Variable thickness filter

FIGURE 15.23 Comparison of the results of a PI controller with and without filtering on the measured value of the controlled variable. Thick line = with filtering, thin line = without filtering. 

Variable Spike Filters. Multilayer interference filters of the longwave-pass type as used for order elimination were discussed on p. 37. By variation in the design of the coatings it is possible to make filters which pass only a narrow band of wavelengths. Such a device is called a spike filter. Recent developments in coating techniques have led to the availability of variable spike filters. In these, the thickness of the layers is varied as a function of angular position on a circularly shaped disc. If a narrow beam from an infrared source impinges on the disc the... 

We now turn attention towards the ease of eonstant-rate filtration. When sludge is fed to a filter by means of a positive displaeement pump, the rate of filtration is nearly constant, i.e., dV/dx = constant. During constant-rate filtration, pressure increases with cake thickness. As sueh, the principal filtration variables are pressure ind filtrate volume, or pressure and filtration time. Integrating the filtration equation for a constant-rate process, we find that the derivative dV/dx ean simply be replaeed by V/x, and we obtain ... 

The sole purpose of the filter support and any applied extracellular matrix is simply to provide a surface for cell attachment and thus to provide mechanical support to the monolayer. However, the filter and matrix also can act as serial barriers to solute movement after diffusion through the cell monolayer. The important variables are the chemical composition of the filter, porosity, pore size, and overall thickness. In some cases, pore tortuosity also can be important. It is desired that the filter, with or without an added matrix, provide a favorable surface to which the cells can attach. However, in some cases these properties can also result in an attractive surface for nonspecific adsorption of the transported solute. In these instances, the appearance of the solute in the receiver compartment of the diffusion cell will not be a true reflection of its movement across the mono-layer. Such problems must be examined on a case-by-case basis. 

The membrane filters have a variability (in the porosity and thickness), although this is believed to be only of the order of 2-5% [16]. 

The Boyden chamber is a simple apparatus used to test for chemotaxis, especially of leukocytes. It can also be used to assess tumor cell transmigration across an endo-thehal monolayer in vitro. It consists of two compartments separated by a MiUi-pore filter (3-8 pm pore size). A chemotac-tic factor is placed in one compartment, and a gradient develops across the thickness of the filter (ca. 150 pm). Cell movement into the filter is measured after an incubation period less than the time taken for the gradient to decay. Cell motility can be measured in Boyden chambers containing filters precoated with different materials, for example fibronectin or fibronectin fragments. The method, when apphed to malignant and non-mahgnant cell hnes, shows that the variable invasive potentials of these cells correlate with their abihty to disrupt the endothelial cell monolayer. 

Four PMTs detect the transmitted light under different projection angels. The four detector signals are connected to the individual channels of a four-module TCSPC system. In practice, variable neutral-density filters and long-pass filters are placed in front of the detectors to compensate for different intensity at different breast thickness and to reduce the daylight sensitivity. 

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