Coil sensitivity

The factor Bi/i in Eq. (2.5.6), the magnetic field per unit current, is defined to be the coil sensitivity. For almost all coil geometries, the value of Bi/i is inversely... 

Providing that hid remains constant, the RF coil sensitivity is inversely proportional to the coil diameter for solenoidal coils of diameter greater than about 100 p,m. It can be shown theoretically and has been demonstrated experimentally that the sensitivity of a solenoidal coil is about three times that of a saddle coil of the same size [3], In order to translate the coil sensitivity into measurement of SIN defined by Equation 7.6, we also have to consider the noise characteristics of NMR microcoils. For coils of diameter less than 3 mm, the major noise source is from the resistance of the coil itself, and the noise from even lossy biological samples can be neglected [10]. 

This equation indicates that the coil sensitivity varies inversely with the diameter of the coil (for a fixed length-to-diameter ratio). For diameters below 3mm, the AC resistance of the coil itself acts as the major noise source, even for lossy biological samples. The resistance depends on both the winding geometry (including wire diameter, number of turns, and turn spacing) and the resistivity of the conductor. 

To date, two arrangements of NMR coils, the solenoidal radio frequency (RF) coil (Figure 2A) and the saddle-type (Helmholtz) RF coil (Figure 2B) have been employed as on-line NMR detectors with CE and CEC. Theoretical studies have shown that reduction of the diameter of the RF coils increases the coil sensitivity [32], The miniaturized versions of saddle types are commonly used in commercial probes. As a major development, a saddle coil which houses 1.7-mm-diameter sample tubes has been introduced by Varian. Another significant contribution is the designing of an inverse coil to accommodate 3-mm-diameter sample tubes with a detection volume of 60 pL and a total volume of 140 pL [33], Fabrication procedures hinder further reduction of diameter of saddle-type coils that are optimized for sample volumes smaller than —1 pL. 

The coil sensitivity of solenoidal probes is inversely related to the coil diameter [32], The S/N per unit volume for solenoidal coils with diameter greater than 100 pm is approximated by Eq. (2) [34] ... 

The coil sensitivity is characterized by means of the reciprocity theorem by the ratio where is the virtual field induced by a coil carrying... 

Keywords eddy current core-less coil sensitivity inductance... 

In contrast to a direct injection of dc or ac currents in the sample to be tested, the induction of eddy currents by an external excitation coil generates a locally limited current distribution. Since no electrical connection to the sample is required, eddy current NDE is easier to use from a practical point of view, however, the choice of the optimum measurement parameters, like e.g. the excitation frequency, is more critical. Furthermore, the calculation of the current flow in the sample from the measured field distribution tends to be more difficult than in case of a direct current injection. A homogenous field distribution produced by e.g. direct current injection or a sheet inducer [1] allows one to estimate more easily the defect geometry. However, for the detection of technically relevant cracks, these methods do not seem to be easily applicable and sensitive enough, especially in the case of deep lying and small cracks. 

A method has been worked out for eddy current testing of surfaces and surface cracks or corrosion under dielectric or non-magnetic metal layer of up to 10 mm. The method is based on excitation of eddy currents by a coil with U - type core and information reading by a sensitive gradientometric element located on a axis of symmetry of the core (fig. 1). 

The first of them to determine the LMA quantitatively and the second - the LF qualitatively Of course, limit of sensitivity of the LF channel depends on the rope type and on its state very close because the LF are detected by signal pulses exceeding over a noise level. The level is less for new ropes (especially for the locked coil ropes) than for multi-strand ropes used (especially for the ropes corroded). Even if a skilled and experienced operator interprets a record, this cannot exclude possible errors completely because of the evaluation subjectivity. Moreover it takes a lot of time for the interpretation. Some of flaw detector producers understand the problem and are intended to develop new instruments using data processing by a computer [6]. 

The absolute measurement of areas is not usually usefiil, because tlie sensitivity of the spectrometer depends on factors such as temperature, pulse length, amplifier settings and the exact tuning of the coil used to detect resonance. Peak intensities are also less usefiil, because linewidths vary, and because the resonance from a given chemical type of atom will often be split into a pattern called a multiplet. However, the relative overall areas of the peaks or multiplets still obey the simple rule given above, if appropriate conditions are met. Most samples have several chemically distinct types of (for example) hydrogen atoms within the molecules under study, so that a simple inspection of the number of peaks/multiplets and of their relative areas can help to identify the molecules, even in cases where no usefid infonnation is available from shifts or couplings. 

Such relays are normally instantaneous, highly sensitive and operate at low spill cuiTents. Since they detect the residual current of the system, the current may contain third-harmonic components (Section 23.6) and operate the highly sensitive relay in a healthy condition. To avoid operation of the relay under such conditions, it is a normal practice to supply the relay coil with a tuned filter, i.e. a series L-C circuit to filter out the third-harmonic components. The capacitance of the filter circuit may also tame a steep rising TRV (Section 17.10.3) during a momentary transient condition and protect the relay. 

---