Transit-Time Technique

FIGURE 5-6 Schematic representation of a single-path contrapropagation 

The speed of the sound can also be measured from the transit times. Let Tu = tu-lm and Td = td- tm. Then 

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Many experimental techniques now provide details of dynamical events on short timescales. Time-dependent theory, such as END, offer the capabilities to obtain information about the details of the transition from initial-to-final states in reactive processes. The assumptions of time-dependent perturbation theory coupled with Fermi s Golden Rule, namely, that there are well-defined (unperturbed) initial and final states and that these are occupied for times, which are long compared to the transition time, no longer necessarily apply. Therefore, truly dynamical methods become very appealing and the results from such theoretical methods can be shown as movies or time lapse photography. 

The technique just described requires the porous medium to be sealed in a cell, so It cannot be used with pellets of irregular shape or granular material. For such materials an alternative technique Introduced by Eberly [64] is attractive. In Eberly s method the porous pellets or granules are packed into a tube through which the carrier gas flows steadily. A sharp pulse of tracer gas is then injected at the entry to the tube, and Its transit time through the tube and spreading at the exit are observed. A "chromatographic" system of this sort is very attractive to the experimenter,... 

A variation on the transit time method is the frequency-difference or sing-around method. In this technique, pulses are transmitted between two pairs of diagonally mounted transducers. The receipt of a pulse is used to trigger the next pulse. Alternatively this can be done using one pair of transducers where each acts alternately as transmitter and receiver. The frequency of pulses in each loop is given by... 

In most ultrasonic tests, the significant echo signal often is the one having the maximum ampHtude. This ampHtude is affected by the selection of the beam angle, and the position and direction from which it interrogates the flaw. The depth of flaws is often deterrnined to considerable precision by the transit time of the pulses within the test material. The relative reflecting power of discontinuities is deterrnined by comparison of the test signal with echoes from artificial discontinuities such as flat-bottomed holes, side-drilled holes, and notches in reference test blocks. This technique provides some standardized tests for sound beam attenuation and ultrasonic equipment beam spread. 

Air and Gas. In the regions where air and natural gas are used as the principal drilling fluids, the potential oil and gas production zones usually have low pore pressure, or require well stimulation techniques to yield commercial production. In these production zones, air drilling (or natural gas drilling) is continued into the production zone and the initial produced formation fluids are carried to the surface by the circulating air or natural gas. This is nearly the same situation as in mud drilling, except that in air (or gas) drilling the transit time for the initial produced formation fluids to reach the surface is much shorter. In mud... 

A more usual procedure for overcoming the disturbances from contaminants is current reversal chronopotentiometry here the current is reversed at the initial transition time tf of the forward reaction and the next transition time xb of the backward reaction is measured as a rule the reversal wave will not be influenced by the contaminant because it will react either before the forward or after the backward reaction of the analyte (see Fig. 3.60a) the entire procedure can be even repeated as cyclic chronopotentiometry (see Fig. 3.60b), which may provide a further check on the reliability. The reversal technique can be applied to initial reduction followed by re-oxidation and also to initial oxidation followed by re-reduction79. 

By means of special precautions Bos74 prevented this difficulty in his additional capacity-current device, which also allows short transition times (down to 0.1 s) at low concentrations. Thus, together with the current reversal technique, the analysis of 10 5 M solutions appeared possible. 

Barreiro MA, McKenna RD, Beck IT. Determination of transit time in the human jejunum by the single-injection indicator-dilution technique. Am J Dig Dis 1968 13(3) 222—233. 

Caride VJ, Prokop EK, Troncale FJ, Buddoura W, Winchen-bach K, McCallum RW. Scintigraphic determination of small intestinal transit time comparison with the hydrogen breath technique. Gastroenterology 1984 86 714-720. 

In addition, monoclonal antibodies have been produced using the ELP fusion and inversion transition cycling technique described before by Floss et al. (2008). MAh directed against HIV-1 and coupled to ELP exhibited enhanced stability and simplified recovery, while at the same time displaying no deleterious effects on biological activity, assembly, and folding of the monoclonal antibody. 

Measurements of the transit times of weak shock waves ( 10Q bar) were used to obtain sound wave velocities in larger specimens than listed in Table II. In the arrangement of Fig 3 a cylinder (or slab) of the expl was immersed in a Plexiglas container filled with water. Initiation of the detonator produced a shock wave which arrived nearly plane thru the water at the surface of the expl specimen. The motion of wave was recorded by a smear camera using a shadowgraph technique. Plots of Us up relationships showed that the resulting curves were nearly straight lines and that for particle velocities, up, from 0.3 to 1.2 mm/ftsec, shock wave velocities are ... 

The preceding characterization of anomalously dispersive transit pulses aroused considerable interest, both from a theoretical and an experimental viewpoint. Attention focused on the latter was stimulated by the possibility of using the log-log display technique to identify ft in cases where the dispersion was such as to obscure any change of gradient in conventionally displayed transit pulses. However, it became necessary to question the validity of such measurements of ft under conditions where individual carrier transit times vary over such a wide range. 

The IFTOF technique requires the current mode of operation in which the various Resistance-capacity product (RC) time constants in the bridge are much smaller than the transit time tj. If the resistance Ri is larger than Rj, then the signal across the amplifier will be simply Rj i(t), where i(t) is the transient photocurrent. 

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