100 h) is the most important factor in determining the EMC for wood. A curve showing EMC as a function of percent relative humidity or relative vapor pressure at constant temperature is called a moisture sorption isotherm. 

The effects of temperature and relative humidity on the kinetics of moisture sorption in epoxy materials for microelectronics encapsulation are not generally known. In a previous paper QJ we examined moisture sorption as a function of temperature under conditions of 100 percent relative humidity. Conjugate sorption measurements were combined with mechanical, dielectric and thermal methods of analysis to examine moisture related micro-structural alterations. 

In the static method, a sample of known mass is allowed to reach equilibrium with air maintained at a constant relative humidity and temperature. The moisture content of the sample at equilibrium is measured and is defined as the EMC. The constant relative humidity environments are usually created using saturated salt solutions in containers [13]. A container with a 10- to 15-g sample suspended in the environment above the saturated salt solution is kept at a constant temperature. The experiment must be repeated at several temperatures and relative humidities. The sample is weighed at a regular interval of 3-12 h until the change in sample mass between two successive readings is less than 0.01 g (at this stage it is assumed that the sample has reached equilibrium). Depending on the vapor pressure of the moisture in the seeds and the vapor pressure of the... 

They are usually measured by gravimetric methods, where a known amount of solid of known initial moisture content is exposed to a gas of specified relative humidity. After sufficient time, the new weight of the sample is measured, which allows one to calculate the actual moisture content. 

Alden et al. extended the method by estimating the equilibrium relative humidity based on the wet-bulb temperature (TJ, the partial pressure of water at the solid s temperature, and the saturated vapor pressure of water at 7 [46]. Thus, the moisture content of a given material can be inferred from the estimated temperature difference (AT) and the equilibrium relative humidity. This method has been successfully implemented to control the endpoint in batch fluid-bed dryer for aqueous and aqueous-alcohol granules. An pirical method was used to determine the desired AT value by frequent sampling of the dried material and moisture content determination. The realtime AT was estimated using a computer program based on the temperature measurements this value was then compared with the desired value and used as the drying-endpoint indicator. 

Changes in the water transport rate of treated PET fabrics have been measured by a vertical wicking test in which the rising height of water in a strip of fabric is determined [3, 33, 77], Measurements of the dissipation of a drop of water on fabric [ 11, 38,62], liquid retention capacity (the ratio of the amount of liquid to the dry fabric quantity), and moisture regain (the amount of water a dry fiber absorbs from the air at a defined relative humidity) [43, 44, 51, 60] also aim to determine changes in the water absorption behavior of treated PET fabrics. Incomplete removal of enzymes adsorbed to the PET surface can, however, easily lead to incorrect results obtained using these methods [11, 23, 102]. 

In the static method, a sample of known mass is allowed to reach equilibrium with air maintained at a constant relative humidity and temperature. The moisture content of the sample at equilibrium is measured and is defined as the EMC. The constant relative humidity environments are usually created using saturated salt solutions in containers [13]. 

Water absorbing capacity was determined gravimetrically using a modification of the method described by Elizalde et al (//), with some modification. Briefly, all samples were equilibrated in a 10% relative humidity (RH) chamber for 48 hours. Then, samples were transferred into a 98% RH chamber and exposed to moisture for 5 min. The dry matter and the absolute amount of absorbed water were determined. All measurements were made in triplicate. The results were expressed as the mean SD in mg water absorbed by per gram of psyllium per minute. 

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