A Comparison of Multiple Frequency and Pulsed Eddy Current Techniques

In principle, the same information should be obtainable from. either pulsed or multiple frequency eddy current techniques, provided they utilize comparable frequency ranges. In practice, there are important differences and advantages for each method. Pulse instrumentation is generally cheaper, simpler, and less sophisticated. On the other hand, there has been greater development of theory and instrumentation using sinusoidal eddy currents, so that the equipment is generally more quantitative at present. The basic problem of determining certain paramenters when others may also be varying can be solved by measuring enough quantities to eliminate the unwanted variables, for example, by measuring the pulse response at various time delays or the sinusoidal response at various frequencies. In practice, the number of useful frequencies is strictly limited. Little additional information is obtainable from frequencies for which the skin depth is much greater or much less than the thickness of the sample. Since the frequencies must be spaced to. permit separation by filters, this puts a practical limit of about four on the number of frequencies useful for a given problem. This is not a serious limitation, since one can measure two quantities for each frequency and the total number of pertinent parameters rarely exceeds six. Pulse equipment can more readily handle a wide range of frequencies, but the instrumentation tends to become more elaborate, especially if high frequencies are needed for a particular application, and the repetition rate becomes low if low frequencies are necessary. The reproducibility of pulses is a problem which can be circumvented by the use of bridge techniques, differential coils and other standard techniques. New computer programs and microprocessor equipment have been developed which now make it possible to set up tests and measure parameters directly and precisely without the lengthy optimization calculations once necessary, though the latter will continue to be useful for the design of optimized coils and experiments

Design of a Temperature-Compensated Induction Extensometer

By proper choice of materials, dimensions and circuit parameters, it is possible to design a linear displacement transducer, or extensometer, to have zero net thermal drift over any given temperature range. The chief limitation is the inability of wires and insulation to withstand very high temperatures. An extensometer has been designed and tested which could theoretically measure displacements up to 150 mm with a maximum error of ±0.15 mm caused by thermal effects over the temperature range from 0° to 1000°C. Experimental limitations prevented testing at temperatures higher than 500°C, but measured and theoretical results were in good agreement over that range. The principles involved in the temperature compensation will be discussed

A Pulsed Eddy Current Method for Examining Thin-Walled Stainless Steel Tubing

A bellows is fabricated from a 12-in. section of type 321 or type 216 stainless steel tubing. In order to ensure that the bellows will survive the rigors of the production environment, it is essential that the tubing be free of all “scratch like” defects. A feasibility study was conducted to determine if an eddy current method could be developed to nondestructively examine this tubing

A Determination of the Wave Forms and Laws of Propagation and Dissipation of Ballistic Shock Waves

Experiments to ascertain the wave forms and laws of propagation and dissipation of ballistic shock waves to large distances (80 yards) from the bullet trajectory are described. Calibers 0.30, 0.50, 20 mm, and 40 mm were studied. In every case an N‐shaped wave profile was observed consisting of a sudden rise in pressure, the “head discontinuity,” followed by an approximately linear decline to a pressure about equally far below atmospheric and then a second sudden return, the “tail discontinuity,” to atmospheric pressure. The peak amplitudes of this disturbance are found to diminish about as the inverse 3/4 power of the miss‐distance (perpendicular distance from the trajectory) while the period T′ (measured between the discontinuous fronts) increases about as the 1/4 power of the miss‐distance for calibers 0.30, 0.50, and 20 mm. For 40‐mm shells the amplitude decays a little faster, about as the inverse 0.9 power of miss‐distance over the range studied. A theory taking account of the dissipation of the N‐wave energy into heat is developed to explain the observed behavior. A method of measuring absolute N‐wave amplitudes by observing the rate of change of period T′ with propagation is described. The theory leads to an absolute relationship at large distances between distance, amplitude, and period in which no arbitrary constants appear

THE STRUCTURE OF EMPLOYMENT AND UNEMPLOYMENT IN A DECLINING RURAL COMMUNITY

Labor and Human Capital,

Scaling relations for eddy current phenomena

Formulas are given for various electromagnetic quantities for coils in the presence of conductors, with the scaling parameters factored out so that small-scale model experiments can be related to large-scale apparatus. Particular emphasis is given to such quantities as eddy current heating, forces, power, and induced magnetic fields. For axially symmetric problems, closed-form integrals are available for the vector potential and all the other quantities obtainable from it. For unsymmetrical problems, a three-dimensional relaxation program can be used to obtain the vector potential and then the derivable quantities. Data on experimental measurements are given to verify the validity of the scaling laws for forces, inductances, and impedances. Indirectly these also support the validity of the scaling of the vector potential and all of the other quantities obtained from it. (auth

Scaling relations for eddy current phenomena

Formulas are given for various electromagnetic quantities for coils in the presence of conductors, with the scaling parameters factored out so that small-scale model experiments can be related to large-scale apparatus. Particular emphasis is given to such quantities as eddy current heating, forces, power, and induced magnetic fields. For axially symmetric problems, closed-form integrals are available for the vector potential and all the other quantities obtainable from it. For unsymmetrical problems, a three-dimensional relaxation program can be used to obtain the vector potential and then the derivable quantities. Data on experimental measurements are given to verify the validity of the scaling laws for forces, inductances, and impedances. Indirectly these also support the validity of the scaling of the vector potential and all of the other quantities obtained from it. (auth

Dihydrodinophysistoxin-1 Produced by Dinophysis norvegica in the Gulf of Maine, USA and Its Accumulation in Shellfish

Dihydrodinophysistoxin-1 (dihydro-DTX1, (M-H)−m/z 819.5), described previously from a marine sponge but never identified as to its biological source or described in shellfish, was detected in multiple species of commercial shellfish collected from the central coast of the Gulf of Maine, USA in 2016 and in 2018 during blooms of the dinoflagellate Dinophysis norvegica. Toxin screening by protein phosphatase inhibition (PPIA) first detected the presence of diarrhetic shellfish poisoning-like bioactivity; however, confirmatory analysis using liquid chromatography-tandem mass spectrometry (LC-MS/MS) failed to detect okadaic acid (OA, (M-H)−m/z 803.5), dinophysistoxin-1 (DTX1, (M-H)−m/z 817.5), or dinophysistoxin-2 (DTX2, (M-H)−m/z 803.5) in samples collected during the bloom. Bioactivity-guided fractionation followed by liquid chromatography-high resolution mass spectrometry (LC-HRMS) tentatively identified dihydro-DTX1 in the PPIA active fraction. LC-MS/MS measurements showed an absence of OA, DTX1, and DTX2, but confirmed the presence of dihydro-DTX1 in shellfish during blooms of D. norvegica in both years, with results correlating well with PPIA testing. Two laboratory cultures of D. norvegica isolated from the 2018 bloom were found to produce dihydro-DTX1 as the sole DSP toxin, confirming the source of this compound in shellfish. Estimated concentrations of dihydro-DTX1 were \u3e0.16 ppm in multiple shellfish species (max. 1.1 ppm) during the blooms in 2016 and 2018. Assuming an equivalent potency and molar response to DTX1, the authority initiated precautionary shellfish harvesting closures in both years. To date, no illnesses have been associated with the presence of dihydro-DTX1 in shellfish in the Gulf of Maine region and studies are underway to determine the potency of this new toxin relative to the currently regulated DSP toxins in order to develop appropriate management guidance