Variation Of Electrical Impedance Research Articles

Electrical impedance variation with water saturation in rock

We measured the electrical impedances of 22 sandstone samples during oil‐driving‐water tests using the two‐electrode method. Experiments show that the imaginary part X of the impedance (R + iX) of rock may respond well to water saturation in the frequency range 100 Hz–15 MHz. We found that the maximum −X values and their corresponding interfacial polarization frequencies are linear with water saturation. The lower critical frequency is found to vary with water saturation, in an unclear relationship with the characteristic length of rock. The dissipation factor at the interfacial polarization frequency remains quite stable and may be an indicator of the pore structure of rock. We used an equivalent circuit to explain the dispersive behavior of rock. More than one interfacial polarization frequency in the impedance Argand plot is predicted and can be observed if the measurement frequency range is wide enough.

THE LOOP TECHNIQUE FOR STRAIN GAGE ROSETTE SIGNAL CONDITIONING

Background The classical approach for signal conditioning individual resistance strain gages is to connect each installed gage with three lead wires to a set of three fixed completion resistors, all arranged electrically in a Wheatstone bridge circuit. An alternative approach for strain gage signal conditioning, sometimes called the Anderson loop, was developed at NASA's Dryden Flight Research Center. This technique offers some significant advantages when used with the strain gage rosettes employed in experimental stress analysis. Invented to eliminate the measurement uncertainties due to random electrical impedance variations in lead wires and connections found in harsh test environments, NASA's loop circuit approach delivers twice the equivalent bridge output level, and simplifies wiring, temperature compensation and data analysis as well. Circuitry The enabling technology for this technique is the dual-differential subtractor measurement circuit function defined in Fig. 1. As illustrated in eq. 1, a subtractor's output is the difference between two independent floating differential inputs.

Measurement of tidal volume by using transthoracic impedance variations in rats.

The application of impedance pneumography for monitoring respiration in small animals has been limited by problems with calibration. With improved instrumentation, we describe the calibration of tidal volume in anesthetized rats. The detection of changes in voltage, reflecting the electrical impedance variations associated with respiration, was optimized by using disposable adhesive silver-silver chloride electrodes, advanced circuitry, and analog-to-digital recording instrumentation. We found a linear relationship between change in impedance and tidal volume in individual rats (R2 >/= 98%), which was strongly influenced by rat weight. Consequently, a calibration equation incorporating change in impedance and rat weight was derived to predict tidal volume. Comparison of the predicted and true tidal volumes revealed a mean R2 >/= 98%, slopes of approximately 1, intercepts of approximately 0, and bias of approximately 0.07 ml. The predicted volumes were not significantly affected by either frequency of respiration or pulmonary edema. We conclude that impedance pneumography provides a valuable tool for the noninvasive measurement of tidal volume in anesthetized rats.

Bladder volume measurement with electrical impedance analysis in spinal cord-injured patients.

The purpose of the study contained herein was to determine the usefulness of electrical impedance for measurement of bladder volume in spinal cord-injured patients, with an assessment of the relationship between electrical impedance and bladder volume exclusively. The study was performed during urodynamic studies to match the exact bladder volume. Thirteen patients with complete spinal cord injuries were recruited. We used silver-silver chloride compound electrodes composed of one pair of current and amplitude electrodes to minimize the influence of superficial skin impedance. Each compound electrode was attached on the lower abdomen bilaterally after skin cleansing. Constant current (60 kHz-1.0 mA), converted from 9 V of direct current, was applied, and corresponding electrical impedance (omega) was measured at "pre" (before urodynamic empty bladder), "full" (with a urodynamic filled bladder), and "post" (after urodynamic empty bladder) status. Electrical impedance at the full status was definitely lower than that at the pre and post statuses in all subjects, with a statistically significant difference (P < 0. 001). The correlation between electrical impedance and bladder volume was negative (r = -0.7988), and the fact of how much the variation in electrical impedance could be explained by variation in bladder volume was estimated (r2 = 0.6381). From these findings, we have determined that the electrical impedance analysis technique can be an alternative measure of bladder volume indirectly.

Origin of cardiac-related thoracic electrical impedance variations in lambs

Cardiac-related deflections in thoracic electrical impedance have been thought to correlate sufficiently well with cardiac stroke volume to be used as the basis for a noninvasive estimation of cardiac output. To determine more precisely the physiological origin of the impedance deflection (DZ), we regarded right ventricular stroke volume (SVa) as the sum of two components: 1) that part of SVa responsible for the transient increment in pulmonary blood volume within a cardiac cycle, SVa-v and 2) the remaining part of SVa, (SVa-SVa-v). SVa-v was measured in lambs by integration of the difference between pulmonary arterial and pulmonary venous flow. SVa and its components were varied experimentally by opening and closing an aorticocaval shunt or by inflating and deflating a cuff implanted around the pulmonary artery. DZ was measured using a tetrapolar disk electrode system. Multivariate linear regression analysis revealed that SVa-v had a significant positive effect on DZ, and, at the same time, (SVa-SVa-v) had a significant negative effect on DZ. In the pulmonary artery occluder model, the positive effect of SVa-v dominated the opposing negative effect of (SVa - SVa-v) so that the net effect of SVa on DZ was positive and significant. In the aorticocaval shunt model, these effects opposed each other to the extent that there was no significant correlation between SVa and DZ. These results shed new light on the physiological origin of DZ. They also demonstrate that use of DZ to measure acute changes in cardiac output may yield misleading results. Changes or the lack of changes in thoracic electrical impedance do not necessarily reflect cardiac output status.

Electrical impedance changes and water content in O/W emulsions during evaporation

Synopsis A method for measuring the variation of electrical impedance (Z) of thin layers (0.1 mm) of O/W emulsions during evaporation in a controlled environment is presented. The evaporation of an emulsion base (A) either as such and containing 1% glycerin (B) or 1% sodium pyrrolidonecarboxylate (C) was investigated for 120 min at constant air temperature (25 degrees C) and relative humidity (55%). The measured impedance of the emulsion layers increased by three orders of magnitude during the evaporation cycles. The relationship between impedance and water content of the evaporating emulsion layers was investigated. Each emulsion showed a complex linear relationship between log Z and water content, the relationship being represented by two (three, for emulsion A) linear portions in the graph. Plots of water content vs. time, temperature vs. time and rate of water loss vs. water content of the emulsion layers are presented. The data appear to indicate that evaporation proceeds via distinct stages, whose correspondence to different mechanisms of water evaporation is discussed. The potentiality of the present method for water analysis, and for investigating structural changes in emulsions during evaporation, is also discussed.

A new method for determining the blood quantity in the eye in a unit of time.

The principles of rheography with "infinite time constant" are utilized. After applying a suction cup electrode mechanism, by gradual increases of the vacuum, ocular pressure rises above venous pressure, and without modification of arterial inflow, the venous outflow is blocked; in consequence an increase of blood accumulation in the interior of the eye occurs. The electrical impedance variation, as a result of the latter, gives the recorded tracing a "slow" rising curve, that maintains the same pulse morphology. After being analyzed and elaborated on with certain mathematical formulae, this curve makes it possible to calculate the blood flow in the eye in a unit of time. Statistics on normal patients of various ages are reported.

Transthoracic electrical impedance variations associated with respiration.

Transthoracic electrical impedance variations associated with respiration.

Variations of transthoracic electrical impedance in relation with hemodynamic changes of pulmonary circulation.

Applying electrodes on bilateral chest walls, variations of electrical impedance were recorded during breath holding under various manipulations of the pulmonary hemodynamics with dogs. Pulsatile variations of the impedance recorded either directly from lobe surfaces or chest walls were in constant coupling with ECG and pressure waves in the pulmonary artery. These impedance pulses and their first differential derivatives decreased their height and area when the pulmonary blood flow was decreased, and increased those when the flow was increased. These changes disappeared when the manipulations were released and they returned to the original shapes. These results suggest that the impedance measurement on the chest wall gives informations concerning changes of pulmonary circulatory dynamics and can possibly be employed as a non-traumatic and continuous monitoring techninque for the pulmonary blood flow both experimentally and clinically.

Physiological identification of variation sources of transthoracic electrical impedance during breath holding.

Variations of transthoracic electrical impedance and their relations with physiological events within the thorax were studied by using electrodes applying constant sinusoidal current at 55kHz on bilateral chest walls of anesthetized dogs during breath holding. The level of the impedance lowered during decrease of oxygen and elevated during ether gas evaporation in the lungs with steady pulmonary blood flow. The impedance decreased when concentrated saline was selectively infused into the pulmonary vasculature but not into the left heart and the great vessels. The impedance levels also were related inversely with the change of blood flow and resultant blood volume in the lung when the pulmonary blood flow was decreased as during occlusion of the venous return to the right heart, pulmonary trunk and unilateral pulmonary artery. The impedance levels on the bilateral chest walls varied independently by manipulations mentioned above which was employed in the individual lung of each hemithorax. These results clearly suggest that the current flux applied on the chest wall well penetrates through the chest cage and passes preferentially through the lung parenchyma, the conductivity of which depends on the changes in the amount of electrically conductive or resistive materials associated with physiological events within the lungs.

Motional Impedance Analysis Applied to a Dynamic Microphone

The receiver analysis developed by R. D. Fay gives a method for calculating the complete performance of an electromagnetic speaker from a plot of electrical impedance measured when the speaker is loaded by a closed tube of variable length. This of course includes the performance of the speaker as a microphone, although the statement is not explicitly made. The present article applies the above analysis to a dynamic microphone and compares the sensitivity thus obtained with an independent calibration at lower audiofrequencies. This method yields a primary calibration, in that neither calibrated source nor comparison microphone is required. The high mechanical resistance of a dynamic microphone and its low coupling constant, as compared with an electromagnetic speaker, result in a much smaller variation in electrical impedance, with changing air load. The variation is of the order of 5 percent of the minimum impedance, as compared with perhaps 80 percent change in speaker impedance. The cross-sectional area of the terminating tube is found to be of considerable importance. The effect of this choice of tube area on over-all accuracy is discussed.