Point tracer measurements

Tracer Type. A discrete quantity of a foreign substance is injected momentarily into the flow stream and the time interval for this substance to reach a detection point, or pass between detection points, is measured. From this time, the average velocity can be computed. Among the tracers that have historically been used are salt, anhydrous ammonia, nitrous oxide, dyes, and radioactive isotopes. The most common appHcation area for tracer methods is in gas pipelines where tracers are used to check existing metered sections and to spot-check unmetered sections. 

The theory necessary for understanding two-station tracer measuring techniques is outlined in Appendix 1. An arbitrary, but unimodal, impulse of tracer is created in a system inlet and the outlet response recorded, see Fig. 21 (Appendix 1). Then, the mean, Mj, of that which resides between the points at which inlet and outlet pulses are observed and recorded is equal to the difference in means of these two signals. Similarly, the variance, T2, and the skewness, T3 are equal to the differences in these respective moments between inlet and outlet. This enables the system transfer function to be defined in terms of a few low-order moments via eqns. (A.5) or (A.9) of Appendix 1, this in turn defining the system RTD. Recall that system moments and moments of the system RTD are one and the same. 

If a pulse of tracer is injected into a flowing stream, this discontinuity spreads out as it moves with the fluid past a downstream measurement point. For a fixed distance between the injection point and measurement point, the amount of spreading depends on the intensity of dispersion in the system, and this spread can be used to characterize quantitatively the dispersion phenomenon. Levenspiel and Smith (L16) first showed that the variance, or second moment, of the tracer curve conveniently relates this spread to the dispersion coefficient. 

Several tracers have been used in experiments describing axial mixing in fluidized beds of porous particles, e.g. acetone [37,57], Tryptophane [47], NaCl [49,56], radioactive tracers [58] and dextrane blue [59], It should be noted at this point, that measurement of RTD is not only important for determining possible domination of the chromatographic result by liquid mixing, Bo may as well be taken as a measure for the existence of a stable classified fluidized bed which is ready for sample application. Measurement of RTD in this case will provide a rational basis for the decision to start a large scale protein purification using a fluidized bed or to take measures for improvement of bed stability before application of valuable material. 

The mechanism of extraction in tracer rare earth concentrations using HDEHP was investigated by Peppard et al. (7,8,9). Freezing-point depression measurements of toluene by the addition of HDEHP showed that HDEHP is dimeric in solution (9). On the basis of all their findings... 

In micro- and nanoscale fluid mechanics, measurements of mass transport and fluid velocity are used to probe fundamental physical phenomena and evaluate the performance of microfluidic devices. Evanescent wave illumination has been combined with several other diagnostic techniques to make such measurements within a few hundred nanometers of fluid—solid interfaces with a resolution as small as several nanometers. Laser Doppler velocimetry has been applied to measure single-point tracer particle velocities in the boundary layer of a fluid within 1 pm of a wall. By seeding fluid with fluorescent dye, total internal reflection fluorescence recovery after photobleaching (FRAP) has been used to measure near-wall diffusion coefficients and velocity (for a summary of early applications, see Zettner and Yoda [2]). 

The use of stable isotope labels in metabolic studies self-evidently involves the measurement of isotope ratios. Isotope ratios are translated into label amounts or tracer to tracee ratios following IDMS principles. IDMS per se is a primary method, that is, it is possible to identify all sources of uncertainty in the analysis and to state the combined uncertainty in the measurement result [60]. When IDMS is used for element quantification by means of an isotopically enriched spike, the uncertainty in the isotope ratio measurement is usually the major contributor to the uncertainty of the analysis [61]. At a precision of 0.5% for the actual isotope ratio measurement, which can be achieved comfortably, even with basic ICP-MS instruments, combined relative measurement uncertainties of 1-2% can easily be attained. This points to measurement precision not being a major obstacle in stable isotope-based metabolic studies. Most modern ICP-MS instruments are capable of measuring isotope ratios at that level of precision. However, as is often the case, reality turns out to be much more complex on second sight. 

Not all mixed valence compounds show intervalence bands. Those that show not the slightest evidence for them are often called class 1. In class 2 intervalence compounds the intervalence-transfer band may well dominate the visible spectrum, swamping the spectra of the individual ions. However, these may be seen the important point is that the different ions retain their chemical individuality. Soluble Prussian blue is a case in point. This contains Fe and Fe bridged by CN ligands. Isotopic tracer measurements show that there is no doubt—the Fe" is bonded to the N atom of the CN ligand and the Fe to the C. If radioactive Fe is used in the preparation and the compound subsequently decomposed, the activity remains in the iron of the same valence state in which it was incorporated. A class 3 also exists, exemplified by the IJ anion, in which it is not possible to associate a unique valence state to individual metal ions typically, but as our example shows, not always, they are structurally indistinguishable. When they form an... 

Vail et al. [68] used a steady state technique involving steady injection of aqueous NaCl solution at a given level of the column, to monitor the axial mixing of the liquid. The fluidized bed contained air, water and 0.87 mm spheres with a density of 2700 kg/m. The concentration profiles of tracer measured below the injection point provided information about axial dispersion coefficients and the characteristic mixing length (ratio of the axial dispersion with respect to the fluid velocity). It was observed that the presence of solids and the increase of the gas and liquid velocities were all factors promoting axial mixing of the liquid phase. 

Standard tracer tests are typically used at relatively small field scales. Estimates of dispersivity at scales larger than several hundred of meters usually rely on different methods, either using historical contamination data or exploiting natural variations in the chemistry of natural recharge of the aquifer. However, estimates of advection and dispersion based on data from contaminant plumes or environmental tracer measurements are less reliable than field tracer tests, since there is a larger uncertainty in the location and the intensity of source zone. Often, there is also an inadequate number of sampling points. 

The hard point with respect to high and documented precision is to obtain an accurate figure of the sensitivity of the measuring position, relative to the tracer used. 

Cross Correlation. Considerable research has been devoted to correlation techniques where a tracer is not used. In these methods, some characteristic pattern in the flow, either natural or induced, is computer-identified at some point or plane in the flow. It is detected again at a measurable time later at a position slightly downstream. The correlation signal can be electrical, optical, or acoustical. This technique is used commercially to measure paper pulp flow and pneumatically conveyed soHds. 

For a large amount of dispersion or small value of Np, the pulse response is broad, and it passes the measurement point slowly enough for changes to occur in the shape of the tracer curve. This gives a non-symmetrical E-curve. 

For example, the type test of a laboratory fume hood includes determination of the concentration at various points across the opening of the hodd by using various tracer source locations inside the hood. The commissioning test could concentrate on the measurements taken at one point in the opening with one source location. 

FIGURE 10< 108 The procedure to measure the capture efficiency by the tracer gas method, aj The measurement of the reference concentration in the duct, when the tracer is released direcdy into the duct, fb) The measurement of the concentration in the duct, when the tracer is released from the source, / -= sampling point, 2 = pump, J = analyter, 4 - injection of tracer, 5 = tracer gas flow meter, 6 = tracer gas cylinder. 

Several experimental techniques may be used, such as acid/base titration, electrical conductivity measurement, temperature measurement, or measurement of optical properties such as refractive index, light absorption, and so on. In each case, it is necessary to specify the manner of tracer addition, the position and number of recording stations, the sample volume of the detection system, and the criteria used in locating the end-point. Each of these factors will influence the measured value of mixing time, and therefore care must be exercised in comparing results from different investigations. 

LDV is the traditional method using tracer particles to measure velocity and one-point statistics of turbulent properties [2]. It is still a very useful technique and has the advantage that it can measure closer to walls compared to PIV. An inherent problem with LDV is that it does not measure at a specific point but rather at places... 

Radioactive-tracer log Tracer fluid movements are measured to produce a radioactive-tracer log. This shows the flow of fluid in the casing, tubing and the annulus, and helps to estimate flow rates, leaks, and other points of exit or entry for fluid into the borehole... 

Fig. 10.3 Photographs of a homogenous, saturated sand pack with seven dye tracer point injections being transported, under a constant flow of 53 mL/min, from left to right times at (a) t=20, (b) t= 105, (c) t= 172, (d) t=255 min after injection. Internal dimensions of the flow cell are 86 cm (length), 45cm (height), and 10cm (width). Reprinted from Levy M, Berkowitz B (2003) Measurement and analysis of non-Fickian dispersion in heterogeneous porous media. J Contam Hydrol 64 203-226. Copyright 2003 with permission of Elsevier...

---