Saturation moisture content measurements

Cake Dryness In dewatering, usually the cake needs to be as diy as possible. Cake diyness is commonly measured by the solids fraction by weight W or by volume ,. The moisture content is measured by the complement of W or ,. The volume fraction of the pores and void in the wet cake is measured by the cake porosity (= 1 — ,) whereas the volume fraction of the liquid in the pores of the cake is measured by the saturation S. For welf-defined solids in the cake with sohd density (bone diy) p, and hquid density Pl, and given that the cake volume... 

This is defined as the percentage ratio of the water vapour pressure in the atmosphere compared to that which would saturate the atmosphere at the same temperature. Alternatively, the difference in temperature between the ambient atmosphere and that to which it would have to be cooled before moisture condensed from it, is also used as a measure of moisture content. This difference in temperature is called the dew point depression. The actual temperature at which condensation takes place is known as the dew point. The relative humidity is then expressed as ... 

In Table I results of Rn-226 activity measurements cn geological samples are shewn together with measurements cn Rn exhalation rates from the sanples. The exhalation rates varies considerably with the moisture content of material. The exhalation rate is lew for dry samples and when the moisture content increases, the exhalation rate increases until it reaches a plateau. When the moisture content increases further, a rapid increase in radon exhalation occur. When the saturation level of moisture is reached, the exhalation rate drops dramatically. The exhalation rates given in Table I are obtained by assuming that the most probable moisture content is whithin the plateau of exhalation rate/moisture curve. (Stranden et al, 1984, Stranden et al, 1984a). 

Another possible solution to the problem of analyzing multiple-layered membrane composites is a newly developed method using NMR spin-lattice relaxation measurements (Glaves 1989). In this method, which allows a wide range of pore sizes to be studied (from less than 1 nm to greater than 10 microns), the moisture content of the composite membrane is controlled so that the fine pores in the membrane film of a two-layered composite are saturated with water, but only a small quantity of adsorbed water is present in the large pores of the support. It has been found that the spin-lattice relaxation decay time of a fluid (such as water) in a pore is shorter than that for the same fluid in the bulk. From the relaxation data the pore volume distribution can be calculated. Thus, the NMR spin-lattice relaxation data of a properly prepared membrane composite sample can be used to derive the pore size distribution that conventional pore structure analysis techniques... 

This method involves preparation of a standard curve by equilibration of a specific amount of dry standard material in duplicate or triplicate over different saturated salt solutions. The standard curve is a plot of aw versus water content of the standard material. The standard must be stable during reuse of the material, and the chamber used should be exactly the same as will be used later. For the measurement, the size of each standard should be in a controlled narrow range, (e.g., —1.6 0.1 g). Once the standard curve is made, dry aliquots of the same mass of standard are equilibrated over a large quantity of the food material (-10 to 20 g). The moisture content of the standard material is then measured (e.g., by mass gain) and aw is estimated from the standard curve (Vos and Labuza, 1974). This method avoids preparation or storing of saturated salts for each determination moreover, use of a standard shortens the equilibration, thus requiring less time for measurement, and less abuse of temperature for sample as well as standard. 

The swell measurements were obtained from readings off the strain gauge dial on the Rheovlbron according to our previously developed procedure (3,4). While swelling of the epoxy Is not directly proportional to Its moisture content. It certainly Is Indicative of the relative degree of moisture saturation (9). 

The psychrometric chart (oT humidity chart) conlam values of a number of process variables for air-water vapor systems at 1 atm. The values listed on the chart include dry-bulb temperature (the temperature measured by common temperature-measurement instruments), moisture content or absolute humidity (mass ratio of water vapor to dry air), relative humidity, humid volume (volume per mass of dry air), wet-bulb temperature (the temperature reading on a thermometer with a water-saturated wick around the bulb immersed in a flowing stream of humid air), and enthalpy per mass of dry air. If you know the values of any two of these variables for humid air at or near 1 atm, you can use the chart to determine the values of the other four, which can greatly simplify material and energy balance calculations. 

At the cellular level, the true density of dry cell wall substance (i.e., within the cell wall) has been determined to be about 1.5 g/ cm, varying to some extent with the method of measurement and species (2). There are voids within the dry wood cell wall, but the void volume here (i.e., micropores) is reported to be only about 2-4%. However, this figure would be expected to increase as wood moisture content is increased to the fiber saturation point (28). 

Sorption isotherms cannot generally be predicted from theory. They need to be measured experimentally. The simplest method of measuring a sorption isotherm is to generate a series of controlled-humidity environments by using saturated salt solutions, allow a solid sample to equilibrate in each environment, and then analyze the solid for moisture content. 

The basic apparatus is shown in Fig. 12-12, and a table of salts is shown in Table 12-7. It is important to keep each chamber sealed and to be sure that crystals are visible in the salt solution to ensure that the liquid is saturated. Additionally, the solid should be ground into a powder to facilitate mass transfer. Equihbration can take 2 to 3 weeks. Successive moisture measurements should be used to ensure that the sample has equilibrated, i.e., achieved a steady value. Care must be taken when measuring the moisture content of a sample this is described later in the chapter. 

Electrical moisture meters provide a quick and reasonably accurate nondestructive alternative. The direct-current resistance of the timber is measured or either the alternating-current capacitance or power loss can be measured. Direct-current resistance moisture meters are more common a pair of needles, a fixed distance apart, is driven into the wood across or along the grain (depending on the manufacturer s instructions) and the electrical resistance measured. The procedure is reasonably accurate between the fibre saturation point (defined later) at 30% and about 6% moisture content (at which point the resistance becomes too great to measure with reasonable accuracy). In this moisture content range, the relationship between electrical resistance and moisture content is represented by a log-log plot. 

There is a difficulty with the definition of the fibre saturation point. Strictly it corresponds to the moisture eontent when the cell walls are fully saturated (which would occur where the relative humidity is 1.0) and all the lumens are free of water. However at high relative humidities, above about 0.98, the lumen tips and pits begin to fill by capillary condensation, eausing a sharp upward break in the sorption curve. In praetiee it is extremely hard to measure moisture content at these humidities and extrapolation from lower relative humidities is not particularly accurate. 

Figure 3.5. NMR spectrum of wood as a function of moisture content (Nanassy, 1974). The spectral intensity of the broad component is due to hydrogen nuclei in dry wood tissue (6% of oven-dry mass) and to a monolayer of strongly adsorbed water molecules. The intensity of this component increases somewhat with moisture content, at least to the fibre saturation point. This implies that new internal surfaces are being created as the moisture content increases. The narrow component corresponds to the more mobile multilayers of adsorbed water. The narrow component is truncated because the idea was to record the broad spectmm which required expanding the vertical scale, so that the peak of the narrow spectmm is well off scale. With quantitative NMR techniques the areas under the broad and narrow components of the spectmm provide a measure of the number of hydrogen atoms in these two states.

Often, only the easily determined moisture content is measured and reported. Such data normally show a wide scatter which may result in misleading conclusions. After introducing the pellet porosity and calculating the liquid saturation, the scatter is reduced to yield more reproducible data (compare Table 11). At optimum operation of the granulation disc, pellets feature nearly constant saturations between approximately 80 and 95 7o. The lower than expected saturation figures are caused by air entrapped in the structure by pores that are completely filled with liquid and surround such areas. If moisture contents deviate considerably from these values, agglomerates with low quality are produced (compare Figure 128). 

If the air dry moisture content of the soil is known and water added to prepare the paste has been measured, then saturation percentage is calculated from... 

---