Sampling surface moisture

Example 4 Calculation of Sample Weight for Surface Moisture... 

Example 4 Calculation of Sample Weight for Surface Moisture Content An example is given with reference to material with minimal internal or pore-retained moisture such as mineral concentrates wherein physically adhering moisture is the sole consideration. With this simphfication, a moisture coefficient K is employed as miiltipher of nominal top-size particle size d taken to the third power to account for surface area. Adapting fundamental sampling theory to moisture sampling, variance is of a minimum sample quantity is expressed as... 

A direct implication of this experiment becomes apparent It is quite possible to determine favorable and unfavorable times for vapor flux sampling, based on moisture events. Furthermore, it may sometimes be possible to artificially enhance sampling by appropriate local introduction of moisture.7 Otherwise, it may prove more effective, in some environments, to sample surface soil particles rather than vapor. 

Moisture in coal takes three forms (l)free or adherent moisture, essentially surface water (2) physically bound or inherent moisture (thai moisture held by vapor pressure and other physical processes) and (3) chemically bound water (water of hydration or combined" water). The ASTM defines total moisture as a loss in weight in an air atmosphere under rigidly controlled conditions of temperature, time, and air flow. Total moisture represents a measurement of all water not chemically combined. Total moisture is determined by a two-slep procedure, involving air-drying for removal of surface moisture from the gross sample, division and reduction of Ihc gross sample, and determination of residual moisture in the prepared sample. An algebraic calculation is used to obtain the total moisture value. 

There appears really to be no method by which true external moisture may be determined, that is, no method by which only the surface moisture is removed in such fashion that the residual solvent in the powder is found to be the same both before and after the powder has been allowed to take up moisture. Samples of powder were taken and residual solvent was determined by the several methods indicated in the next table. The samples were exposed 2 weeks to an atmosphere practically saturated with water vapor, and residual solvent was again determined as before. The surprising result was secured in every case, as indicated, that the amount of residual solvent was less after the powder had been exposed to the moist atmosphere than it was before it had been exposed. Yet the powder had taken up large quantities of moisture during the exposure. It is clear that the exposure to the moist atmosphere had made the volatile matter of the interior of the grains more accessible to desiccating influ-... 

Variation in sample height on a non contact optical unit can be a major analytical limitation. There are at least three American and three European near-infrared moisture monitoring systems which use water absorption at 1940 nm with one or more reference wavelengths. These are designed to be mounted above a conveyor belt with clearance between the optical window and the sample surface. Since water is a strong absorber the coefficients used in the analytical expression are small and the uncertainty associated by multiplying the sampling noise may be acceptable. For analytes where absorbance differences are less, the precision may not be sufficient. 

The role of water in coal and the quantitative measurement of water are complicated because water is present within the coal matrix in more than one form (Allardice and Evans, 1978). Thus, the total moisture includes both the surface moisture and the residual moisture remaining in the sample after determining the air-dry loss (ASTM D-3302). Thus,... 

Distributed circuit methods use coaxial lines, waveguides and resonant cavities at microwave frequencies. The circuits are designed for measuring an attenuation factor and a phase factor, from which sample dielectric properties can be calculated. The sample may form the dielectric medium between the two conductors of a coaxial line (Scaife et al, 1971), or an open coaxial line is brought into contact with the sample surface (Roussy and Pearce, 1995). Fagan et al, (2004) used an open coaxial line method to demonstrate that the moisture and salt contents of processed cheese could be predicted by measuring dielectric properties over a range of frequencies. 

The rays reach the sample detector via an interference filter. A reference detector receives the rays reflected by the wall of the Ulbricht s sphere. The concentration of the analyte can be inferred from the difference between the two reflectance values. In addition, by this formation of difference values the optical system equalizes fluctuations of the xenon flashlight. A sapphire window placed before the sample and reference detector minimises excessive evaporation of surface moisture from the test area of the test strip. The interference filter, which is also situated before the detectors, is method-dependent and is inserted into the instrument with the plug-in module (Fig. 47). 

The response of the supported nickel oxide on alumina to an in situ experiment is very different from that presented earlier for the supported tungsten oxide, as shown in Figure 13. The Raman band of the supported nickel oxide on alumina is not affected by the removal of the moisture present on the sample surface (compare Figures 13(a) and 13(d)) and suggests that water molecules do not coordinate to the supported nickel oxide. 

The appearance of the conjugate sorption data presented so far qualitatively indicates a shift from a more to a less Fickian character as the initial surface moisture content increases. That is, the appearance of the conjugate sorption isotherms obtained by totally immersing the samples violate two of the criteria by which Fickian behavior is defined. The same cannot be said for those samples exposed to less than 100% R.H., particularly at 25°C. This qualitative trend for PMC is further demonstrated by Figures 16 and 17. Here MjVWq is presented as a function of time. For both the thick and the thin sample, as either temperature or relative humidity is increased, the character of the curves progresses towards pure Case II description. That is, the moisture uptake becomes linear with time up to the point where a plateau is achieved in the behavior. 

The CsCl (>99.9% purity) is dried at 200° under high vacuum and then melted in a silica container. The latter step reduces the possibility of recontamination by surface moisture and makes weighing and complete sample transfer easier in the dry box. 

The metal surface probably acquires an electrochemical inhomogeneity, i.e. energy discontinuity [38], on contact with condensed moisture inhibited by Tet. The initial acts of Tet adsorption take place on active centers with maximum positive charge density. These centers are concentrated on anodic areas of the Fe surface, where Fe(II) ions are generated. Subsequently, two processes take place simultaneously on the sample surface ... 

Both previous methods suffer from the requirement of initially small sample volumes. In the case of pour plates, the volume of inoculum must be limited, lest the agar not become sufficiently solid, whereas in the spread-plate method, excess surface moisture permits spreading and coalescence of colonies. Because of this, it is necessary to plate a range of dilutions in order to have countable plates. 

In order to increase the accuracy and reliability or results, Karl Fischer results are usually compared to TG results. The moisture results should be nearly equivalent from these two methods since they are both measures of total bound and surface moisture in the sample cake [17]. Vial-to-vial moisture variability in the samples is usually inherent in freeze-dried samples since each vial is unique with respect to freeze-drier shelf and position on the shelf. Both the Karl Fischer and TG methods are frequently capable of measuring the moisture content of one vial and therefore vial-to-vial variability for one lot since one test requires either 20 or 5 mg of sample, respectively. For these two methods also the relative standard deviation is near 10%. Low vial-to-vial variability is produced by a well-controlled sample lyophilization process. After the freeze-drying of U.S. National Reference Preparation for a-Fetoprotein in Mid-Pregnancy Maternal Serum [51], 85 samples were chosen at random. The mean residual moisture content of the 85 samples was 0.55% with a standard deviation of 0.19%. 

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