Density, must measurement

Dislocation density is measured as the total length of dislocation lines in a unit volume of crystal, meters per meter cubed. However, experimentally it is often simpler to determine the number of dislocations that intersect a surface, so that a common measure of dislocation density is the number of dislocation lines threading a surface, that is, the number per meter squared. In a fairly typical material there will be on the order of 108 dislocation lines crossing every square centimeter of solid. However, it is known that if a solid is deformed, the dislocation density rises, perhaps by a factor of 103 or 104. Clearly, dislocations must be able to multiply under the conditions that lead to deformation. 

The active strand concentration v is obtained from measurements of the initial modulus using Eq.(7.2) with g = 1. Values of v for samples at the same crosslink density but with different primary molecular weights are extrapolated to 1/M=0, giving vc + ve. Values of vc + ve obtained at two or more crosslink densities provide ve, and hence Me, by extrapolation. An advantage of this method is that only relative values of crosslink density must be known absolute values of vc are not required. If absolute values of vc are known, the g factor can be evaluated as well. 

The density of CO2 in the absorption cell, however, is a function of both concentration and bulk air density. In normal process analyzers, where temperature and pressure within the absorption cell are controlled, measurements can be easily referred to gas density by a simple calibration curve. In an open path system, changes in bulk air density must be measured. Indeed, one of the major problems faced in testing the sensor was the development of test facilities where we could control the temperature, pressure and CC>2 more accurately than the sensor could measure. Even the small changes in building pressure associated with ventilation system fluctuations resulted in output signal changes three to four times the sensor signal to noise level. In operation, pressure and temperature near the open cell are measured and used to calculate gas density. 

For porous solids such as coal, there are five different density measurements true density, apparent density, particle density, bulk density, and in-place density. The true density of coal is the mass divided by the volume occupied by the actual, pore-free solid in coal. However, determining mass of coal may be deemed as being rather straightforward, but determining volume presents some difficulties. Volume, as the word pertains to a solid, cannot be expressed universally in a simple definition. Indeed, the method used to determine volume experimentally, and subsequently, the density, must be one that applies measurement rules consistent with the adopted definition. 

Methods of measurement of coal density include use of a gas pycnometer and particle density by mercury porosimetry. However, the difference in density values using different gases must be recognized since, for example, density values measured by nitrogen may be greater than those obtained when helium is used. Density measurement depends on adsorption of gas molecules, and differences (between nitrogen and helium) may be due to nitrogen adsorption on the coal surface. 

In the complete theory the measured elution volume is shown to be a function of the product of the particle mass and density difference between particle and carrier liquid. The particle density must be accurately known before a mass and diameter can be calculated. The density of a colloidal-sized particle is not always well-known or conveniently measured. For example, significant adsorption of surfactant or entrapment of carrier within the particle can lead... 

These are made from one piece of material and have no compensating added materials. They are required when a precise measurement of density must be made. These can be made of multiple materials for purposes of correcting the weight. This can be done by adding material or adding rings or hooks. The added material must not be able to separate from the weight. 

Low-vapor-pressure oils can be substituted for some operations, but calibrations for density must be made so that their measurements can be interpreted as millimeters of mercury, torr, or Pa. Unfortunately, vacuum measurements can take a considerable amount of time when using oil because it takes a long time for a film of oil to settle from the walls of a manometer. 

Silt density index measures suspended solids, particularly colloids, such as alumina- or iron silicates, clay, iron corrosion products, and microbes, that have a great potential for fouling RO membranes (see Chapter 3.8 for more details about SDI). The SDI should be as low as possible to minimize fouling of the membranes, but must be less than 5 to meet warranty requirements set by the membrane manufacturers (best practices call for SDI in RO feed water to be less than 3). Note that there is no direct correlation of turbidity to SDI, other than high turbidity usually means high SDI (the converse is not always true). 

Fig. 8.20 shows the dependence of the band tail luminescence intensity on the defect density as measured by the g = 2.0055 ESR resonance in undoped a-Si H. The luminescence intensity drops rapidly when the defect density is above 10 cm" , becoming unobservable at defect densities above 10 cm" (Street et al. 1978). These data establish that the defect provides an alternative recombination path competing with the radiative band tail transition. At the low temperatures of the measurements, the electrons and holes are trapped in the band tails and are immobile. Both the radiative and the non-radiative transitions must therefore occur by tunneling. Section... 

In order to separate properties of the film from those of the crystal, admit-tance-vs-frequency (Y-vs-f) measurements are made on the TSM resonator before and after deposition of a film. Fitting the equivalent-circuit model to measurements made on the uncoated device is crucial, allowing extraction of all of the circuit elements except Z, — the impedance element arising from the film (Figure 3.7). Once the uncoated resonator has been characterized, the impedance element Z arising from a film coating. If measurements at only a single harmonic are used, film thickness and density must be known to extract G and G". 

From the example described above we can appreciate that we must have a well-defined geometry in order to be able to deduce charge densities from measurements with probes. The situation is necessarily more complicated when charge is distributed through the thickness of a specimen rather than just residing on a single surface (Blythe, 1975b). 

The molecular formula is some whole-number multiple of the empirical formula. To determine the molecular formula, you must know the approximate molar mass of the compound under study. From Avogadro s hypothesis, the ratio of molar masses of two gaseous compounds is the same as the ratio of their densities, provided that those densities are measured at the same temperature and pressure. (This is true because a given volume contains the same number of molecules of the two gases.) The density of the welding gas from Example 2.4 is 1.06 g at 25°C... 

When the angles are all 90° (so that their cosines are 0), this formula reduces to the simple result V = abc for the volume of a rectangular box. If the mass of the unit cell contents is known, the theoretical cell density can be computed. This density must come close to the measured density of the crystal, a quantity that can be... 

A plot of R vs. a is given in Fig. 9-15 for typical values involved in the 111 reflection from aluminum with Cu Aa radiation, namely,/It = 1.0 and 0 = 19.25°. (Values of 100/i are tabulated in [G.l 1, vol. 2, p. 307] and values of 1 /R by Taylor [G.19].) Figure 9-15 shows that the integrated intensity of the reflection decreases as a increases in the clockwise direction from zero, even for a specimen containing randomly oriented grains. In the measurement of preferred orientation, it is therefore necessary to divide each measured intensity by the appropriate value of the correction factor R in order to arrive at a figure proportional to the pole density. From the way in which the correction factor R was derived, it follows that we must measure the integrated intensity of the diffracted beam. To do this with a fixed... 

So far then, if one knew the rates of the important chemical reactions involved and if one had the manifold of response curves, one could predict the behavior of simultaneously relaxing and reacting networks, presumably on any time scale. However, the direct measurement of the manifold of response curves is at best time consuming and almost always impractical. This is still true even though it has been shown that, in the case of a continuous chemical reaction, only the response at discrete crosslink densities must actually be known (7). Thus we are presently investigating Indirect methods by which this information can be obtained. 

The most important technique for perfusion culture methods is to separate the concentrated cells and conditioned medium from the suspended culture broth. As noted above, the separation methods chiefly used are filtration with tubular and flat membranes as well as ceramic macroporous filters. These membrane reactors can be employed for both anchorage-dependent and suspension growing cells. Static maintenance type systems are commercially available for disposable reactors, and small size unit reactors from 80 ml to 1 liter are used for continuous production of monoclonal antibodies with hybridoma cells. The maintainable cell densities are about 10 -10 cells/ ml which is essentially mouse ascites level. However, in these systems, the cell numbers cannot be counted directly because the cells adhere to membranes or hollow fibers. Therefore, the measurement of cell density must use indirect methods. Such indirect methods include the assaying of the quantities of glucose consumption and the accumulation of lactate. The parameters of scale-up have not yet been established for these static methods. 

---