Impedance changes

At sufficiently high frequency, the electromagnetic skin depth is several times smaller than a typical defect and induced currents flow in a thin skin at the conductor surface and the crack faces. It is profitable to develop a theoretical model dedicated to this regime. Making certain assumptions, a boundary value problem can be defined and solved relatively simply leading to rapid numerical calculation of eddy-current probe impedance changes due to a variety of surface cracks. 

Once y/" has been determined, the impedance change in an eddy-current coil due to the crack can be calculated using the following integral over the crack mouth [3] ... 

The measurements were made along the cracks with an average step size of 3 mm. The predictions were calculated from a position -15 mm to + 15 ram for set 1, from -40 mm to + 40 mm for set 2 and from -25 mm to + 25 mm for set 3. The impedance change has been calculated at 1mm intervals in the range. Taking into account the symmetry of the configuration, only half of the predictions need to be calculated. 

The analysis of the curves obtained in the thin-skin regime ean lead to a simple determination of slot length depending on the dimension of the probe chosen for the inspection. If the size of the probe (outer diameter) is smaller than the defect length we can notice 5 zones relative to the relationship between the position of the probe, the interaction of the induced eddy current and the slot, and the impedance change for the probe. 

Zone 1 The probe is far away form the slot the interaction with the slot is low. The impedance change is small. This situation is tme until the probe reaches the edge of the slot. The range of the zone is [- x 1 ]... 

Zone 4 The area of coverage interacting with the slot increase. The impedance change increases. This situation is true over the probe depth, that is to say until the all the length of the probe is over the slot. The range of the zone is [x3 x4]... 

The signal is reflected from the product surface because there is an abmpt impedance change in the sensor at the air—product interface. Because the electromagnetic field extends outside the two sensor conductors, the sensor impedance depends on the dielectric constant of the surrounding medium. In... 

DNA immobilized by organosilane chemistries has proven to be an effective method for measuring impedance changes upon hybridization using both gold and platinum electrodes [21,50]. The effect of different silane chemistries creates differences in the hydrophobicity and hydration levels of the modified surface. The organosilane treatment along with the ssDNA... 

Chemical Microsensors Based on Surface Impedance Changes... 

Figure 10.6 A waveguide section between two partial sections, a) Physical picture indicating traveling waves in a continuous medium whose wave impedance changes from R0 to Ri to R2. b) Digital simulation diagram for the same situation. The section propagation delay is denoted asz- T. The behavior at an impedance discontinuity is characterized by a lossless splitting of an incoming wave into transmitted and reflected components.

Yamamoto and Yamamoto [8,9] showed that not only is the skin s impedance profile quite complex, but, in addition, it was demonstrated that the skin s impedance changes in a very complex manner. In particular, the skin s impedance depends on the season, the time of day at which the impedance is determined, the state of the subject studied, the site at which the impedance is measured, and the electrode paste used to make the impedance measurements. Other investigators have confirmed and amplified on these observations [10-12,14,15,17]. For example DeNuzzio and Berner [12] demonstrated the important influence that the type of electrolyte present around the electrodes have on the skin s impedance Clar et al. [10] showed that the time variation occurred not only during the day but from day to day and Panescu et al. [14] demonstrated that the average impedance of the forearm was less than that for the palm. 

Skin tissues isolated from rats, rabbits, and humans have been monitored in vitro to predict penetration and irritation. The rat epidermal slice technique has been validated as a screen for corrosive substances. The electrical impedance changes as the integrity of the stratum corneum is altered. The use of this technique to predict irritancy is being investigated in the United Kingdom. Another method studies enzyme changes when a substance is applied to a slice whose lower surface is bathed in culture medium. Enzyme changes separate irritant and nonirritant chemicals. 

Figure 2-9 Typical EIS device. Auxiliary, working, and reference electrodes are shown in gold, bine, and green, respectively. In a solntion (gray transparent cube), a redox compound snch as Fe(CN)g will complete the circnit between the auxiliary and working electrodes, bnt upon application of a protein that wonld bind to a SM immobilized on the working electrode, the access of the redox solnte may be severely limited, resnlting in impedance changes. (See insert for color representation of figure.)...

On the other hand, the impedance changes with the battery age and this can be used as an SoH indicator. According to Feder et al. [10], conductance is in good correlation with available capacity. Therefore, if the SoC is known by another method, the conductance can be used for SoH determination. 

This complex relationship means that if the cavity is held at resonance and the spectral line is swept, e.g. by Stark or Zeeman modulation, although other modulation schemes are possible, the cavity impedance changes and the reflected power incident on the Gunn device changes in sympathy with the spectral scan. This causes a current to flow in the Gunn oscillator circuit related to the spectral absorption profile, and therefore to its amplitude and area. That current can be readily transformer-coupled out of the Gunn bias circuit and detected S)mchro-nously with the modulation frequency. 

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