Magnetic transmittance

Fig. 23.5. Scatter plot shows the correlation between estimated and true volumes of a barostat bag positioned in the human stomach after scanning by a 3D ultrasound system. Scanning was performed with a 3.5-MHz transducer attached to a Bird system after stepwise instillation and aspiration of the test meal. The magnetic transmitter was positioned just behind the back of the examined subject and within the performance range of the sensor (60 cm)...

A mechanics-free airborne sound location system is used in order to record the probe movement and the rotation direction of the probe relative to the weld. Two airborne sound transmitters are arranged on the probe holder and two receivers are fixed on a 50 cm long rail equipped with two magnetic pads The exact probe position and rotation direction is continuously determined by system. 

The essential features of an NMR spectrometer shown m Figure 13 5 are not hard to understand They consist of a magnet to align the nuclear spins a radiofrequency (rf) transmitter as a source of energy to excite a nucleus from its lowest energy state to the next higher one a receiver to detect the absorption of rf radiation and a recorder to print out the spectrum... 

Flow is an important measurement whose calibration presents some challenges. When a flow measurement device is used in applications such as custody transfer, provision is made to pass a known flow through the meter. However, such a provision is costly and is not available for most in-process flowmeters. Without such a provision, a true cahbration of the flow element itself is not possible. For orifice meters, calibration of the flowmeter normally involves cahbration of the differential pressure transmitter, and the orifice plate is usually only inspected for deformation, abrasion, and so on. Similarly, cahbration of a magnetic flowmeter normally involves cahbration of the voltage measurement circuitry, which is analogous to calibration of the differential pressure transmitter for an orifice meter. 

In order for electronic transmission systems to be less susceptible to interference from magnetic fields, current is used for the transmission signal instead of voltage. The signal range is 4 to 20 miUiamps. In each circuit or current loop, there can be only one transmitter. There can be more than one receiver, but not an unlimited number. For each receiver, a 250 ohm range resistor is inserted into the current loop, which provides a 1- to 5-volt input to the receiving device. The number of receivers is hmited by the power available from the transmitter. 

Fig. 5. Effect of magnet current on electrical resistivity and transmittance of SnOj film. Microwave power of 1600 W.

Fig. 6. Effect of microwave power on electrical resistivity and transmittance of Sn02 film. Magnet current of 160 A.

A pulse is a burst of radiofrequency energy that may be applied by switching on the Rf transmitter. As long as the pulse is on, a constant force is exerted on the sample magnetization, causing it to process about the Rf vector. 

To study NMR spectra of compounds, apparatus is required that consists of three sets of components. These are a radio-frequency transmitter, a homogeneous magnetic field and a radio-frequency receiver. In addition to these, the apparatus includes a unit to sweep the magnetic field over a small range, a mere few parts per million. 

A long capillary with a computer-controlled switching valve (the instruments must be separated by 2-3 metres because of the strong magnetic field) connects the exit from the HPLC with the probehead. The latter is completely different in its construction from conventional probeheads instead of the NMR tube there is a small flow cell, the volume of which is 40-100 pi. The transmitter and receiver coils are attached directly to the cell in order to maximize the sensitivity. 

These systems work by placing a sample between the pole pieces of a magnet (electromagnet or permanent), surrounded by a coil of wire. Radio frequency (r.f.) is fed into the wire at a swept set of frequencies. Alternatively, the magnet may have extra coils built onto the pole pieces which can be used to sweep the field with a fixed r.f. When the combination of field and frequency match the resonant frequency of each nucleus r.f. is emitted and captured by a receiver coil perpendicular to the transmitter... 

The basic components of the solid state spectrometer are the same as the solution-phase instrument data system, pulse programmer, observe and decoupler transmitters, magnetic system, and probes. In addition, high-power amplifiers are required for the two transmitters and a pneumatic spinning unit to achieve the necessary spin rates for MAS. Normally, the observe transmitter for 13C work requires broadband amplification of approximately 400 W of power for a 5.87-T, 250-MHz instrument. The amplifier should have triggering capabilities so that only the radiofrequency (rf) pulse is amplified. This will minimize noise contributions to the measured spectrum. So that the Hartmann-Hahn condition may be achieved, the decoupler amplifier must produce an rf signal at one-fourth the power level of the observe channel for carbon work. 

The basic instrumentation used for spectrometric measurements has already been described in Chapter 7 (p. 277). The natures of sources, monochromators, detectors, and sample cells required for molecular absorption techniques are summarized in Table 9.1. The principal difference between instrumentation for atomic emission and molecular absorption spectrometry is in the need for a separate source of radiation for the latter. In the infrared, visible and ultraviolet regions, white sources are used, i.e. the energy or frequency range of the source covers most or all of the relevant portion of the spectrum. In contrast, nuclear magnetic resonance spectrometers employ a narrow waveband radio-frequency transmitter, a tuned detector and no monochromator. 

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