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Automatic balancing dc and ac resistance ratio bridges are the primary measurement devices used by National Measurement Institutes for the calibration of standard platinum resistance thermometers (SPRTs) on the International Temperature Scale of 1990 (ITS-90). Performance assessment of these resistance ratio bridges is critical to the determination of both the uncertainties of the measurements and for identifying if a resistance bridge is exhibiting non-compliant behavior. NIST and NRC investigated the performance of 18 resistance ratio bridges consisting of 14 ASL F18s, 2 ASL F900s, and 2 MI 6010Bs. The assessment techniques included the use of an AEONZ resistance bridge calibrator, an ASL ratio test unit, and complements checks. Additionally, the possible effects from using different ac frequencies were investigated. This paper presents the methods of assessment employed, indicates the performance results obtained for the 18 resistance ratio bridges, describes the determination of measurement uncertainties, and assesses the contribution to the calibration uncertainty of SPRT measurements at ITS-90 fixed points. © 2003 American Institute of Physics [ABSTRACT FROM AUTHOR]