1" sensitivity transient response

The development of an SCR system for vehicle applications requires precise calibration of the amount of urea injected as a function of the quantity of NO emitted by the engine, exhaust temperature and catalyst characteristics. Although model simulations can help in the control, it is necessary to use specific NO sensors which, however, still have problems of sensitivity and transient response. Installing a clean-up catalyst for ammonia would provide more latitude and obtain higher NO conversion ratios without re-emission of ammonia into the atmosphere. 

Recently there has been a growing emphasis on the use of transient methods to study the mechanism and kinetics of catalytic reactions (16, 17, 18). These transient studies gained new impetus with the introduction of computer-controlled catalytic converters for automobile emission control (19) in this large-scale catalytic process the composition of the feedstream is oscillated as a result of a feedback control scheme, and the frequency response characteristics of the catalyst appear to play an important role (20). Preliminary studies (e.g., 15) indicate that the transient response of these catalysts is dominated by the relaxation of surface events, and thus it is necessary to use fast-response, surface-sensitive techniques in order to understand the catalyst s behavior under transient conditions. 

In critical cases it may well be worthwhile to make a complete analysis of stability. In many cases, however, enough can be learned by studying what Bilous and Amundson (B7) called parametric sensitivity. These authors derived formulas for calculating the amplification or attenuation of disturbances imposed on an unpacked tubular reactor originally in a steady state, with the idea that if the disturbances grow unduly the performance of the reactor is too sensitive to the conditions imposed on it, that is, to the parameters of the system. The effect of feedback from a control system was not considered. As pointed out by the authors, it would be a much more complicated task to apply their procedure to a packed reactor, but it still would entail far less computation than a study of the transient response. 

A plot of Cf 2 versus s thus gives a straight line of slope 4/[tm Pe) and intercept 2 on the ordinate axis. The main advantages of the above methods, compared with the normally-used method of central moments, are (a) the validity of the model may be easily assessed, and (b) the sensitivity to experimental errors in the determination of transient response is greatly reduced, provided suitable s-values are used. Michelsen and Ostergaard68 showed that the last method can also be applied to the N-tanks-in-series model. Very recently, Pham and Keey,82 by working with the general definitions of t/g, Uu and U2 as ... 

Although the basic principles of type III potentiometric sensors are apphcable for gaseous oxide detection, this should not obscure the fact that these sensors still require further development. This is especially true in view of the kinetics of equilibria and charged species transport across the solid electrolyte/electrode interfaces where auxiliary phases exist. Real life situations have shown that, in practice, gas sensors rarely work under ideal equilibrium conditions. The transient response of a sensor, after a change in the measured gas partial pressure, is in essence a non-equilibrium process at the working electrode. Consequently, although this kind of sensor has been studied for almost 20 years, practical problems still exist and prevent its commercialization. These problems include slow response, lack of sensitivity at low concentrations, and lack of long-term stability. " It has been reported " that the auxiliary phases were the main cause for sensor drift, and that preparation techniques for electrodes with auxiliary phases were very important to sensor performance. ... 

FIG. 5 Photocurrent transient responses obtained at a dye-sensitized water]DCE junction for two types of mechanisms (a) homogeneous photoinduced electron transfer followed by ion transfer, and (b) photoinduced heterogeneous electron transfer. Curve (a) was obtained in the presence of ZnTPP and benzoquinone in the DCE phase (Reprinted from Ref 64 with permission from Elsevier Science). Panel (b) depicts the heterogeneous quenching of the heterodimer ZnTPPS-ZnTMPyP by decamethylferrocene at two different Galvani potential differences. Reprinted with permission from Ref 7. Copyright (1999) American Chemical Society. 

The transient response curve of the 10263 film to ethanol gas is shown in Figure 9.23. The variation of sensitivity was measured at the optimized temperature of 400°C and ethanol concentration of 900 ppm. Upon injection of ethanol gas, the sensitivity reached saturation in about 4 min, and when dry air was introduced, the sensitivity returned to the initial value after 12 min. The response and recovery times are in several minutes and the obtained value of sensitivity is very low when compared to the Sn02 gas sensors. The sensitivity and transient response of the In203 sensor toward ethanol are poor, which may be improved by incorporating some suitable dopants. In the present... 

The response and recovery times are important parameters for tailoring sensors for desired applications. The response time is defined as the time required for the sensitivity to reach 90% of the equilibrium value after the test gas in injected. The recovery time is the time necessary for the sensor to attain sensitivity 10% above the original value in air. The transient response curve of the Mgln204 sensors before and after and Li+ implantation are shown in Figure 9.26. The variations... 

Many time-resolved methods do not record the transient response as outlined in the earlier example. In the case of linear systems, all information on the dynamics may be obtained by using sinusoidally varying perturbations x(t) (harmonic modulation techniques) [27], a method far less sensitive to noise. In this section, the complex representation of sinusoidally varying signals is used, that is, A (r) = Re[X( ) exp(I r)]> where i = The quantity X ( ) contains the amplitude and the phase information of the sinusoidal signal, whereas the complex exponential exp(I )f) expresses the time dependence. A harmonically perturbed linear system has a response that is - after a certain transition time - also harmonic, differing from the perturbation only by its amplitude and phase (i.e. y t) = Re[T( ) exp(i > )]). In this case, all the information on the dynamics of the system is contained in its transfer function which is a complex function of the angular frequency, defined as [27, 28]... 

Analysis of currently available data on intrinsic threshold approaches " indicates they are limited to less than about 5x10 pels due to relatively high voltage or power requirements, slow transient response times, or long page refresh times. Uniformity of appearance will also be a problem due to high sensitivity of the intrinsic LC threshold to variations in LC layer thickness or in the substrate LC interface. 

The response of a typical T-F pH electrode develops over several days with maximum daily drift of about 5 mV (typically 1 mV). Its minimum longevity is 40-50 days and the success of fabrication varies from 10% (Pt/Au contact) to 50% (Fe contact). The transient response for a decade increase of HCl concentration has a time constant from several seconds (10 to lO M HCl) to about half a minute (10 to 1 M HCl) pNa sensitive glasses also give longer time constants in higher concentrations. 

Capsaicin is used topically for symptomatic relief of pain. Unlike other pain medications that decrease inflammation (NSAIDs) or prevent transmission (local anesthetics) and perception of pain (narcotics), capsaicin works on a special type of nociceptors at the origin of the pain signal. Capsaicin activates the capsaicin or vaniloid 1 receptor, which is a special type of temperature-sensitive transient receptor potential (TRP) non-selective ion channel (TTRPl). Capsaicin also induces release of substance P, which is responsible for runny nose, watery eyes, sweating, and gastric juice production. Capsaicin also releases endorphins however, it appears that this does not play a major role in pain relief by capsaicin. 

For enclosed bodies of water the probable maximum storm surge should be derived by using validated one or two dimensional mathematical models. The critical portion of the wind field, after being adjusted for any overland effects, is used as input for this analysis. The selection of coefficients and boundary conditions should be based on conservative assumptions. When one dimensional models are used, the transverse or crosswind set-up or a transverse seiche component is calculated separately and added to the longitudinal wind set-up. If the water body is sensitive to resonance, the transient responses should also be considered separately in a one dimensional model. If the water body is considered to be relatively insensitive to resonance, an analysis should be performed to substantiate this. The two dimensional transient mathematical models automatically take into account the transverse components and resonance effects. Components of the probable maximum still water levels are the longitudinal wind set-up, the transverse or cross-wind set-up and the reference water level. 

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