Tracer-probe technique

Detector-probe technique It can find the location of a leak when the tracer gas is on the inside of the piece in question. This technique is time-consuming. Because the probe is collecting both air and helium, it is about 10% as sensitive as the tracer-probe technique. It cannot tell you the size of the leak. 

Aside from permeability and absorbency complications, other universal concerns of helium leak detection are factors such as source operating pressure, spraying patterns (for tracer-probe technique), response time, clean-up time, and cold trap usage. Pump use and general helium leak detector maintenance operations are also fairly universal. 

The source operating pressure is the vacuum necessary to operate the leak detection device. This pressure is not specific, rather it is a pressure range within the leak detector which works. Optimistically, we want the helium leak detector, and the system to which it is connected, to have the greatest possible vacuum. This gives the tracer-probe technique the maximum sensitivity with the quickest response time. As an added benefit, when one is operating at a very high vacuum,... 

Most helium leak detectors will not operate with pressures above lO"4 torr to 10 5 torr. At these greater pressures, the main element to the mass spectrometer will bum out. Fortunately most, if not all, helium leak detectors have various safety check mechanisms that automatically shut off the current to the main filament if the pressure goes above a set limit. So, you must depend on alternate leak detection methods, or use the detector-probe technique to discover large leaks. Once large leaks have been discovered and closed, you can concentrate on the smaller leaks that can be found with the tracer-probe technique. 

Fig. 7.65 A suggested pattern for spraying helium on a vacuum system when using the tracer-probe technique. The pattern is a compromise between proximity to the leak detector and spraying high before low areas.

Cleanup time is almost always longer than response time because of the difficulty in desorption of helium from a vacuum system, compounded by helium permeation into porous materials. So, when using the tracer-probe technique do not overspray helium onto your system. The more helium that enters your system, the more that can permeate into porous materials and the longer the cleanup time. 

Fig. 3.1.10 Molecular lifetimes xintra and. aii in H-ZSM-5 crystallites obtained using the NMR tracer desorption technique and calculated via Eq. (3.3.15), respectively. Tracing by probe molecules (methane, measurement at 296 K) after an H-ZSM-5 catalyst has been kept for different coking times in a stream of n-hexane (filled symbols) and mesitylene (open symbols) at elevated temperature. The inserts present the evidence provided by a comparison of xintra and r]1,]]], with respect to the distribu-...

The dynamic tracer pulse technique used in this work facilitates the study of BV and how BV might be altered because of high levels of organic compounds or humidity. Based on competitions of the various, selected probes, cursory information about surface sites can be obtained for a prospective adsorbent such information is especially useful for multifunctional polymers. This technique can also permit the fine tuning of the environmental collection strategy through the examination of retentions on mixed adsorbents or multiple adsorbent beds. The use of dynamic TPC with the polyimides demonstrated that these materials are more sensitive, in terms of BV of the tested probe molecules, to the effects of humidity than is Tenax-GC. 

The procedure recommended by Hochman and McCord [87] is to use two tracer probes to directly measure the bottom-to-top residence time, x-y, by the first appearance from an impulse injection at z = 0, and the top-to-bottom residence time, i2, with an impulse injection at z = L. Then, the recycle parameter, r, can be found from ft, = V/F g, Eq. (k) and the crossflow parameter, k-pO , from parameter estimation techniques can be used, as to be described in Sec. 12.5.C. 

A study of the residence time distribution (RTD) analysis of liquid phase has been performed. The liquid RTD is determined by means of the tracer response technique. An approximated 8-Dirac pnilse of tracer solution (NaCl) is injected into the reactors at a certain time (t = 0) and the outlet signal is detected by conductivity probe and recorded by the acquisition system. The tracer is injected as quickly as possible to obtain as closely as practical a 8-pulse of tracer at the inlet. 

According to what has been stated above, good results have been obtained as a result of a spectrophotometric technique that entails a colored tracer. Two measuring probes are set up one at the inlet and the other at the outlet of the device. The acquisition time is set to 0.12 s. The operating protocol adopted during RTD experiments is as follows ... 

Dispersion in packed tubes with wall effects was part of the CFD study by Magnico (2003), for N — 5.96 and N — 7.8, so the author was able to focus on mass transfer mechanisms near the tube wall. After establishing a steady-state flow, a Lagrangian approach was used in which particles were followed along the trajectories, with molecular diffusion suppressed, to single out the connection between flow and radial mass transport. The results showed the ratio of longitudinal to transverse dispersion coefficients to be smaller than in the literature, which may have been connected to the wall effects. The flow structure near the wall was probed by the tracer technique, and it was observed that there was a boundary layer near the wall of width about Jp/4 (at Ret — 7) in which there was no radial velocity component, so that mass transfer across the layer... 

Spectroscopic techniques, such as ultra-violet (9), Infrared (25), Nuclear Magnetic Resonance (24), and Fluorescence spectroscopies (5-8), constitute direct probes of specific events occurring at the molecular scale. When a quantitative interpretation is possible, spectroscopy provides very detailed microscopic information. Unfortunately however, the interpretation of spectra in terms of molecular events is often complex. Yet another approach that probes events at the molecular scale involves the use of tracers, such as chromophores (1-225). Again, the complexity of the tracer imposes limitations on the extent to which the data can be interpreted quantitatively. 

Improvement of the techniques for monitoring local instantaneous concentrations down to the viscous dissipation microscale (e.g. spatial and time resolution of conductivity probes), development of new techniques (e.g. optical, radioactive tracer methods). 

EQCM can be combined with other techniques, e.g., with -> probe beam deflection (PBD) [viii], radiotracer methods (- tracer methods) [ix], and - scanning electrochemical microscopy [x]. 

The extent of gas dispersion can usually be computed from experimentally measured gas residence time distribution. The dual probe detection method followed by least square regression of data in the time domain is effective in eliminating error introduced from the usual pulse technique which could not produce an ideal Delta function input (Wu, 1988). By this method, tracer is injected at a point in the fast bed, and tracer concentration is monitored downstream of the injection point by two sampling probes spaced a given distance apart, which are connected to two individual thermal conductivity cells. The response signal produced by the first probe is taken as the input to the second probe. The difference between the concentration-versus-time curves is used to describe gas mixing. 

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