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John A Pearce

Publications and source records attributed to John A Pearce.

7 recordsLinked to original sources
PubMed2005

Microwave radiometry for continuous non-contact temperature measurements during microwave heating.

Temperature measurement during microwave heating in industrial and commercial processes can improve quality, throughput, and energy conservation. Conventional ways of measuring temperature inside a microwave oven cavity are costly, inconvenient, or unsuitable for high-volume industrial applications. In this paper, we describe the theory of microwave radiometry as applied to the measurement of temperature during microwave heating. By extending the theory of radiative transfer to the case of thermal microwave radiation inside a cavity, we show that the same characteristics which make a microwave cavity suitable for heating materials also assist in obtaining meaningful temperature data with microwave radiometry. We present experimental data from the heating of liquid and solid materials which confirm the essential features of the theory, and show agreement between this method and more conventional methods of +/-4 degrees C.

Computer Simulation↗
PubMed2005

Nonlinear conductance-volume relationship for murine conductance catheter measurement system.

The conductance catheter system is a tool to determine instantaneous left ventricular volume in vivo by converting measured conductance to volume. The currently adopted conductance-to-volume conversion equation was proposed by Baan, and the accuracy of this equation is limited by the assumption of a linear conductance-volume relationship. The electric field generated by a conductance catheter is nonuniform, which results in a nonlinear relationship between conductance and volume. This paper investigates this nonlinear relationship and proposes a new nonlinear conductance-to-volume conversion equation. The proposed nonlinear equation uses a single empirically determined calibration coefficient, derived from independently measured stroke volume. In vitro experiments and numerical model simulations were performed to verify and validate the proposed equation.

Animals↗
PubMed2004

Electrical properties of soils at ism radio frequencies, from 1 to 40 MHz.

Soil electrical properties, conductivity and permittivity, vary with moisture content and temperature. The goal of the work was to generate working correlative functions of the electrical properties of soil over practical ISM radio fJequencies (6.78 to 40.68 MHz) from the Kirtland Air Force Base RF soil remediation project, and its constituent soil types, to facilitate numerical model studies. Correlates were developed to facilitate numerical model temperature and moisture dependence of the electrical properties. Measurements were made in continuous spectra from 1 to 40 MHz for varying moisture content and temperatures of sand, kaolinite, Austin landfill clay, sandy loam and Kirtland Air Force Base soil samples. The measurements were conducted in a 1 5/8" coaxial sample holder with flush mounted center conductor. Impedance data were collected with a Hewlett-Packard 4194A Impedance Analyzer from which calculations of permittivity were made. In general, the relative permittivity and effective loss factor are sensitive functions of water content and temperature.

Electric Impedance↗
PubMed2004

Electric field penetration depth of myocardial surface catheters and the measurement of myocardial resistivity.

The in vivo measurement of cardiac conductance in mice offers a method to generate an instantaneous left ventricular volume signal. In order to translate the measured conductance to volume, it is necessary to determine the resistivity of the myocardial tissue. This is done using tetrapolar surface conductance catheters, placed on the surface of the left ventricle. It is important to determine the depth of penetration of the electric field of this surface catheter to ensure that the electric field is confined within the myocardium and does not extend into the left ventricle blood volume. The depth of penetration is experimentally determined by a technique described by Foster et al. [2].

Cardiography, Impedance↗
PubMed2004

Design of instrumentation and data-acquisition system for complex admittance measurement.

Instantaneous left ventricular volume measurements have been made for many years using a tetrapolar conductance catheter. The main objective is to determine the efficiency of the beating heart, using a tetrapolar catheter inserted in the left ventricle of transgenic mice. The effect of the parallel myocardium contribution must be removed from the total measurement. A dual-frequency technique involving 1 kHz and 100 kHz was chosen because it has been established that the imaginary part (the capacitive reactance) of the complex admittance of the cardiac muscle is much smaller in the lower frequency than at the higher frequency. The design involves generation of an accurate frequency source for both the frequencies careful selection of operational amplifiers for the current conversion stage so that the current is not too large to kill the mouse and that it is capable of performing at high frequencies. The band pass filter stage involved careful design with minimal overlap of the pass bands of both the channels. The overall circuit was designed so that there is minimal shift in the phase due to the circuit elements alone. Work also involved design of GPIB--based data acquisition system using LabVIEW and a digital oscilloscope for effective data acquisition even at high frequencies, which are normally limited by the sampling frequency. This data acquisition system is currently being used in laboratory studies in vivo.

Animals↗
PubMed2002

Dynamic impedance measurements during radio-frequency heating of cornea.

Hyperopia affects approximately 25% of the population. The aim of different heating modalities for the treatment of hyperopia is to steepen the central curvature of the cornea. Conductive keratoplasty (CK) involves the placement of radio-frequency (RF) lesions around a 7-mm-diameter ring concentric with the pupil of the eye. Dynamics of lesion formation during CK depend on corneal electrical impedance, which is expected to change during each 600-ms-long macropulse. The purpose of this study was to measure impedance dynamics during CK. RF lesions were made in in vitro porcine eyes at different power settings. Voltage and current measurements were acquired using a high-speed computer-based data acquisition system. Root-mean-square voltages (VRMS) and curre (IRMS) were calculated for each micropulse, and impedance was determined by calculating the quotient VRMS/IRMS. Initial corneal impedance in vitro was approximately 2000 ohms. During the macropulse, impedance decreased initially due to increased mobility of conductive ions. At higher power settings (e.g., > 70%, or maximum peak-to-peak voltage of 233 V), impedance increased after the initial decrease, indicative of local water vaporization and/or tissue coagulation. Preliminary impedance data obtained for in vivo porcine eyes were similar in magnitude to the in vitro values.

Animals↗