General Compressive Measurements

H2.1 General Compressive Measurements H2.2 Textural Measurements with Special Fixtures H2.3 Texture Profile Analysis... 

Chapter H2 describes the measurement of textural properties of solid-like foods. The first unit in that chapter, unit H2.i, describes a general procedure commonly used to evaluate the texture of solid foods. This method involves the compression of the food material between two parallel plates. There are a number of empirical textural parameters which can be evaluated with this technique. Simple compressive measurements do not provide a complete textural picture of some foods untthi.i presents variations to the parallel plate compression method with the use of special fixtures. For example the use of a puncture probe or a wire cutting device provide data that may relate more directly to the consumer s evaluation of texture for products like apples and cheese, unit m.3 describes a general protocol for the evaluation of a number of sensory texture parameters. This protocol is... 

The basis for estimating z in this manner is the empirical law of corresponding states, which holds that the values of certain physical properties of a gas—such as the compressibility factor— depend to great extent on the proximity of the gas to its critical state. The reduced temperature and pressure provide a measure of this proximity the closer Tx and r are to 1, the closer the gas is to its critical state. This observation suggests that a plot of z versus Tx and Px should be approximately the same for all substances, which proves to be the case. Such a plot is called the generalized compressibility chartJ... 

Solids are usually ground with a material such as potassium bromide and compressed into a pellet. Moisture must be absent, and the transparent disk is placed in the window of the spectrometer. In general, however, measurement of intensity of absorption in the solid phase is unreliable due to scattering and reflection losses, and a uniform distribution of sample in the pellet is difficult to achieve. One method of handling solutions is to allow them to soak into a KBr wedge, evaporate the solvent, and compress the tip into a microdisk or pellet (C26) alternatively the sample (1 /tl) may be either placed directly on the KBr disk (B13) or mixed with a little KBr powder and subsequently incorporated in the disk. These microdisk techniques can be used for the examination of gas chromatograph effluent. For multiple analyses, an enclosed turntable system loaded with the disks can be used. 

The value of p/r0 can be deduced experimentally from compressibility measurements, for we can see in a general way from equation (3.03) that a small value of this quantity represents a rapid variation of repulsive force with distance and a correspondingly small compressibility. Values of p/r0 for the alkali halides so obtained vary between about 0 09 and 0 12, confirming the rapid variation with distance which we have presupposed and indicating, from equation (3.04), that the contribution of the repulsive forces to the lattice energies of these salts is of the order of 10 per cent of that due to the forces of electrostatic attraction. 

Ambient condition vapor phase reactants are stored in gas cylinders, generally in a compressed state. Subsequent to pressure regulation, their flows generally are measured with mass flow controllers which give high accuracy and permit microprocessor control of vapor phase flows. 

COMMENTS Note that there is significant deviation for each approach. Of them, the hydrogen-specific compressibility chart shonld be the most accurate since it is based on directly measured experimental data. The corrected generalized compressibility chart values were also quite close. Considering the vehicle range is directly related to the mass of fuel that can be stored, use of the ideal gas law alone would greatly underestimate the storage volume required, by almost 30% in this case. 

Given two of the measured properties P, v, and T, calculate the value of the third using cubic equations of state (e.g., van derWaals, Redlich-Kwong, Peng-Robinson), the virial equation, generalized compressibility charts, and ThermoSolver software. Apply the Rackett equation, the thermal expansion coefficient, and the isothermal compressibility to find molar volumes of liquids and solids. 

Creep tests require careful temperature control. Typically, a specimen is loaded in tension or compression, usually at constant load, inside a furnace which is maintained at a constant temperature, T. The extension is measured as a function of time. Figure 17.4 shows a typical set of results from such a test. Metals, polymers and ceramics all show creep curves of this general shape. 

In addition to freedom from bottoming out , most people prefer a seat which effectively provides a soft surface with a firm interior. One measure of the relationship between such surface softness and inner support is the sag factor or support factor. In one commonly used test this is obtained by dividing the force required to compress a foam by 65% of its height by the force needed to obtain 25% sample compression. This generally increases with density but is typically <2.5 for a conventional slabstock foam but >2.5 for a high-resilience foam. 

The work of the present section shows that shock-compression experiments provide an effective method for determination of higher-order elastic properties and that, by the same token, the effects of nonlinear elastic response should generally be taken into account in investigations of shock compression (see, e.g., Asay et al. [72A02]). Fourth-order contributions are readily apparent, but few coefficients have been accurately measured. 

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