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.     

A reevaluation of the NIST low-frequency standards for AC-DCdifference in the voltage range 0.6-100 V

A reevaluation of the NIST standards of ac-dc difference was undertaken in an effort to reduce the calibration uncertainty offered by NIST for thermal voltage converters (TVC's) at frequencies below 100 Hz. This paper describes the measurements taken in support of this effort, as well as the devices used for the reevaluation process and the analysis of the uncertainty of the measurements. This reevaluation of the NIST low-frequency standards will permit a significant reduction in uncertainty for ac-dc difference calibrations at 10 Hz in the voltage range from 0.6-100 V     

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.     

Automation and Evaluation of Two Different Techniques to Calibrate Precision Calibrators for Low Frequency Voltage Using Thermal Devices

Low frequency (LF) voltage and current are important parameters in electrical metrology. The standards for LF voltage and current are established by assigning AC–DC transfer difference to thermal devices, i.e. thermal converters or thermal transfer standard along with current shunts. Automated calibration systems have been developed based on Null method and measurement technique developed by Budovsky for calibration of precision calibrator in LF voltage and current against thermal devices. The technique based on the Algorithm developed by Dr. Ilya Budovsky (National Metrology Institute (NMI), Australia) has been compared with the conventional null technique. Indigenously developed software has been used to calibrate the precision calibrator in the entire LF voltage and current range using Holt thermal converters and current shunts. Calibration results at 1 V, 10 V in the frequency range from 10 Hz to 1 MHz as well as calibration results of 1 A in the frequency range from 40 Hz to 10 kHz are presented in this paper. These result shows that the measurement technique developed by Budovsky has reduced the complexity of AC–DC transfer measurements, measurement time and the uncertainty in measurement.     

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     

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.     

Improved High-Input-Impedance mV-Amplifiers With Gain Factors From 10 to 900

New millivolt-amplifiers (mV-amplifiers) driving planar multijunction thermal converters (PMJTCs) at their output have been developed to measure the output voltage of micropotentiometers $(muhbox{pots})$. Using a twin-stage design with gains of 10, 20, and 30 per stage, total gain factors ranging from 10 to 900 are achieved. This results in a usable range of input voltages from 100 $muhbox{V}$ up to 100 mV at frequencies ranging from dc to 1 MHz. A novel offset compensation results in small reversal differences, and this allows for reliable operation at higher gain factors. There is a sophisticated protection circuit to avoid damage of the PMJTC in the case of overload. The ac–dc transfer difference is flat within 100 $muhbox{V/V}$ up to 100 kHz.      

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.      

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     

Theoretical Analysis of the AC/DC Transfer Difference of the NPL Multijunction Thermal Convertor over the Frequency Range DC to 100 kHz

Measurements of the rms values of alternating currents are frequently made by means of thermal transfer instruments. At low and midfrequencies Thomson heating can have an important influence on the ac/dc transfer difference of these devices, whereas at higher frequencies the variation with frequency of the effective resistance of their heaters is the most decisive factor. The present paper examines the influence of both Thomson heating and the reactive components of the heater on the performance of the NPL multijunction convertor, and sets out the theoretical analysis that supports the claim that its ac/dc transfer difference does not exceed 1 or 2 ppm up to a frequency of 100 kHz.     

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     

Interlaboratory Comparison in Measuring Low Levels of AC Voltage between Italy and Egypt

The paper describes an interlaboratory comparison program between the National Institute for Standards (NIS), Egypt and the Istituto Nazionale di Ricerca Metrological (I.N.Ri.M.), Italy for measuring low ac voltages. The aim of this program is to demonstrate the technical competence of both institutes. The interlaboratory comparison has been carried out under the framework of the executive program of scientific and technological cooperation between Italy and Egypt. A Fluke model 792A has been used as a travelling standard, which was calibrated against the reference standard of NIS and I.N.Ri.M. at 10, 20, 50, 100 and 200 mV at 40 Hz, 1 kHz, 10 kHz and 20 kHz. The standards of the two institutes, NIS and I.N.Ri.M., have been used to calibrate the traveling standard at 10, 20, 50, 100 and 200 mV at frequencies of 40 Hz, 1 kHz, 10 kHz  and 20 kHz. The ac–dc transfer difference results of the traveling standard are evaluated then compared at the intended frequencies. In this paper, the comparison results are discussed in detail. The performance of this test is also judged by calculating the relative error normalized with respect to the uncertainty of the measurement. The comparison results and the efficiency test E _n values show a good agreement between the NIS and the I.N.Ri.M. systems in assigning ac–dc transfer difference.     

AC/DC Thermal Converters of the ETL

In order to establish alternating current and voltage standards, various types of thermal current and voltage converters have been constructed. From their detailed analysis, the specifications and characteristics have been determined. These converters have been used in a circular international comparison. A precise comparator of 0.1-ppm resolution has been also constructed. The estimated accuracy of current converters is better than 10 ppm up to 100 kHz and that of voltage converters is better than 25 ppm up to 50 kHz and 30 ppm at 100 kHz.     

A sensitive differential capacitance to voltage converter forsensor applications

There is a need for capacitance to voltage converters (CVC's) for differential capacitive sensors like pressure sensors and accelerometers which can measure both statically and dynamically. A suitable CVC is described in this paper. The CVC proposed is based on a symmetrical structure containing two half ac bridges, is intrinsically immune to parasitic capacitances and resistances, is capable of detecting capacitance changes from dc up to at least 10 kHz, is able to handle both single and differential capacitances, and can easily be realized with discrete components. Its sensitivity is very high: detectable capacitance changes of the order of 2 ppm of the nominal value (24 aF with respect to a nominal capacitance of 12 pF) result in a measured output voltage of 1.5 mV. However, due to drift the absolute accuracy and resolution of the CVC is limited to 3.5 ppm. A differential accelerometer for biomedical purposes was connected to the CVC and showed a sensitivity of 4 V/g. The measured rms output voltage noise in the frequency range of 2-50 Hz is 750 μV, resulting in a signal to noise ratio of 75 dB at an acceleration of 1 g in the frequency range of 2-50 Hz     

Integrated Calibration System for Accurate AC Current Measurements up to 100?kHz

This paper describes the establishment of an integrated calibration system for accurate AC current measurements at National Institute of Standards, Egypt. The measurement system consists of a new assembled thermal current converter (TCC) associated with an appropriate hardware and automation software. The system has been used for a wide range of AC current from 5?mA to 20?A at frequencies from 10?Hz to 100?kHz. The assembled TCC at rated current of 5A was modified and recalibrated in PTB, Germany to enhance the system reliability. The estimated uncertainty budget of this system is presented in this paper.     

Improvement of the Voltage Dependence of High-Voltage AC–DC Transfer Differences at the NMIJ

A new ac–dc comparator system of 20–1000 V has been developed at the National Metrology Institute of Japan (NMIJ) for the calibration of ac–dc thermal voltage converters (TVCs), which are used as national ac–dc transfer standards. The ac–dc transfer differences of high-voltage transfer standards were evaluated by a traditional step-up procedure. The voltage dependence of the ac–dc transfer difference was observed in the earlier step-up procedures over 300 V, as reported by the latest international intercomparison. The experimental results for high-voltage TVCs with several different range resistors at the NMIJ suggest that the voltage dependence may primarily be caused by input connectors of range resistors and the change in the resistance value of the resistor and TVC modules due to heating from resistors. This paper describes the voltage dependence improvement of ac–dc transfer differences over 300 V at the NMIJ.      

Compact inductance standard

A portable Maxwell-Wien bridge as a part of the Korea Research Institute of Standards and Science (KRISS) inductance standard has been developed. Two auxiliary resistive-capacitive networks (analogous to a "Wagner ground") provide excellent stability of the bridge balance and impose less strict requirements on the components of the networks. Removable capacitance and ac/dc resistance standards used in the bridge arms make it possible to realize the inductance unit in terms of capacitance and resistance in the frequency range 500 Hz to 3 kHz. Investigations of the standard and results of preliminary (trial) comparison with the Mendeleyev Institute for Metrology demonstrate that the bridge can be used for measurement of 10 and 100 mH inductance standards with an uncertainty within (1-3) /spl mu/H/H at frequencies of 1 and 1.6 kHz. The use of this bridge as a constituent part of a transportable standard gives an opportunity to eliminate any uncertainty arising from instability of the standard inductors.     

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.     

A 1000 A/20 kV/25 kHz-500 kHz Volt-Ampere-Wattmeter for loads withpower factors from 0.001 to 1.00

Many industrial processes and scientific experiments utilize large amounts of ac power at frequencies from 3 kHz to 500 kHz. The phase angle and the impedances of these loads often vary over a wide range. This paper describes an instrument to provide an accurate measurement of currents (1 A to 1000 A), voltages (100 V to 20 kV), and powers (100 W to 20 MW) over the frequency range from 25 kHz to 500 kHz. It deals with loads having power factors down to nearly zero and with load impedances from 10 Ω to 20 kΩ. The paper contains practical details of both the design and the calibration of the front-end voltage and current transducers. The characteristics of a nearly ideal broadband current transducer are presented. Overall instrument calibration, verification and traceability problems are considered in detail     

Development and Implementation of Current Tee for AC High Current Calibration

Thermal current converters or thermal transfer standard with AC–DC current shunts act as the reference standards for accurate and precise measurements of low frequency current in the frequency range from 40 Hz to 10 kHz. At present CSIR-NPL, India has the AC–DC current transfer difference (δ _x) calibration facility upto 20 A. To extend our AC current calibration range from 20 A to 100 A, a current Tee for AC high current using LC connectors has been indigenously designed and developed. This paper presents the development of current Tee for AC high current calibration. The calibration results for assigning ‘δ _x’ at 30 A current shunt with respect to 20 A are shown and the same measurement technique has been used to extend the current range up to 100 A.