Time Lag Measurements

Most experimental determinations of Dt, D2 reported to date 13 15 16 23 26,55,56) have been obtained by the method described above but sorption kinetics 16,17,27) and permeation time lag measurements 29,57 58> have also been employed for this purpose. 

Diffusivity. Diffusivity values derived from time lag measurements of permeation (D = P/6t) vs. membrane composition of PVC/EVA show the same general tendency as the permeability-composition data. The D values are, however, less reproducible, probably owing to short time lag values, and therefore are not reported here. Solubility values can also be calculated (S = P/D), but they are not reported for the same reason. No separate solubility measurements were made. 

The seriousness of this oversight is apparent in Sefcik and Schaefer s analysis of Toi s transport data (24) in terms of their NMR results (28) The value of the so-called "apparent" diffusion coefficient calculated from Toi s time lag data increases by 25% for an upstream pressure range between 100 mm Hg and 500 mm Hg On the other hand, the value of Deff(c) calculated from Toi s data changes by 86% over the concentration range from 100 to 500 mm Hg The difference in the two above coefficients arises from the fact that Da is an average of values corresponding to a range of concentrations from the upstream value to the essentially zero concentration downstream value in a time lag measurement Deff > on t le other hand, has a well-defined point value at each specified concentration and is typically evaluated (independent of any specific model other than Fick s law) by differentation of solubility and permeability data (22) ... 

The above discussions have dealt with data obtained without tte refinements introduced by the concept of partial immobilization. The low pressure limit of Eq. (11) corr onds to tl effective diffiisivity measured by low pressure time lag measurements, i.e. 

Fig. S. Comparison of the true Henry s law mode diffusion co licfent, Di> with the diffusion coefficient determined from time lag measurements at low nessure...

This is called the time lag. This time is a measure of the time scale that molecules diffuse through the medium. Since the time lag measurement is a rather straightforward task, it is a preferred method to many others to determine the Knudsen diffusivity as no further analysis is required on the data to obtain the... 

J. C. Jansen, K. Friess, E. Drioh, Organic vapour transport in glassy perfluoropolymer membranes. A simple semi-quantitative approach to analyze clustering phenomena by time lag measurements, submitted. 

In another study of the effects of imiaxial orientation on PVC and other essentially amorphous glasses (139), a wide range of penetrants has been considered and permeability, solubility, and diffusivity effects caused by various orientation variables have been explicitly determined (Fig. 36) (139). The orientation, measured by birefringence, leads to improved barrier properties, and appears to have an effect on both the apparent solubility coefficient = C/p and the apparent diffusivity determined by time-lag measurements. 

Experimental diffusion coefficients, as obtained from time-lag measurements, report a transport diffusion coefficient which cannot be obtained from equilibrium MD simulation. Comparisons made in the simulation literature are typically between time-lag diffusion coefficients (even calculated for glassy polymers without correction for dual-mode contributions and self-diffusion coefficients. 

The statistical study of breakdown time-lags in liquids has been the subject of numerous studies Lewis (1985) and his coworkers have proved the existence of a field-dependent "statistical time-lag", and showed how curves like the one of Fig. 1 could be predicted. The influence of the conditioning process of the electrodes and of the test method on the time-lag distribution density functions are extensively discussed in the book of Gallagher (1975). From breakdown time-lag measurements under rectangular voltage, it is possible to separate the initiation time (its statistical nature being questionable if it is field-dependent) from the so-called "formative time-lag", which is in fact the streamer propagation time (at least, in most of the experimental situations). With AC or DC, another aspect of initiation phenomena will be presented. 

Additional CWI parameters concern the similarity between the reference and current CFs and temporal averaging. For the application at Okmok, the peak of the time-windowed correlation is required to exceed 0.9 in order for the associated time-lag measurement within the moving time window to be considered when... 

Two emerging areas of research in the use of seismic interferometry for monitoring changes at volcanoes are highlighted in this section. The first example addresses the use of new sources besides repeating earthquakes and the oceanic microseism. The second example concerns the use of decorrelation between repeating signals in addition to traditional time-lag measurements. 

MBRS Molecular beam spectroscopy [158] A modulated molecular beam hits the surface and the time lag for reaction products is measured Kinetics of surface reactions chemisorption... 

Carbon Dioxide Transport. Measuring the permeation of carbon dioxide occurs far less often than measuring the permeation of oxygen or water. A variety of methods ate used however, the simplest method uses the Permatran-C instmment (Modem Controls, Inc.). In this method, air is circulated past a test film in a loop that includes an infrared detector. Carbon dioxide is appHed to the other side of the film. AH the carbon dioxide that permeates through the film is captured in the loop. As the experiment progresses, the carbon dioxide concentration increases. First, there is a transient period before the steady-state rate is achieved. The steady-state rate is achieved when the concentration of carbon dioxide increases at a constant rate. This rate is used to calculate the permeabiUty. Figure 18 shows how the diffusion coefficient can be deterrnined in this type of experiment. The time lag is substituted into equation 21. The solubiUty coefficient can be calculated with equation 2. 

In an ideal first-order system, only one capacity causes a time lag between the measured quantity and the measurement result. Typically, an unshielded thermometer sensor behaves as a first-order system. If this sensor is rapidly moved from one place having temperature Tj to another place of temperature T2, the change in the measured quantity is close to an ideal step. In such cases, the sensor temperature indicated by the instrument has a time histoty as shown in Fig. 12.13. 

It is implicit in the idea of a series elastic component that there should be a lag between the activation of the contractile apparatus and the rise of force measured between the ends of the muscle. The observed time lag is also commensurate with the idea that the series component is largely due to the crossbridges themselves. 

Include a first-order time lag in the temperature measurement, using the... 

In the majority of methods described thus far, the interfacial kinetics are deduced by measuring concentration changes in the bulk of the solution rather than at the interface, where the reaction occurs. This introduces a time lag, limiting the resolution of the measurement in the determination of interfacial kinetics. A more direct approach is to identify the interfacial flux. This can be achieved in the electrolyte dropping electrode, via the current flow, but this method is only applicable to net charge-transfer processes at externally polarized interfaces. 

D can then be calculated from the measured time lag, which is the intercept on the time axis, by... 

The advantage of using the time lag method is that the partition coefficient K can be determined simultaneously. However, the accuracy of this approach may be limited if the membrane swells. With D determined by Eq. (12) and the steady-state permeation rate measured experimentally, K can be calculated by Eq. (10). In the case of a variable D(c ), equations have been derived for the time lag [6,7], However, this requires that the functional dependence of D on Ci be known. Details of this approach have been discussed by Meares [7], The characteristics of systems in which permeation occurs only by diffusion can be summarized as follows ... 

Barrer (19) has developed another widely used nonsteady-state technique for measuring effective diffusivities in porous catalysts. In this approach, an apparatus configuration similar to the steady-state apparatus is used. One side of the pellet is first evacuated and then the increase in the downstream pressure is recorded as a function of time, the upstream pressure being held constant. The pressure drop across the pellet during the experiment is also held relatively constant. There is a time lag before a steady-state flux develops, and effective diffusion coefficients can be determined from either the transient or steady-state data. For the transient analysis, one must allow for accumulation or depletion of material by adsorption if this occurs. 

As pDNA and mRNA transfection differ in both the timing of mRNA expression and the gross amount of mRNA delivered to the cell, it is important to identify a suitable time point to measure miR-mediated repression. We observe that at any time point after transfection, pDNA transfections have higher measurable levels of miR-mediated repression compared to mRNA transfections (Fig. 6.2C). This difference may, in part, reflect a time lag of active miR-protein-complex formation relative to the onset of translation of the transfected Renilla luciferase mRNA. For single time point experiments, we decided to measure miR-mediated repression in mRNA and DNA transfections at 16 and 24 h, respectively. 

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