Time-temperature history, Small

This calculational scheme has been applied to the Small Boy detonation. The test was made to compare fractionation data from this well-studied event (2) with calculated values. Using 25 particle size fractions that approximate the fallout from the event and the time-temperature history given in Table III, which was derived from generalized time-temperature event histories, the fission product contents of the nuclide chains 80 through 150 in the particle size fractions were calcu-... 

Fiber orientation uniformity is also affected by small-scale or timewise variations in polymer viscosity, related to breakage of polymer chains during the extrusion process. The degradation occurs as a result of residual moisture that immediately reacts to break chains, and by thermal degradation that occurs more gradually over time. Different residence times and temperature histories within the laminar flow streamlines lead to different viscosities, and hence different average orientation levels in the different fibers. 

When reactions are fast relative to the mixing rate, not only are the apparent reaction rates affected but the whole time and temperature history of the reaction mechanism is also affected, yielding different selec-tivities and yields, depending on the intensity of the mixing. This often leads to a scale-up/scale-down problem, where yields of the desirable products in a plant-scale reactor are not as good as those in a small-scale reactor in the laboratory or the pilot plant. If the yield drops from the pilot-scale to the plant-scale reactor when all other important variables (temperature, pressure, and composition) have been held constant, then there is a mixing problem. Fast... 

Since the heat capacity is measured using a differential scanning calorimeter, the dependence of Cp on time and temperatures is complicated because Cp is measured continuously while the sample is being heated or cooled at a constant rate. Therefore, the time for the heat capacity measurement is not well defined, and thermal history effects complicate the shape of the step at 7. To overcome this difficulty, one can approximate the real situation by changing the temperature in small discrete steps AT at time intervals At. Stephens and Moynihan et al. follow this procedure and calculate the change in the sample s enthalpy H and its heat capacity from Cp =LH/l T. However, we already have an explicit expression for the equilibrium enthalpy and heat capacity. Since the result for Cp, (8.2), depends partially on the rate of change of p with respect to T, we need only include the effect of p falling out of equilibrium (i.e., dp/dT tCS) for T< 7 in that result. ... 

The reactive mixture is assumed to remain at a constant volume over its reaction time, and the induction time is defined in terms of its temperature history. Most of the mixtures underwent a large temperature increase of more than 1000 K, and the induction time is defined as the time of maximum rate of temperature increase. In most cases, this coincides approximately with the time at which the temperature has completed about half of its total increase. This is not, strictly speaking, a true induction period, often defined as the time required for a small (i.e. 1-5%) temperature or pressure increase, but it represents a time scale for the release of a significant amount of energy. In addition to the induction time t, it is useful to define the induction length A e t(Dcj-ui), which represents a characteristic length scale in the post-shock unreacted gas mixture. 

Time-temperature indicators are used for products where the storage and transport in appropriate conditions is very important. This indicator informs on the difference between the optimal temperature and the actual temperature and how long a product was exposed to inappropriate temperature conditions. An indicator must be able to monitor one or more conditions chill temperature, frozen temperature, temperature abuses, partial, or full history. The principle operation is based on the polymerization reaction or enzymatic fat hydrolysis or diffusion of the chemically modified dye from encapsulation layer. i Color change must be correlated to food shelf-life. The time-temperature indicator could be placed as a label or as a small packaging in shipping containers (Figure 61.3). 

The determination of the thermal conductivity of grain is based on the comparison of the temperature history data obtained by using the line heat source probe with the approximate analytical and numerical methods [35,54]. The analytical method has the advantage of being quick in calculating thermal conductivity. This method, however, requires a perfect line source and a small diameter tube holding the line heat source. In reality, this requirement is difficult to meet. Therefore, a time-correction procedure has been introduced [52,54,56]. Another objection to the analytical method is that it cannot easily be used to calculate the temperature distribution in the heated grain and to compare it with the measured one. Such a comparison can be easily accomplished by a numerical method, where the estimated accuracy for thermal conductivity is determined and the thermal conductivity of the device is taken into account [54]. 

An important characteristic of a potentiometric indicator electrode is the response time. The response time is the time required for the establishment of an equilibrium electrode potential. According to an lUPAC recommendation [24], the response time is the time interval from the moment of inserting the potentiometric set-up into the test solution to the moment when the potential deviates from the equilibrium potential by 1 mV. This time interval can span from milliseconds to minutes or hours and depends on many conditions, e.g. concentration of the measuring ion (small exchange currents), speed of stirring, temperature, history and pretreatment of the indicator electrode and so on. 

For those not familiar with this type information recognize that the viscoelastic behavior of plastics shows that their deformations are dependent on such factors as the time under load and temperature conditions. Therefore, when structural (load bearing) plastic products are to be designed, it must be remembered that the standard equations that have been historically available for designing steel springs, beams, plates, cylinders, etc. have all been derived under the assumptions that (1) the strains are small, (2) the modulus is constant, (3) the strains are independent of the loading rate or history and are immediately reversible, (4) the material is isotropic, and (5) the material behaves in the same way in tension and compression. 

Control urine should be collected from individuals who have no apparent past history of exposure to the active ingredient. This control urine must be stored frozen until used for field fortification purposes. The urine is then thawed, shaken well, and a certain amount should be aliquoted into a small jar/bottle to use for field fortification. The active ingredient is then added to the urine using a 1-mL volumetric pipet, the solution is shaken well, and the sample is immediately frozen. Occasionally, the fortified sample can be left at room temperature or at some lower temperature in a liquid state to simulate field storage during collection of the urine sample. After leaving the sample at such temperatures for the prescribed length of time, the sample is immediately stored frozen. 

The practices of isocratic and gradient sorptive chromatography are very different. Isocratic chromatography tends to be very sensitive to the details of mobile phase preparation, temperature, pump speed, and sample composition. Gradient chromatography is usually more tolerant of small variations in these factors but may be extremely sensitive to column history, equilibration time, and gradient preparation. 

---