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Abstract and Manuscript Management System - Abstract Detail |
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| Speaker: | George Jones |
| Title: | Measurement Uncertainties in Resistance Scaling due to Power Loading Effects in Precision Resistors |
| Topic Group: | Electrical I |
| E-mail: | george.jones@nist.gov |
| Co-Authors: | Randolph E. Elmquist |
| Abstract: | Five manganin alloy Thomas-type 1 ? resistors serve as primary working standards at the National Institute of Standards and Technology (NIST) in the precision potentiometer DC current comparator (DCC) system used for special 1 ? customer calibrations. To maintain and predict the values of these resistors, the value of this bank is compared to the quantized Hall resistance (QHR) standard at NIST approximately twice a year. This procedure begins with comparing a bank of five precision 100 ? resistors against the QHR using a cryogenic current comparator (CCC) system. Next, several precision 1 ? resistors manufactured using the resistance alloy Evanohm by the Australian National Measurement Laboratory are measured against the 100 ?? resistors through a second CCC system. The current in these CCC systems is ramped from one polarity to the other over a period of several seconds. The transfer resistors are then measured in the DCC system, where a measurement current of 100 mA is applied continuously and its polarity is switched within milliseconds. Over many years of careful monitoring, the relative values of these transfer resistors were seen to have discrepancies that were not related to the drift in the value of the resistors and exceeded the uncertainty in the measurement systems. Some of these variations were believed to be due to power loading in the transfer resistors. Recent experiments on different types of precision 1 ? resistors have demonstrated that conditions of power dissipation within the resistors and the duty cycle of the power applied to the resistors have significant effect on the uncertainty of the measurements. This paper describes the experimental results and measurement uncertainty due to these power loading effects. Additional measurements have examined loading effects in Thomas-type and certain other precision 1 ohm resistors. The relationship between the loading effect and the temperature coefficient will be described, as well as possible thermoelectric contributions to the changes of resistance observed in these measurements. |
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