Power comparator based on-site calibration of isolating current transformers

A portable system for on-site calibrations of isolating current transformers in stationary energy meter test systems is described. The uncertainties of the calibration system are in the order of 0.005 % for the ratio error and 0.01 crad for the phase displacement of the current transformer at power frequencies. The power comparator based measurements can be done at test currents from 50 mA to 120 A.     

Backside calibration chronopotentiometry: using current to perform ion measurements by zeroing the transmembrane ion flux

A recent new direction in ion-selective electrode (ISE) research utilizes a stir effect to indicate the disappearance of an ion concentration gradient across a thin ion-selective membrane. This zeroing experiment allows one to evaluate the equilibrium relationship between front and backside solutions contacting the membrane by varying the backside solution composition. This method is attractive since the absolute potential during the measurement is not required, thus avoiding standard recalibrations from the sample solution and a careful control of the reference electrode potential. We report here on a new concept to alleviate the need to continuously vary the composition of the backside solution. Instead, transmembrane ion fluxes are counterbalanced at an imposed critical current. A theoretical model illustrates the relationship between the magnitude of this critical current and the concentration of analyte and countertransporting ions and is found to correspond well with experimental results. The approach is demonstrated with lead(II)-selective membranes and protons as dominating interference ions, and the concentration of Pb(2+) was successfully measured in tap water samples. The principle was further evaluated with calcium-selective membranes and magnesium as counterdiffusing species, with good results. Advantages and limitations arising from the kinetic nature of the perturbation technique are discussed.     

Comparison of the quantized Hall resistance and ohm NRC

A report is given of the progress towards the establishment of a quantized Hall resistance (QHR) measurement system suitable for maintaining the NRC (National Research Center of Canada) representation of the ohm. A system using a cryogenic current comparator bridge is described and compared to the previously reported 15 T, 20-mK potentiometric system. General problems concerning the use of the quantized Hall resistance to realize a representation of the ohm are discussed     

Accuracy of Raman lidar water vapor calibration and its applicability to long-term measurements

A Raman lidar calibration method adapted to the long-term monitoring of atmospheric water vapor is proposed. The accuracy of Raman lidar water vapor profiles is limited by that of the calibration process. Typically, calibration using in situ balloon-borne measurements suffers from the nonsimultaneity and noncollocation of the lidar and in situ measurements, while calibration from passive remote sensors suffers from the lower accuracy of the retrievals and incomplete sampling of the water vapor column observed by lidar. We propose a new hybrid calibration method using a combination of absolute calibration from radiosonde campaigns and routine-basis (off-campaign) partial calibration using a standard lamp. This new method takes advantage of the stability of traceable calibrated lamps as reliable sources of known spectral irradiance combined with the best available in situ measurements. An integrated approach is formulated, which can be used for the future long-term monitoring of water vapor by Raman lidars within the international Network for the Detection of Atmospheric Composition Change and other networks.     

Examination of precipitation in the aluminum alloy AA6111 using electrical resistivity measurements

The evolution of electrical resistivity during the aging of aluminum alloy AA6111 has been measured over a wide range of annealing temperatures. From these measurements, the solubility product for the Q phase in aluminum has been proposed and using a new approach, the contribution of precipitates to the overall resistivity is quantified.     

An AC-bridge for low-frequency measurements of the quantized Hallresistance

A simple circuit which makes possible the use of a resistance-ratio bridge based on a cryogenic current comparator (CCC) with both AC and DC is described. The different sources of uncertainty associated with the use of AC in a CCC bridge are discussed. It is shown that they should have an effect which does not exceed a few parts in 10 ⁹ of the resistance-ratio being measured, if the frequency is limited to a few hertz. This analysis is confirmed by experimental results of resistance-ratio measurements between the quantized Hall resistance (QHR) and a 100 Ω resistance standard carried out at DC, 1, 2, and 4 Hz. These measurements are, to the author's knowledge, the first accurate DC measurements of the QHR. They demonstrate that the quantization of the Hall resistance, observed with AC and for the frequency range studied here, remains complete to within a few parts in 10⁹ or better     

Determination of the time-dependence of ΩNBS usingthe quantized Hall resistance

The quantum Hall effect is being used to monitor the US legal representation of the ohm, or as-maintained ohm, Ω_NBS. Measurements have been made on a regular basis since August 1983. Individual transfers between the quantized Hall resistance R _H and the five 1-Ω resistors which comprise Ω_NBS can be made with a total of one standard deviation (1σ) uncertainty of ±0.014 p.p.m. This uncertainty is the root-sum-square of 32 individual components. The time-dependent expression for R_H in terms of Ω_NBS is: R_H=25812.8[1+(1.842±0.012)×10^-6 =(0.0529±0.0040)(t-0.7785)×10^-6 /year] Ω_NBS, where t is measured in years from January 1, 1987. The value of Ω_NBS is, therefore, decreasing at the rate of (0.0529±0.0040) p.p.m./year     

Comments on "Accuracy of Raman lidar water vapor calibration and its applicability to long-term measurements"

In a recent publication, Leblanc and McDermid [Appl. Opt., 47, 5592 (2008)]APOPAI0003-693510.1364/AO.47.005592 proposed a hybrid calibration technique for Raman water vapor lidar involving a tungsten lamp and radiosondes. Measurements made with the lidar telescope viewing the calibration lamp were used to stabilize the lidar calibration determined by comparison with radiosonde. The technique provided a significantly more stable calibration constant than radiosondes used alone. The technique involves the use of a calibration lamp in a fixed position in front of the lidar receiver aperture. We examine this configuration and find that such a configuration likely does not properly sample the full lidar system optical efficiency. While the technique is a useful addition to the use of radiosondes alone for lidar calibration, it is important to understand the scenarios under which it will not provide an accurate quantification of system optical efficiency changes. We offer examples of these scenarios. Scanning of the full telescope aperture with the calibration lamp can circumvent most of these limitations. Based on the work done to date, it seems likely that the use of multiple calibration lamps in different fixed positions in front of the telescope may provide sufficient redundancy for long-term calibration needs. Further full-aperture scanning experiments, performed over an extended period of time, are needed to determine a "best practice" for the use of multiple calibration lamps in the hybrid technique.     

Direct resistance comparisons from the QHR to100 M/spl Omega/ using a cryogenic current comparator

Measurements of room-temperature 100 M/spl Omega/ standard resistors and cryogenic thin-film resistors based directly on a quantized Hall resistance standard have been made with a cryogenic current comparator (CCC) bridge. This 15 496:2 ratio CCC attains a current sensitivity of 10.7 fA/Hz/sup 1/2/ in measurements of cryogenic thin-film resistors, without extensive shielding or filtering. A resistive primary winding helps the CCC maintain stability in the presence of external noise. The resistive-winding technique may be useful for the absolute measurement of small currents delivered by single-electron tunneling devices.     

Oscillations of the Hall current in a Hall thruster with an anode layer

Burning of an intense discharge in crossed electrical and magnetic fields in a Hall electrojet engine (Hall thruster) is considered. An engine of this type is an azimuthally symmetric device in which the discharge burns in an annular channel between the poles of the magnetic circuit. The anode is usually made in the form of a cavity through which the working gas is supplied (thruster with anode layer, TAL) or in the form of a planar ring mounted in a dielectric channel in a weak magnetic field (stationary plasma thruster, SPT), and the role of the cathode is played by the plasma surrounding the engine. The electrons trapped in the magnetic field between the poles of the magnetic circuit oscillate in the electric field between the anodic and cathodic regions of the discharge, forming the Hall current closed in the azimuthal direction. It is known from practice that discharge of this type is always nonstationary and the main self-sustained oscillations are excited on the atomic time-of-flight frequency (“accelerative” regime). In addition, there exists a region of parameters in which the self-sustained oscillations become stochastic and the discharge current increases sharply (“stochastic” regime). This work is devoted to the induction measurements of the oscillations of the Hall current arising in various regimes of operation.     

Leaching of conductive species: Implications to measurements of electrical resistivity

Electrical tests have been used to characterize the microstructure of porous materials, the measured electrical response being determined by the contribution of the microstructure (porosity and tortuosity) and the electrical properties of the solution (conductivity of the pore solution) inside the pores of the material. This study has shown how differences in concentration between the pore solution (i.e., the solution in the pores) and the storage solution surrounding the test specimen leads to significant transport (leaching) of the conductive ionic species between the pore solution and the storage solution. Leaching influences the resistivity of the pore solution, thereby influencing electrical measurements on the bulk material from either a surface or uniaxial bulk resistance test. This paper has three main conclusions: 1.) Leaching of conductive species does occur with concentration gradients and that a diffusion based approach can be used to estimate the time scale associated with this change. 2.) Leaching of ions in the pore solution can influence resistivity measurements, and the ratio of surface to uniaxial resistivity can be used as a method to assess the presence of leaching and 3.) An estimation of the magnitude of leaching for standardized tests of cementitious materials.     

In situ measurements of the resistivity of Antarctic sea ice

The resistivity of first yea sea ice was measured in situ at two locations in McMurdo Sound, Antarctica using the Wenner array technique at audio frequencies. In addition, salinity and temperature profiles were measured. The results are adequately described by a three-layer model made up of a thin conducting surface layer, an insulating layer and finally sea water. The average resistivity of sea ice was found to lie in the range 50–200 Ω depending on salinity, structure and temperature. The resistivity and thicknesses of the surface layer could not be determined uniquely by the model but a maximum value for the resistivity as low as 4 Ω m was obtained. The resistivity of the surface layer was found to be influenced by the removal of the snow cover. The depth predicted by the Wenner sounding was found to be roughly 50% of the actual depth, a result that is consistent with a conductivity in the vertical direction and parallel to the brine channels of four times the conductivity in the horizontal direction within the bulk layer.