NBS Provides Voltage Calibration Service in 0.1-10-Hz Range Using AC Voltmeter/Calibrator

Prompted by the need to support vibration and pressure measurements at frequencies down to 0.5 Hz (with expected future needs to 0.1 Hz), NBS now offers a calibration service for voltage standards and rms voltmeters in the range of 0.1-10 Hz. The means for the service is an "ac Voltmeter/Calibrator," an NBS-developed instrument containing an rms digital voltmeter and ac and dc voltage calibrators. The methods used to calibrate the ac voltage calibrator are discussed; also, application of the ac Voltmeter/Calibrator to the calibration of customers' voltage and voltmeter standards is described. Finally, a multifrequency voltage reference source with frequency-independent amplitude is proposed as a more suitable transfer standard than thermal voltage converters (TVC's) for the 0.1-10-Hz range.     

Evaluation of a voltage source with three calculable RMS outputs

This paper describes a voltage source which generates three different waveforms. Measurement techniques with such a source are proposed, with emphasis on the uncertainty of each waveform used, when the ac amplitude is defined with a dc calibration of the source. The instrument circuitry and operation is described. Measurement results are also discussed. Emphasis is given to selected parameters of waveforms and to their effect on the measured rms value     

Correction of Systematic Errors Due to the Voltage Leads in an AC Josephson Voltage Standard

The National Institute of Standards and Technology (NIST) has recently reported the first application of a quantum ac Josephson voltage standard for the calibration of thermal transfer standards in the 1- to 10-kHz frequency range. This paper describes preliminary work on extending its frequency calibration range up to 100 kHz by correcting the systematic errors due to voltage leads. A ground loop created by the dc blocks, which is a previously unaccounted source of high-frequency systematic error, has been identified, and its effects are partially mitigated.      

An Automatic System for AC/DC Calibration

An automatic ac/dc difference calibration system using direct measurement of thermoelement EMFs is described. The system operates over a frequency range from 20 Hz to 100 kHz, covering the voltage range from 0.5 V to 1 kV. For all voltages, the total uncertainty (including the uncertainty of the specific reference thermal converters used) is 50 ppm at frequencies from 20 Hz to 20 kHz, inclusive, and 100 ppm at higher frequencies up to 100 kHz. In addition to ac/dc difference testing, the system can be used to measure some important characteristics of thermoelements, as well as to calibrate ac voltage calibrators and precision voltmeters. Results of intercomparisons between the new system and the manual NBS calibration system, using single-range, coaxial-type, thermal voltage converters as transfer standards, are reported. The results indicate that the ac/dc differences measured are accurate to well within the combined total uncertainty limits of the two systems.     

An Automatic RMS/DC Comparator

The design of an instrument for the automatic comparison of an ac voltage with a stable dc source is described. A differential multijunction thermal converter is used as an rms/dc converter with an FET-switched input amplifier for ac/dc substitution. The output voltages of the rms/dc converter with ac and dc input voltages are sampled and stored, and the difference amplified and displayed on a panel meter or chart recorder. Accuracy is ±20 ppm of input ranges of 10-200 V at frequencies of 50 Hz-1 kHz, and maximum full scale deflection sensitivity is 0.01 percent of input range. The instrument may be used either as an rms comparator with a linear voltage scale or as a mean-square comparator with a linear power scale.     

An RMS Digital Voltmeter/Calibrator for Very-Low Frequencies

A portable rms digital voltmeter (DVM) has been developed at NBS to support vibration measurements over the ranges of 0.1 to 50 Hz and 2 mV to 10V. A self-contained calibrator provides for self-calibration and may be used for calibrating other VLF voltmeters. The calibrator basically consists of a Kelvin-Varley divider fed by a reference voltage (either dc or sinewave generated by a ROM-DAC combination) A multijunction thermal converter (MJTC) was selected as the sensing device in the rms/dc converter of the DVM since its low ac/dc difference facilitates calibration of the ac calibrator. Factors affecting accuracy and response time are analyzed. The DVM response time is 40 s for the lowest input frequency. Its accuracy (percent of reading) is 0.1 percent above 0.5 Hz and 5 mV and 0.2 percent below these values. The ac calibrator accuracy is 0.02 percent. Measurement accuracy improves by a factor of about 4 for transfer measurements (comparing input voltages with ac calibrator voltages) Means for extending this accuracy to 0.01 percent are discussed.     

The Calibration of dc Voltage Standards at NIST

This document describes the procedures used at NIST to calibrate dc voltage standards in terms of the NIST volt. Three calibration services are offered by the Electricity Division: Regular Calibration Service (RCS) of client standard cells at NIST; the Volt Transfer Program (VTP) a process to determine the difference between the NIST volt and the volt as maintained by a group of standard cells in a client laboratory; and the calibration of client solid-state dc voltage standards at NIST. The operational procedures used to compare these voltage standards to NIST voltage standards and to maintain the NIST volt via the ac Josephson effect are discussed.     

Electric Potential Measurements in Gaseous Insulating Materials by Corona Probe in a Controlled Environment

The experimental determination of electric potential in gaseous insulating materials is performed using a corona probe. This method can be used to measure point values of electric potential from dc to ac at industrial frequency values. The proposed method, after an initial calibration of the probe, allows the determination of the amplitude of the electric potential at the point in space where the tip of the probe is located. The measurement is performed by adjusting the feeding potential of the probe until a complete corona suppression is obtained. To discriminate the corona effect of the probe from other possible corona sources, a device for probe current detection is proposed. The corona probe method has been verified by measuring electric potential, under both dc and 50-Hz ac conditions, in known field configurations and in natural and controlled environments. Finally, the proposed method has been applied to determine the space distribution of the electric potential around high-voltage insulating structures under dc and 50-Hz voltages in a natural environment.     

Recent Advances in Precision AC Measurements

Development of a new monolithic thermal sensor, a unique high-impedance attenuator, a precision sample-and-hold circuit, and a novel ac-error-correction technique have brought significant improvements in the accuracy of ac measurements. A precision thermal truerms measurement system is described that offers a transfer stability of 25 parts/106 in the frequency band 40 Hz-kHz. A microprocessor-based system uses a nonvolatile memory for software calibration; this enhances dc, ac, and resistance measurement accuracy, including production and repair.     

Optimization of QHE-devices for metrological applications

In the framework of an European project aiming at the realization of a system for the calibration of capacitance standards based on the quantum Hall effect (QHE), optimized QHE devices for the metrological application as dc as well as ac standards of resistance are developed. The present paper describes the dc characterization of a large number of devices with different layouts, contact configurations, carrier concentrations, and mobilities. The results demonstrate the influence of the device parameters on the critical current, the width of the quantized plateaus, the longitudinal voltages along the device and the quantized Hall resistance. Recommendations are given for the layout and mobility of QHE devices in view of their use as dc standards of resistance     

Stability of some DC reference standards

Some 15 years ago, after 80 years of utilization of the Weston cell as voltage standard, commercial designs of sources with Zener diodes, the so-called “dc reference standards” appeared on the market. They have much more favorable properties compared to Weston Standard Cells. The stability of one commercial dc reference standard Fluke 732A at the 10 V output was analyzed in relation to the calibration data obtained in the Fluke, NIST, and PTB laboratories during a period of seven years. Two approaches to accessible data for regression line calculation were compared. In the first approach all the data were taken into consideration, whereas in the second approach only the mean values of particular calibration expressed for the mean date were taken into account and were calculated with weights. Both approaches show a very good agreement. The voltage of the analyzed source at the 10 V output changes for about 0.1×10^-6/year, which is several times less than the value given in the manufacturer's specifications     

A Current Source for Picoammeter Calibration

A current source, which is to be employed in the calibration of low-current meters (picoammeters and electrometers), is presented here. The output current range is 100 fA to 100 pA and is directly traceable to calibrated standards of dc voltage, capacitance, and time period. The source is based on a low- frequency (ap1 mHz) trapezoidal signal generator, which charges and discharges a gas-dielectric capacitor; the voltage is monitored with a voltmeter that is triggered by a precision time base. The source has been employed during March 2006 for the Italian participation to the supplementary comparison EUROMET.EM-S24 and will be part of an extension of the Italian national standard of dc current. However, being composed of low-cost electronics and common commercial instrumentation, the source can also find useful application in secondary calibration laboratories.     

Wavelength demodulation of ultrabright green light-emitting diodes for electrical current sensing

We report on a novel electrical current-sensing principle based on wavelength-encoded modulation of the ultrabright green (at 525 nm) light-emitting diode (LED) transducers. It complies with the optical subsystem of a hybrid current transformer . Real-time wavelength demodulation is performed with the passive spectral edge filter OG 530. Linear calibration plots were achieved with -0.33 nm/mA for dc and +0.99 mA/sup -1/ for ac current sensitivity, respectively. A measurement accuracy of 1.3% for 28.4-mA ac peak current range is achieved. A simple theoretical model is outlined. Issues such as electronic and thermal effects on stability performance are also addressed.     

Effective source term in the diffusion equation for photon transport in turbid media

The Green's function for the diffusion equation is widely used to describe photon transport in turbid media. We have performed aseries of spectroscopy experiments on a number of uniform turbid media with different optical properties (absorption coefficient in the range 0.03-0.14 cm(-1), reduced scattering coefficient in the range 5-22 cm(-1)). Our experiments have been conducted in the frequency domain, where the measured parameters are the dc intensity (I(dc)), ac amplitude (I(ac)), and phase (?) of the light intensity wave. In an infinite medium, the Green's function predicts a linear dependence of ln(rI(dc)) and ? on the source-detector separation r. Our measurements show that the intercepts of these straight lines predicted by the Green's function do not agree with the experimental results. To reproduce the experimental results, we have introduced an effective photon source whose spatial extent and source strength depend on the optical properties of the medium. This effective source term has no effect on the slopes of the straight lines predicted by the Green'sfunction at large values of r.     

AC and DC voltages from a Josephson arbitrary waveform synthesizer

We have synthesized and measured ac and dc voltages using a Josephson arbitrary waveform synthesizer. On-chip filtering has enabled the first practical operating margins for ac and arbitrary waveforms. Using a digital voltmeter, we demonstrate the operating margins and linearity of 101 synthesized dc voltages and confirm the flatness of a single voltage step. We present the first ac-dc and ac-ac voltage measurements of the synthesizer at 3.65 mV using an ac-dc thermal transfer standard. This quantum-based standard source can be used to measure the ac-dc difference of thermal transfer standards at small voltages     

A low-voltage and high-frequency ac-dc transfer system usinginductive dividers

A setup for the calibration of high-input-resistance ac-measuring instruments for low voltages (mV) and high frequencies (1 MHz) is presented. The low voltages generated down to one hundredth of an ac standard voltage (0.5 V to 2 V) are traced by means of a one-decade inductive voltage divider (IVD) cascaded by a sub-IVD with a fixed ratio of 10-to-1. Construction details of the main IVD which are necessary to meet the requirements are given. The error characteristics are mainly determined by capacitive currents acting on stray and mutual inductances. The internally built-in resistive dc voltage divider and a special switch allow calibrations to be performed via effective ac-dc transfer at the standard voltage level. The first results for the calibration of a transfer standard, Fluke 792A, at 100 mV and 200 mV were in good agreement with results obtained with very sensitive multijunction thermal converters. Using such converters as standards in the new system will enable ac calibrations down to voltages below 1 mV     

Automatic Self-Certification of a Computer-Controlled Calibration System

The theory, techniques, and apparatus that are used to perform an automatic self-certification of a computer-controlled calibration system are described. This process determines the linearity and bias deviations of the instrumentation of the system. These deviations are stored in the digital computer for subsequent use as corrections to the nominal instrument values when the system is applied to external devices. Dc measurements and stimuli, the uncertainties of which are of the order of 0.001 percent, have been achieved using this technique, exceeding the specifications normally associated with the uncorrected instrumentation. A similar reduction in uncertainty results from the self-certification process in ac voltage and resistance measurements. Using a 1-volt dc source, 1000-ohm resistor, and ac-dc thermal transfer as references, the scales of measure are automatically reestablished over a wide dynamic range in less than five minutes. Laboratory applications of this computer-controlled system have been made to classical metrology problems that include measurements of standard resistors, saturated standard cells, and resistance thermometers. Semi-automatic calibrations of test and measuring instruments have been performed to demonstrate potential productivity and accuracy.     

A simple resistance network for calibrating resistance bridges

This paper describes a simple, passive, low-cost resistance network, closely related to Hamon build-up resistors, that enables the calibration of dc and low-frequency ac resistance and conductance bridges. The network is configured so that the four component resistors can be connected to realize 35 distinct four-terminal resistances, all interrelated by the usual formulas for the series and parallel connections of resistors. Theoretical analysis and experimental results show that with due care in the design, the network can be readily constructed to achieve an accuracy of better than 1 μΩ for resistances of the order of 100 Ω (1:10⁸) for angular frequencies from dc to 10⁴ rad/s     

Attributes of direct current aperiodic and alternating current harmonic components derived from large amplitude fourier transformed voltammetry under microfluidic control in a channel electrode

The flow rate dependencies of the aperiodic direct current (dc) and fundamental to eighth alternating current (ac) harmonic components derived from large-amplitude Fourier transformed ac (FT-ac) voltammetry have been evaluated in a microfluidic flow cell containing a 25 μm gold microband electrode. For the oxidation of ferrocenemethanol ([FcMeOH]/[FcMeOH](+) process) in aqueous 0.1 M KNO(3) electrolyte, standard "Levich-like" dc behavior is observed for the aperiodic dc component, which enables the diffusion coefficient for FcMeOH to be obtained. In experimental studies, the first and second ac harmonic components contain contributions from the double layer capacitance current, thereby allowing details of the non-Faradaic current to be established. In contrast, the higher order harmonics and dc aperiodic component are essentially devoid of double layer capacitance contributions allowing the faradaic current dependence on flow rate to be studied. Significantly, flow rate independent data conforming to linear diffusion controlled theory are found in the sixth and higher ac harmonics at a frequency of 15 Hz and for all ac harmonics at a frequency of ≥ 90 Hz. Analysis of FT-ac voltammograms by theory based on stationary microband or planar electrode configurations confirms that stationary microband and planar electrode configurations and experimental data all converge for the higher order harmonics and establishes that the electrode kinetics are very fast (≥1 cms(-1)). The ability to locate, from a single experiment, a dc Faradaic component displaying Levich behavior, fundamental and second harmonics that contain details of the double layer capacitance, and Faradaic ac higher order harmonic currents that are devoid of capacitance, independent of the volume flow rate and also conform closely to mass transport by planar diffusion, provides enhanced flexibility in mass transport and electrode kinetic analysis and in understanding the performance of hydrodynamic electrochemical cells and reactors.     

The effect of alternating current on the plasma frequency of the tunnel Josephson junctions

The effect of alternating current on the plasma frequency of the tunnel Josephson junctions simultaneously carrying both direct (dc) and alternating (ac) currents has been studied. The presence of the ac component leads to a decrease in the plasma frequency. An analytical expression describing the plasma frequency as a function of the ac current amplitude is proposed. The obtained results agree with the recent experimental data on the properties of tunnel Josephson junctions.