Sample dead time

Figure 24.9. Output response behavior of discrete instruments, t, — sampling dead-time = t2 — h = ti — ts = ta = analytical deadtime = h — tz = h — ti = ts + ta < ta < It, + 2ta, where ta is the total measurement dead-time. Courtesy of the Foxboro Company.

Two sources of dead-time are obvious. Sampling dead-time t, is the time elapsed between the instant the sample is taken and the instant it enters the analyzer. 

For on-line control these techniques must be evaluated for speed, reliability, and sample dead time [4]. As in the MWD techniques, manufacturers are moving more to vard on-line implementation of the more recent methods, but many of these techniques still remain in the control laboratory. 

Two-mode control combines the speed of response of proportional action with the elimination of offset brought about by automatic reset. The proportional mode is just as valuable in a sampled dead-time loop as it was in one without sampling. In fact, proportional action enables any loop whose dead time is less than the sampling interval to he critically damped. Figure 4.22 shows how this is done. 

Dead time. Probably the best example of a measurement device that exhibits pure dead time is the chromatograph, because the analysis is not available for some time after a sample is injected. Additional dead time results from the transportation lag within the sample... 

Sample Transport Transport time, the time elapsed between sample withdrawal from the process and its introduction into the analyzer, shoiild be minimized, particiilarly if the analyzer is an automatic analyzer-controller. Any sample-transport time in the analyzer-controller loop must be treated as equivalent to process dead time in determining conventional feedback controller settings or in evaluating controller performance. Reduction in transport time usually means transporting the sample in the vapor state. 

Scattered radiation. In a transmission experiment, the Mossbauer sample emits a substantial amount of scattered radiation, originating from XRF and Compton scattering, but also y-radiation emitted by the Mossbauer nuclei upon de-excitation of the excited state after resonant absorption. Since scattering occurs in 4ti solid angle, the y-detector should not be positioned too close to the absorber so as not to collect too much of this unwanted scattered radiation. The corresponding pulses may not only uimecessarily overload the detector and increase the counting dead time, but they may also affect the y-discrimination in the SCA and increase the nonresonant background noise. 

Faraday collector, simultaneously with U, U and U during the first sequence. This shortens the analysis routine, consuming less sample. Ion beam intensities are typically larger in MC-ICPMS than in TIMS due to the ease with which signal size can be increased by introducing a more concentrated solution. While this yields more precise data, non-linearity of the low-level detector response and uncertainties in its dead-time correction become more important for larger beam intensities, and must be carefully monitored (Cheng et al. 2000 Richter et al. 2001). 

Before going further, it may be noted that the flipping ratio does not depend either on the Lorentz factor or on absorption in the sample. Certain instrumental parameters such as the polarisation of the neutron beam for the two spin states, the half wavelength contamination of the neutron beam and the dead-time detector can readily be taken into account when analysing the data. On the other hand, the extinction which may occur in the scattering process is not so easy to assess, but must also be included [14]. Sometimes, it is even possible to determine the magnetisation density of twinned crystals [15]. 

As regards the NO breakthrough, when N0/02 adsorption is carried out on the Pt/y-Al203 (1/100 w/w) sample, no dead time is observed, which indicates a negligible storage of NO species on the surface. As a matter of fact, only minor amounts of NO have been stored in this case up to catalyst saturation, which however desorb upon switching off the NO feed flow. 

In the case of the Pt-Ba/alumina ternary sample, the dead time in the NO breakthrough upon a step change of NO in the presence of oxygen is reasonably well predicted by the model but the oxidation of NO to N02 is markedly overestimated. If the Pt dispersion in the model is lowered, the oxidation of NO to N02 is properly described but the NO breakthrough is no longer predicted. Therefore, the literature model is not able to describe at the same time the oxidation of NO to NO, and the NO breakthrough. 

Both TCSPC and TG benefit from operation in SPC mode. SPC results in little or no noise and a high photon-economy [10]. Therefore, TCSPC and TG are ideal for high spatial and lifetime resolution imaging [24], Both techniques offer high image contrast also on dim samples. However, the dead-time of the detectors and the point scanning character limit the throughput of these systems. 

As early GC peaks elute in a few seconds or less, rapid scanning of the mass range of interest is necessary. Fast scanning also allows partially resolved GC peaks to be sampled several times,peak slicing, to facilitate identification of the individual components (Figure 12.5) provided that the dead volume of the interface is small compared to peak volumes. For the speedy interpretation of spectral data from complex chromatograms a... 

The best model in terms of fitting the frequency response results (obtained in MATLAB by fast-Fourier-transforming the input and output signals) is the third-order with a dead time equal to four sampling periods. Fig. 14.9h compares the model frequency response with the data. 

When the deadtime in a process is an integer multiple of the sampling period, the function in the Laplace domain converts easily into z in the z domain, where dead time D = kT. When the dead time is not an integer multiple of the sampling period, we can use modified z transforms to handle the situation. 

Suppose we add the fictitious deadtime element and impulse sampler sketched with dashed lines in Fig. 18.13. Then by letting the dead time vary continuously between 0 and 1, we could obtain a description of x, at any time in between the sampling periods. 

The NMR experiment performed is the simplest one imaginable. The H nuclei in the sample are irradiated with a single square pulse of RF energy ( 1 W) lasting about 10-12/rs. After waiting a dead time of 10/rs, to allow the coil to physically stop oscillating from the power input it experienced, the receiver is... 

One problem is the dead time of the probe-preamplifier subsystem (the combined effect of probe RF ringing and of preamplifier recovery from saturation). While irrelevant in samples with long enough FID (above 0.1 ms or so), it may become a major limitation with fast-decaying FTDs (solid samples and/or samples with very short T2S) because it can obscure a substantial portion of the FID signal. 

Acoustic ringing of the probe assembly after an RF pulse is a pesky problem which often limits the measurements of nuclides with low gyromag-netic ratios (it can also strongly interfere with measurements of samples containing piezoelectric components). The disturbance is often misinterpreted as a particularly long dead-time disturbance, until one notices that, unlike normal dead-time components, it disappears when B is set to zero. It is difficult to remove because it follows the phase of the RF pulse and thus cannot be eliminated by any simple RF phase-cycling. 

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