Clifton B. Childers

AC-DC difference relationships for current shunt and thermal converter combinations

The relationship between the overall AC-DC difference of a thermal converter and current shunt combination and the characteristics of the separate thermal converter and current shunt is described. Predicted and measured results are given for shunts used with different thermal converters. The disagreement is generally small compared to the uncertainty of AC current measurements at the individual frequencies. >

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.

AC-DC difference characteristics of high-voltage thermal converters

This paper describes a study of high-voltage thermal converters (HVTCs) at voltages above 100 V at frequencies up to 100 kHz. Techniques for the construction of HVTCs are described, and the effects of aging and dielectric loss on the resistor, changes in the timing sequence of ac-dc difference tests, relay dead-times, warmup times, and voltage level dependence are given. >

A resistive ratio standard for measuring direct voltages to 10 kV

A highly accurate guarded voltage-ratio standard has been developed for measuring direct voltages from the range of a standard cell to 10 kV. The ratio standard has a resolution capability of 0.1 ppm for ratios of 1:1 to 10 000:1 with an estimated uncertainty of 0.2 ppm. It is designed for operation at a rating of 2 kΩ/V and consists of an adjustable reference section in series with three resistance groups each containing nine nominally equal sections. The resistance ratios are determined by a self-calibration technique using a 1:1 bridge. A series-parallel mode of calibration provides an additional check on the accuracies of the ratios. The standard is housed in a temperature-controlled oil bath whose oil is filtered to remove moisture and contaminants.

High-current thin film multijunction thermal converters and multi-converter modules

High-current, thin-film multijunction thermal converters (FMJTCs) have been fabricated at NIST with heater ranges from a few milliamperes to 1 A. Multiconverter modules containing high-current FMJTCs have also been constructed to measure currents up to 6 A at frequencies up to 100 kHz.

A Semiautomatic System for AC/DC Difference Calibration

A semiautomatic ac/dc difference calibration system is described. The system operates over a frequency range of 20 Hz to 100 kHz, covering the voltage range from 0.5 V to 1 kV. For all voltages at frequencies in the range from 20 Hz to 20 kHz, the total uncertainty is 50 ppm and 100 ppm for voltages at frequencies between 20 kHz and 100 kHz. In addition to ac/dc difference testing, the system can be readily adapted to calibrate precision ac digital voltmeters or ac calibrators. Results of extensive intercomparison testing of the new system against a manual test system, using a multirange thermal transfer instrument as a transport standard, are reported, and the results indicate that the differences obtained are well within the combined total uncertainty limits of the two systems.

Comparison of high voltage thermal converter scaling to a binary inductive voltage divider

High-voltage thermal converters (HVTCs) are used as standards of AC-DC difference and for the measurement and calibration of AC voltage up to 1000 V and 100 kHz. Their multiplying resistors can be compensated to yield small AC-DC differences by using adjustable internal shields; however, the AC-DC differences of HVTCs may vary as functions of warm-up time, applied frequency, and applied voltage. Voltage coefficients between 100 V and 1000 V can be quite significant compared to calibration uncertainties, and can be major sources of error in the buildup process used to characterize the HVTCs. Formal and informal international intercomparisons of HVTCs have revealed variations among the participating laboratories. The present work was undertaken to compare the scaling accuracy of HVTCs to the completely independent principle of a binary inductive voltage divider.

AC-DC difference characteristics of high-voltage thermal voltage converters

This paper describes a study of thermal voltage converters (TVCs) at voltages above 500 V and at frequencies up to 100 kHz. The effects of aging and dielectric loss on the resistor, as well as changes in the timing sequence of AC-DC difference tests, relay dead-times, warmup times and voltage level dependence are described.<<ETX>>