Gradient-based measurements

Measurement of the transepidermal water loss (TEWL) is the standard method to determine SC barrier status. A disturbed skin barrier is characterized by high TEWL.7 The measurement of the TEWL is based on the diffusion principle in an open chamber. The density gradient is measured... 

The MRI-based viscosity measurement relies on local calculations of the velocity gradient based on using MRI velocity profiles to provide a wide range of shear viscosity-shear rate data. Two observations are used to characterize fully developed and steady laminar flow in a MRI-based viscometer the velocity profile and the pressure drop per unit tube length measurements. Viscosity measurements were acquired on strawberry milk samples using H P LG tubing for the sample channel. 

Limitations due to convection. At longer times the buildup of density gradients and stray vibrations will cause convective disruption of the diffusion layer, and usually result in currents larger than those predicted by the Cottrell equation. The time for the onset of convective interference depends on the orientation of the electrode, the existence of a protective mantle around the electrode, and other factors (1, 2). In water and other fluid solvents, diffusion-based measurements for times longer than 300 s are difficult, and even measurements longer than 20 s may show some convective effects. 

ARR residuals may be obtained from a DBG. If ARRs cannot be deduced from a DBG in closed symbolic form because of nonlinear implicit equations indicated by causal paths, ARR residuals are given implicitly. Their numerical values can be obtained by solving the entire DBG model in each step of the parameter estimation iteration. Derivatives of measured variables with respect to time that are needed in the evaluation of the DBG model are to be performed in discrete time. Once residuals are available for the time points of an observation window, the cost function can be built. If a gradient based parameter estimation method is used, the gradient of the least squares cost function can be obtained by using discrete derivatives. 

Estimation of the maximum pressure gradient based on peak velocity measurements (or assessment of maximum acceleration)... 

Diffusion studied by NMR was recently reviewed [727]. Various reviews deal with gradient-based NMR diffusion measurements [722-725,728]. The literature on diffusion is vast and highly mathematical [729-731]. 

Thermocouples are primarily based on the Seebeck effect In an open circuit, consisting of two wires of different materials joined together at one end, an electromotive force (voltage) is generated between the free wire ends when subject to a temperature gradient. Because the voltage is dependent on the temperature difference between the wires (measurement) junction and the free (reference) ends, the system can be used for temperature measurement. Before modern electronic developments, a real reference temperature, for example, a water-ice bath, was used for the reference end of the thermocouple circuit. This is not necessary today, as the reference can be obtained electronically. Thermocouple material pairs, their temperature-electromotive forces, and tolerances are standardized. The standards are close to each other but not identical. The most common base-metal pairs are iron-constantan (type J), chomel-alumel (type K), and copper-constantan (type T). Noble-metal thermocouples (types S, R, and B) are made of platinum and rhodium in different mixing ratios. 

There are a number of NMR methods available for evaluation of self-diffusion coefficients, all of which use the same basic measurement principle [60]. Namely, they are all based on the application of the spin-echo technique under conditions of either a static or a pulsed magnetic field gradient. Essentially, a spin-echo pulse sequence is applied to a nucleus in the ion of interest while at the same time a constant or pulsed field gradient is applied to the nucleus. The spin echo of this nucleus is then measured and its attenuation due to the diffusion of the nucleus in the field gradient is used to determine its self-diffusion coefficient. The self-diffusion coefficient data for a variety of ionic liquids are given in Table 3.6-6. 

By contrast a cathodic protection system based on sacrificial anodes is designed from the outset to achieve the required protection potential. If this is not achieved in practice there is no control function that can be exercised to improve the situation. Some remodelling of the system will be required. Moreover, the currents from each current source (the sacrificial anodes) is modest so that field gradients in the environment are not significant. It is at once clear that potential measurements are less significant in this case and instant-off measurements are neither necessary nor possible. 

Fig. 4 Predicted versus observed summer Anoxic Factor (AF) in (a, b) Foix Reservoir (Spain), (c, d) San Reservoir (Spain), (e, f) Brownlee Reservoir (USA), and (g, h) Pueblo Reservoir (USA). The results have been arranged to place the systems along a gradient of relative human impact (Foix Reservoir at the top, Pueblo Reservoir at the bottom). Predictions are based on linear models using different independent variables (in brackets) Inflow = streamflow entering the reservoir during the period DOCjjiflow = mean summer river DOC concentration measured upstream the reservoir CljjjAow = mean summer river CU concentration measured upstream the reservoir and Chlepi = mean summer chlorophyll-a concentration measured in the epilimnion of the reservoir. The symbol after a variable denotes a nonsignificant effect at the 95% level. Solid lines represent the perfect fit, and were added for reference. Modified from Marce et al. [48]...

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