Pulse travelling time

Check whether the assumption of vertical and lateral homogeneity from the point of tracer injection to the first cascade (1 km downstream) is justified. At the start of the injection, a short pulse of uranin (sodium fluorescein, a fluorescent dye) was added to the river in order to measure the travel time of the water. At each station, the samples for analyzing the halogenated compounds were taken 1.5 hours after the uranin peak had passed by. Based on this information, justify why longitudinal dispersion can be disregarded in the evaluation of the experiment. 

Even if each electron pair were emitted at exactly the same moment, the associated START-STOP time difference would vary from pair to pair. This is because of the different travelling times of the coincident electrons from their places of origin to the electron detector as well as the different processing times of the electron detector, including the construction of the electronic pulse. Hence, the coincident electrons will lead to a certain time spectrum as shown in the section of the spectrum in Fig. 4.48 which is labelled true coincidences. The name true coincidences implies that these are the coincidences in which one is interested. 

Sonic (up to 9,500 Hz) and ultrasonic (10-70 kHz) level sensors operate either by the absorption (attenuation) of acoustic energy as it travels from source to receiver, or by generating an ultrasonic pulse and measuring the time it takes for the echo to return. If the transmitter is mounted at the top of the tank, the pulse travels in the vapor space above the tank contents, and if it is mounted on the bottom, the time of travel reflects the depth of liquid in the tank. In water, at ambient temperature, the ultrasonic pulse travels at 1,505 m/s (4,936 ft/s). 

Changes in the water table of the Mohawk River and a number of adjacent observation wells is reported in Fig. 4.10, adapted from Winslow et al. (1965). The wells followed the river, with a time lag of 4-12 hours (insert in Fig. 4.10). Two possible explanations for this time lag may be envisaged (1) arrival of the hydraulic pulse, or (2) arrival of the recharge front (assuming piston flow section 2.1). To tell the two apart, the time lag observed for these wells by temperature measurements is helpful, as discussed in section 4.8 (see Fig. 4.21). The temperature time lag of, for example, well 58, has been observed to be about 3 months, whereas the water table time lag was only 12 hours. The latter defines the arrival of the hydraulic pulse, whereas the former defines the travel time of the recharge front. The distances given in the insert in Fig. 4.10, divided by the respective time lags, provided the... 

FIGURE 2-4 Transport of a chemical in a river. At time zero, a pulse injection is made at a location defined as distance zero in the river. As shown in the upper panel, at successive times C, t2, and t3, the chemical has moved farther downstream by advection, and also has spread out lengthwise in the river by mixing processes, which include turbulent diffusion and the dispersion associated with nonuniform velocity across the river cross section. Travel time between two points in the river is defined as the time required for the center of mass of chemical to move from one point to the other. Chemical concentration at any time and distance may be calculated according to Eq. [2-10]. As shown in the lower panel, Cmax, the peak concentration in the river at any time t, is the maximum value of Eq. [2-10] anywhere in the river at that time. The longitudinal dispersion coefficient may be calculated from the standard deviation of the concentration versus distance plot, Eq. [2-7]. 

For example, to estimate DL, a pulse of tracer is injected into a river and the longitudinal distribution of the tracer is measured as the river carries it past a downstream location. The spatial standard deviation of tracer and the travel time are determined from tracer concentration data, and DL is computed using Eq. [2-7]. 

Figure 15.7. Typical configurations used in biological MS. In MALDI-TOF, the ions produced by a short laser pulse travel across a flight tube, arriving at different times at the detector. In ESI-triple quadrupole, the first quadrupole (Ql) is used to separate the sprayed ions, in the second (Q2, also called the fragmentation cell) argon atoms collide with the ions the resulting ions (daughter ions) are analyzed in Q3, and subsequently detected.

---