NIST researchers develop improved high-resistance standards.

Researchers at NIST have developed a new generation of high-resistance standards and delivered the standards to the Primary Standards Laboratories of the Army and Air Force. The new standards, with nominal resistances of 1 G[ohm] and 10 G[ohm], have performance characteristics that are substantially better than have been demonstrated by any prior standards at these resistance levels. As a result of this superior performance, NIST was able to deliver these standards with calibration uncertainties reduced by factors of three and five, respectively, below those that can be provided for other high-resistance standards.

The improved performance characteristics will allow the Department of Defense (DoD) standards laboratories to deliver improved high-resistance measurements. Demand for improved high-resistance measurement capability has, in recent years, driven substantial improvement in the measurement instrumentation and techniques. However, performance characteristics of existing high-resistance standards did not allow electrical metrology laboratories to achieve the full potential of the improved instrumentation. A recent NIST-led international comparison of high-resistance measurement capability clearly demonstrated the potential for improved metrology with improved standards. NIST undertook development of these improved standards as a critical step required for DoD laboratories to maintain their high-resistance measurement capabilities at the necessary state-of-the-art.

The NIST standards are based on commercially available film-type resistance elements. The elements underwent heat treatment, which leads to a very large improvement in the long-term stability of their resistance. Sets of the treated elements were carefully selected to form composite standards with net resistance very close to the nominal value, and with minimal residual net drift rate. The composite elements were hermetically sealed in brass cylinders to improve their long-term stability and to mitigate the effects of environmental humidity, and then shock-mounted in aluminum enclosures using highly damped visco-elastic material. Calibrated thermistor probes were permanently mounted inside each enclosure to permit monitoring of the temperature. Mounting of the coaxial terminals on circular polytetrafluoroethylene plates allows the standards to be fully guarded, which suppresses the leakage of current to ground. The resulting stable and robust standards have excellent traveling characteristics and are suitable for use as primary resistance standards. The standards were characterized for their drift with time and for their dependence on temperature and voltage. The 1 G[ohm] resistance standards were found to typically exhibit temperature coefficients of 5 x [10.sup.-6]/[degrees]C, voltage coefficients of 0.003 x [10.sup.-6]/V, and drift rates of less than 10 x [10.sup.-6]/year. The 10 G[ohm] resistance standards were found to typically exhibit temperature coefficients of -3 x [10.sup.-6]/[degrees]C, voltage coefficients of 0.003 x [10.sup.-6]/V, and drift rates of less than 20 x [10.sup.-6]/year. For comparison, high-resistance standards that are presently available commercially have typical thermal coefficients that are one to 10 times larger, and voltage coefficients that are larger by a factor of 100 to 300. The observed drift is improved by a factor of three to five.

CONTACT: Michael Kelley, [phone]; [e-mail].