Extension of the NIST AC-DC Difference Calibration Service for Current to 100 kHz

The NIST calibration service for ac-dc difference of thermal current converters relies on multijunction thermal converters as the primary standards, and various thermal converters and thermoelements (TEs) as the reference and working standards. Calibrations are performed by comparing the ac-dc difference of a customer’s thermal current converter to the ac-dc difference of a NIST standard current converter. Typical artifacts accepted for calibration include single-junction thermoelements, multijunction thermal converters, and transfer shunts for use with TEs. This paper describes the standards on which the calibration service is based and the results of the study to characterize the NIST standards over the extended frequency range from 50 kHz to 100 kHz at currents from 1 mA to 20 A. The general method for the frequency extension at high frequency involves the use of thermoelements in the 5 mA range, with small frequency dependence, as the starting point for build-up and build-down chains to cover the whole range from 1 mA to 20 A.     

NIST Sampling System for Accurate AC Waveform Parameter Measurements

This paper summarizes efforts at the National Institute of Standards and Technology (NIST) to develop a waveform sampling and digitizing system with accuracy comparable to that of an ac-dc thermal transfer standard for ac voltage measurements over the frequency range of 10 Hz to 1 MHz. In the frequency range from 1 kHz to 1 MHz, the sampler's gain flatness is better than that available from the best commercial digital multimeter. In ac-ac comparisons referenced to 1 kHz, the system agrees with a NIST-calibrated thermal transfer standard to within 17 muV/V from 20 Hz to 100 kHz for measurements made at both 1 and 0.1 V. The sampler's excellent dynamic linearity and flat input impedance are also discussed     

The Ampere and Electrical Standards

This paper describes some of the major contributions to metrology and physics made by the NIST Electricity Division, which has existed since 1901. It was one of the six original divisions of the National Bureau of Standards. The Electricity Division provides dc and low-frequency calibrations for industrial, scientific, and research organizations, and conducts research on topics related to electrical metrology and fundamental constants. The early work of the Electricity Division staff included the development of precision standards, such as Rosa and Thomas standard resistors and the ac-dc thermal converter. Research contributions helped define the early international system of measurement units and bring about the transition to absolute units based on fundamental principles and physical and dimensional measurements. NIST research has helped to develop and refine electrical standards using the quantum Hall effect and the Josephson effect, which are both based on quantum physics. Four projects covering a number of voltage and impedance measurements are described in detail. Several other areas of current research at NIST are described, including the use of the Internet for international compatibility in metrology, determination of the fine-structure and Planck constants, and construction of the electronic kilogram.     

A System for Accurate Direct and Alternating Voltage Measurements

A system for calibrating and using a group of dc and ac standards has been developed at NBS to meet increasing needs for greater accuracy in the measurement of direct and rms alternating voltages at audio frequencies. With a group of saturated cells, a universal ratio set as a potentiometer, a simplified self-calibrating volt box, and a new differential-thermocouple ac-dc comparator, an accuracy of 10 ppm can be obtained for direct voltage measurements and 40 ppm for rms alternating voltage measurements between 20 and 20 000 c/s, in terms of the direct volt and the ac-dc transfer standards maintained at NBS.     

Calibration of Voltage Transformers and High- Voltage Capacitors at NIST

The National Institute of Standards and Technology (NIST) calibration service for voltage transformers and high-voltage capacitors is described. The service for voltage transformers provides measurements of ratio correction factors and phase angles at primary voltages up to 170 kV and secondary voltages as low as 10 V at 60 Hz. Calibrations at frequencies from 50–400 Hz are available over a more limited voltage range. The service for high-voltage capacitors provides measurements of capacitance and dissipation factor at applied voltages ranging from 100 V to 170 kV at 60 Hz depending on the nominal capacitance. Calibrations over a reduced voltage range at other frequencies are also available. As in the case with voltage transformers, these voltage constraints are determined by the facilities at NIST.     

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     

Uncertainty in NIST Force Measurements

This paper focuses upon the uncertainty of force calibration measurements at the National Institute of Standards and Technology (NIST). The uncertainty of the realization of force for the national deadweight force standards at NIST is discussed, as well as the uncertainties associated with NIST’s voltage-ratio measuring instruments and with the characteristics of transducers being calibrated. The combined uncertainty is related to the uncertainty of dissemination for force transfer standards sent to NIST for calibration.     

An intercomparison of AC voltage using a digitally synthesizedsource

An AC voltage intercomparison was conducted by the National Institute of Standards and Technology (NIST) to determine the consistency of AC voltage measurements made at various standards laboratories. The transport standard used for this purpose was an NIST-developed, digitally synthesized sinusoidal voltage source whose RMS (root mean square) value was calculated by measuring the DC level of each of the steps used to synthesize the sine wave. The uncertainty of the calculated voltage at approximately 7 V RMS is typically within ±10 parts per million (p.p.m.) from 15 Hz to 7.8 kHz. This approach incorporates a technique of determining AC voltage with reference to a measured standard DC voltage, which is independent of the traditional thermal voltage converter approach. Preliminary measurements made at each of the participating laboratories agree with the calculated value to within ±20 p.p.m. These results indicate that at 7 V, in the low audio-frequency range, the AC voltage measurement techniques implemented at these laboratories are near the state of the art     

Comparison of the NIST and BIPM Standards for Air Kerma in Medium-Energy X-Rays

A comparison has been made of the air-kerma standards for medium-energy x-rays of the National Institute of Standards and Technology (NIST) and the Bureau International des Poids et Mesures (BIPM). The comparison involved a series of measurements at the BIPM and the NIST using the air-kerma standards and three NIST reference-class transfer ionization chamber standards. Reference beam qualities in the range from 100 kV to 250 kV were used. The results show the standards to be in reasonable agreement within the combined standard uncertainty of the comparison of 0.37 %, although a significant trend with radiation quality is observed and the possible sources discussed.     

Improved capacitance measurements with respect to a 1-pF cross-capacitor from 200 to 2000 Hz

This paper describes frequency dependence measurements of fused-silica capacitance standards from 200 to 2000 Hz, using a 1-pF cross-capacitor as the reference. The measured frequency dependence of fused-silica capacitors was found to vary significantly, ranging from a change of less than 0.2 /spl mu/F/F for one standard to a change of 0.8 /spl mu/F/F for another over the frequency range. Increasing capacitances with decreasing frequency from 1592 Hz for all tested fused-silica capacitors indicates that dielectric relaxation due to dielectric bulk and/or interfacial defects is the dominant source of frequency dependence. The relative combined standard uncertainty at 200 Hz (the largest in the frequency range) is 0.07 /spl mu/F/F, which is smaller by about a factor of three than the uncertainty reported previously from the National Institute of Standards and Technology (NIST).     

Thermal Converters for Audio-Frequency Voltage Measurements of High Accuracy

The ac-dc differences of a reference group of thermoelements have been evaluated at audio frequencies to a few parts per million (ppm) at currents from 5 to 20 mA. A technique for comparing the ac-dc differences of two thermoelements with an uncertainty of about 2 ppm has been developed. Two 5 mA thermoelements are used with a plug-in set of resistors of computable reactances to form thermal voltage converters for voltage measurements. With this same technique adjacent ranges of these converters can be compared to step up from 0.5 to 500 V to better than 10 ppm.     

Comparison of the NIST and BIPM Medium-Energy X-Ray Air-Kerma Measurements

The air-kerma standards used for the measurement of medium-energy x rays were compared at the National Institute of Standards and Technology (NIST) and at the Bureau International des Poids et Mesures (BIPM). The comparison involved a series of measurements at the BIPM and the NIST using the air-kerma standards and two NIST reference-class transfer ionization standards. Reference beam qualities in the range from 60 kV to 300 kV were used. The results show the standards to be in agreement within the combined standard uncertainty of the comparison of 0.35 %.     

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.     

A planar Bi-Sb multijunction thermal converter with small ac-dctransfer differences

A planar Bi-Sb multijunction thermal converter was fabricated on the LPCVD Si₃N₄/SiO₂/Si₃N ₄-diaphragm, prepared by silicon bulk micromachining, which thermally isolated a bifilar Pt- or NiCr-heater and the hot junctions of a Bi-Sb thermopile from the silicon substrate. The voltage responsivity, the ac-dc transfer difference, and the fluctuations of the output thermoelectric voltage and heater resistance were discussed to investigate the design factors of a thermal converter. The respective voltage responsivities in air and in a vacuum of the thermal converter with a built-in NiCr-heater were about 14.0 mV/mW and 54.0 mV/mW. The ac-dc voltage and the current transfer differences in air were about ±0.60 ppm and ±0.11 ppm in the dc reversing frequency range from 10 Hz to 10 kHz. They are sufficiently small to be used as practical ac standards. Compared to the thermal converter with a built-in Pt-heater, the thermal converter with a built-in NiCr-heater demonstrated a higher voltage responsivity and smaller ac-dc transfer differences, while exhibiting slightly larger fluctuations in output thermoelectric voltage and in heater resistance     

Comparison of the NIST and PTB Air-Kerma Standards for Low-Energy X-Rays

A comparison has been made of the air-kerma standards for low-energy x rays at the National Institute of Standards and Technology (NIST) and the Physikalisch-Technische Bundesanstalt (PTB). The comparison involved a series of measurements at the PTB and the NIST using the air-kerma standards and two NIST reference-class transfer ionization chamber standards. Results are presented for the reference radiation beam qualities in the range from 25 kV to 50 kV for low energy x rays, including the techniques used for mammography dose traceability. The tungsten generated reference radiation qualities, between 25 kV and 50 kV used for this comparison, are new to NIST; therefore this comparison will serve as the preliminary comparison for NIST and a verification of the primary standard correction factors. The mammography comparison will repeat two previously unpublished comparisons between PTB and NIST. The results show the standards to be in reasonable agreement within the standard uncertainty of the comparison of about 0.4 %.     

AC–DC Transfer Standard Measurements and Generalized Compensation With the AC Josephson Voltage Standard

We present AC-DC transfer standard measurements using the National Institute of Standards and Technology's pulse-driven AC Josephson voltage standard source. We have investigated the frequency dependence for several output voltages up to 200 mV for frequencies from 2.5 to 100 kHz. We found that, as the frequency increases, the ac-dc differences for the two arrays on the same chip do not agree. We explored this deviation in ac-dc difference for the two arrays by investigating different configurations of the probe cabling and wiring, chip carriers, and on-chip circuit design. We found that the circuit design produced the greatest improvement, particularly at the highest frequency (100 kHz), where the deviation in ac-dc difference was reduced by more than 60%. In this paper, we also demonstrate tenfold higher output voltages and improved operating margins for arbitrary (nonsinusoidal) waveforms. These enhancements were accomplished by implementing a more general current bias to the arrays having the same harmonic content as that of the synthesized arbitrary waveform.     

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.     

Materials Research With Neutrons at NIST

The NIST Materials Science and Engineering Laboratory works with industry, standards bodies, universities, and other government laboratories to improve the nation’s measurements and standards infrastructure for materials. An increasingly important component of this effort is carried out at the NIST Center for Neutron Research (NCNR), at present the most productive center of its kind in the United States. This article gives a brief historical account of the growth and activities of the Center with examples of its work in major materials research areas and describes the key role the Center can expect to play in future developments.     

CCE 92-05 intercomparison of ac-dc voltage transfer standards athigh frequencies (1-50 MHz)

From August 1995 to May 1998, the CCE 92-05 intercomparison of ac-dc voltage transfer standards at high frequencies was carried out. Two travel standards were measured by 15 national standards institutes. The results in the frequency range from 1 to 50 MHz show a good agreement between the majority of participants. The span of the majority of the reported ac-dc differences at 50 MHz is less than 1000 μV/V, which is similar to a previous intercomparison but with a greater number of participants     

Leakage current detection in cryogenic current comparator bridges

Several tests have been developed to locate leakage currents in cryogenic current comparator (CCC) resistance ratio bridges used at NIST to measure ratios of 1000 Ω/100 Ω, 6453.2 Ω/100 Ω, and 10 kΩ/100 Ω. The major advantage of the tests is that they can be performed in situ using the sensitivity of the CCC bridge. These test procedures have been used to reduce the leakage error uncertainty of CCC ratio measurements, linking working standards to the quantized Hall resistance (QHR) and to the NIST calculable capacitor experiment. CCC bridges require that the current which passes through a standard resistor must equal the current through the appropriate CCC winding to very high precision. This can be difficult to verify at or below 1 pA because a large number of possible leakage paths exist. Errors due to six important leakage current paths are given, and the calculated changes in the resistance ratio are compared with measurements made with a controlled leakage resistance in a 100 Ω/1 Ω CCC bridge