Pulsed Generator

Occasionally, a nonrotational measurement device can generate pulse outputs. One example is the vortex shedding meter, where a pulse can oe generated when each vortex passes over the detec tor. 

Equipment and reagents Ultrasound generator, pulse generator, photomultiplier tube, oscilloscope, light-insulated cabinet, rare gas source (e.g. argon), 3-ami-nophthalhydrazide (luminol), sodium hydroxide, alcohol or other volatile organic solutes. 

The diffracted X-radiation is detected by Geiger, proportional, or semiproporhonal detectors. See Fig. 10. The detector of each monochromator generates pulses, which are a measure of the intensity of radiation of each wavelength. The pulses are filtered through a discriminator in order to avoid undesired interferences. Pulses shrinking due to an increase of frequency of pulses is automatically compensated. Collected pulses are transferred to a computer for processing and output. See Fig. 11. 

Laser II A femtosecond mode-locked dye laser (Coherent, Satori) synchronously pumped using a cw mode-locked and frequency-doubled Nd YAB laser (Coherent, Antares), generating pulses in 76 MHz repetition rate and 250-fs fwhm. 

Of these, the first option is the most commonly used in process applications. Turbine flowmeters are probably the most common example where pulse inputs are used. Another example is a watthour meter. Basically any measurement device that involves a rotational element can be interfaced via pulses. Occasionally, a nonrotational measurement device can generate pulse outputs. One example is the... 

To emulate the operation of the FeRAM cell of the integrated circuit the measurement setup has to generate pulses of both polarities. The Shunt method as it is described in Section 3.2.2 is useful to exclude the influence of the sense capacitor and to reach high speed. 

Figure 2 Diagram of a generalized 2D-PIV setup showing all major components flow channel with the particle seeded fluid flow, laser sheet pulses illuminating one plane in the fluid, a CCD camera imaging the particles in the laser-illuminated sheet in the area of interest, a computer with PIV software installed, a timing circuit communicating with the camera and computer and generating pulses to control the double-pulsed laser. The PIV software setups and controls the major components, and analyses the images to derive a vector representation of flow field (see Plate 4 in Color Plate Section at the end of this book).

Position can be detected by optical or other encoders that generate pulses in proportion to the amount of movement detected. Encoders can provide incremental or absolute readings. An incremental encoder outputs a number of pulses corresponding to the amount of motion. There are usually two outputs, called A and B, that are separated in time by 90° to indicate the direction of travel. This is shown in Figure 3.156. 

The Coulter technique is a method of determining the number and size distribution of particles suspended in an electrolyte by causing them to pass through a small orifice on either size of which is immersed an electrode. The changes in electrical impedance as particles pass through the orifice generate pulses whose amplitudes are proportional to the volumes of the particles. The pulses are fed to a pulse height analyzer where they are scaled and counted and, from the derived data, the size distribution of the suspended phase is determined. 

Thom et. al. [43] used a magnified version of the Coulter Counter and, by drawing spheres through the aperture on nylon threads, mapped out the generated pulses on different streamlines (Figure 9.4). Pulse distortion was eliminated when the edges of the orifiee were rounded to form a conical... 

Some reactions are difficult to study directly because the required instrumentation is not available or the changes in standard physical properties (light absorption, conductivity etc.) typically used in kinetic measurements are too small to be useful. Competition kinetics can provide important information in such cases. In some situations, the chemistry itself makes direct measurement inconvenient or even impossible. This is the case, for example, in studies of slow reactions of free radicals. Because of the ever-present radical-depleting second-order decomposition reactions, slow reactions of free radicals with added substrates are possible only at very low, steady-state radical concentrations. The standard methods of radical generation (pulse radiolysis and flash photolysis) are not useful in such cases, because they require micromolar levels of radicals for a measurable signal. The self-reactions usually have k > 10 M s , so that the competing reactions must have a pseudo-first-order rate constant of lO s or higher (or equivalent, if conditions are not pseudo-first order) to be observed. Competition experiments, on the other hand, can handle much lower rate constants, as described later for some reactions of C(CH3)20H radicals with transition metal complexes. 

Figure 2. Dependence of rate constant on -AG° for electron transfer from B to A in A-Sp-B " generated pulse radiolytically in 2-methyltetrahy-drofuran. B = 4-biphenylyl, A = 2-naphthyl (1), 9-phenanthryl (2), 1-pyr-enyl (3), hexahydronaphthoquinon-2-yl (4), 2-naphthoquinonyl (5), 2-benzoqui-nonyl (6), 5-chlorobenzoquinon-2-yl (7), 5,6-dichlorobenzoquinon-2-yl (8). (Adapted from Ref. [31]).

Note that at this point we selected the function g arbitrarily, because we did not specify the shape of the source-generated pulse. To determine the function gi, we should substitute solution (13.68) into equation (13.64) ... 

Figure 4 Sketch of the electrochemical STM for short-pulse surface modifications. The potential of the working electrode (WE) is controlled by a low-frequency potentiostat (Pot) versus the reference electrode (RE) via the counterelectrode (CE). The tunneling voltage (Ut) is supplied via the I/U converter of the STM (A) and a low-pass filter (LP). To apply the short pulses to the STM tip, a high-frequency pulse generator (Pulse) is switched onto the tip for a few milliseconds.

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