Experimental monitoring techniques temperature measurements

As noted above, not all techniques which provide information regarding crystallinity are useful to follow the rate of crystallization. In addition to sufficient sensitivity to monitor small changes, the method must be rapid and suitable for isothermal regulation, quite possibly over a range of different temperatures. Specific volume measurements are especially convenient for this purpose. We shall continue our discussion using specific volume as the experimental method. 

Frequency dependent complex impedance measurements made over many decades of frequency provide a sensitive and convenient means for monitoring the cure process in thermosets and thermoplastics [1-4]. They are of particular importance for quality control monitoring of cure in complex resin systems because the measurement of dielectric relaxation is one of only a few instrumental techniques available for studying molecular properties in both the liquid and solid states. Furthermore, It is one of the few experimental techniques available for studying the poljfmerization process of going from a monomeric liquid of varying viscosity to a crosslinked. Insoluble, high temperature solid. 

The only way to validate kinetic models is to measure experimentally the degree of cure as a function of time and temperature. It can be done on both macroscopic and microscopic levels by monitoring chemical, physical (refractive index [135], density [136], and viscosity [137]), electrical (electrical resistivity [138,139]), mechanical, and thermal property changes with time [140,141]. The most-used techniques to monitor cure are presented in the next two subsections. 

A variety of experimental techniques have been used for the determination of uptake coefficients and especially Knudsen cells and flow tubes have found most application [42]. Knudsen cells are low-pressure reactors in which the rate of interaction with the surface (solid or liquid) is measured relative to the escape through an aperture, which can readily be calibrated, thus putting the gas-surface rate measurement on an absolute basis. Usually, a mass spectrometer detection system monitors the disappearance of reactant species, as well as the appearance of gas-phase products. The timescale of Knudsen cell experiments ranges from a few seconds to h lindens of seconds. A description of Knudsen cell applied to low temperature studies is given [66,67]. 

Emission measurement techniques have in many applications proven very useful in providing an alternative to the absorption method. Emission measurements free the experimenter from the time and position restraints imposed by a celestial source and remove the complications imposed by the necessity to position a remote source in line with the gas sample of interest. One example of the application of emission measurements and their effectiveness, is their use to measure the effluents from sources such as smoke stacks (57). In this application there is usually a temperature differential which allows discrimination between the target and the ambient atmosphere. This type of measurement is most effective in monitoring target gases when they are in close proximity to the source since the target gas temperature soon becomes the same as the ambient atmosphere and their measurement becomes much more difficult if not impossible. 

In this technique, adsorption is allowed to take place at one temperature and the crystal is then rapidly heated to the desired desorption temperature. This requirement for rapid heating is experimentally very demanding and for this reason, the technique is not often used. The desorption rate can be monitored by measuring the desorption flux as a function of time Kohrt and Gomer [214] used a field emission tip as a flux detector. Alternatively, an adsorbate-sensitive physical property of the surface, such as electron-stimulated desorption [215] or work function [216], can be used. 

Thermocouples are the most common instrument to measure temperature in the chemical industry. They are versatile, compact, are applicable over a wide temperature range, and are relatively inexpensive. Not only are they used as a means to monitor the process, they are used to troubleshoot, identify areas of non-uniformity, and even determine the hydrodynamic pattern of fluids in motion. The technique of detecting flow maldistribution based on measuring the concentration is demonstrated in Figures 2.10 and 2.11. In Figure 5.8, both the concentration profile and temperature profile of an experimental methanation fluidized bed reactor are illustrated ... 

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