Temperature change, time constant

For capacity measurements, several techniques are applicable. Impedance spectroscopy, lock-in technique or pulse measurements can be used, and the advantages and disadvantages of the various techniques are the same as for room temperature measurements. An important factor is the temperature dependent time constant of the system which shifts e.g. the capacitive branch in an impedance-frequency diagram with decreasing temperature to lower frequencies. Comparable changes with temperature are also observed in the potential transients due to galvanostatic pulses. 

This WLF equation enables us to calculate the time (frequency) change at constant temperature, which - as far as the dynamic-mechanical behaviour is concerned - is equivalent to a certain temperature change at constant time (frequency). For temperatures below Tg deviations from the WILF equation are of course to be expected. This has been stated, for instance, by Rusch and Beck (1969). 

If we ignore transpiration and assume uniform tissue temperatures, the time constant for temperature changes equals... 

Fig. 7. Effects of fast temperature changes during tail currents upon repolarization to V=0 (resting potential) following 6 s at V=30 mV. Superimposed records at constant temperature (above, 20°C below, 10 C) and during short (ca. 1-s) temperature changes (above, two cold pulses to 10°C below, three warm pulses to 20 C). The temperature in the vicinity of the node of Ranvier (10 mm upstream at continuous superfusion) was recorded with a small thermocouple of limited temporal response the true temperature step achieved by a fast change of perfusates and estimated to be completed within 0.1 s, induced step changes in current and immediate changes in the time constants of tails. At constant temperatures the time constants of tail currents were 0.9 s (20 C) and 3.8 s (10 C). (With permission from Ulbricht 1969b) ...

Schematic illustrations of the effect of temperature and surface density (time) on the ratio of two isotopes, (a) shows that, generally, there is a fractionation of the two isotopes as time and temperature change the ratio of the two isotopes changes throughout the experiment and makes difficult an assessment of their precise ratio in the original sample, (b) illustrates the effect of gradually changing the temperature of the filament to keep the ratio of ion yields linear, which simplifies the task of estimating the ratio in the original sample. The best method is one in which the rate of evaporation is low enough that the ratio of the isotopes is virtually constant this ratio then relates exactly to the ratio in the original sample.

The physical constants of furfuryl alcohol are Hsted in Table 1. When exposed to heat, acid or air the density and refractive index of furfuryl alcohol changes owing to chemical reaction (51), and the rate of change in these properties is a function of temperature and time of exposure. 

Adaptive Control. An adaptive control strategy is one in which the controller characteristics, ie, the algorithm or the control parameters within it, are automatically adjusted for changes in the dynamic characteristics of the process itself (34). The incentives for an adaptive control strategy generally arise from two factors common in many process plants (/) the process and portions thereof are really nonlinear and (2) the process state, environment, and equipment s performance all vary over time. Because of these factors, the process gain and process time constants vary with process conditions, eg, flow rates and temperatures, and over time. Often such variations do not cause an unacceptable problem. In some instances, however, these variations do cause deterioration in control performance, and the controllers need to be retuned for the different conditions. 

Temperature Measurement shift. Measurement not representative of process. Indicator reading varies second to second. Ambient temperature change. Fast changing process temperature. Electrical power wires near thermocouple extension wires. Increase immersion length. Insulate surface. Use quick response or low thermal time constant device. Use shielded, twisted pair thermocouple extension wire, and/or install in conduit. 

Filtration constants K and C can be experimentally determined, from which the volume of filtrate obtained over a specified time interval (for a certain filter, at the same pressure and temperature) can be computed. If process parameters are changed, new constants K and C can be estimated from Equations 14 and 15. Equation 16 may be further simplified by denoting tg as a constant that depends on K and C ... 

The dump temperature of the compound was varied by changing the mixer s rotor speed and fill factor while keeping the other mixing conditions and the mixing time constant. Under the assumption that the final dump temperature is the main parameter influencing the degree of the sUanization reaction, the effect of the presence of ZnO on the dynamic and mechanical properties of the compound was investigated. ZnO was either added on the two-roll mill or in the mixer. 

The strain temperature sweep measurement is conducted with a preselected amplitude for the applied strain (y) and a constant frequency (f). The changing parameter is the temperature T, which is given in a temperature-time profile [T = T(t)]. This test method serves to illuminate the structural build-up, the softening, the melting and the gelation of pectins influenced when the temperature changes. 

For a thermometer to react rapidly to changes in the surrounding temperature, the magnitude of the time constant should be small. This involves a high surface area to liquid mass ratio, a high heat transfer coefficient and a low specific heat capacity for the bulb liquid. With a large time constant, the instrument will respond slowly and may result in a dynamic measurement error. 

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