AC-DC transfer standards-present status and future directions

Precision ac-dc transfer standards, for use at frequencies up to 100 kHz, are reviewed with emphasis on developments over the past eight years. Primary standards and most precise ac-dc transfer instruments are based on single- and multi-junction thermal converters. Sources of error in thermal converters at midfrequencies are mainly associated with thermoelectric effects, and these sources are briefly discussed. Other sources of error and the performance and limitations of standards and instruments used for the ac-dc transfer of voltage and current, up to 1200 V and 20 A, are also considered. Calibration and intercomparison of these instruments in national laboratories is particularly costly and time consuming, and in recent years this has led to the development of automatic testing systems and procedures. It is unlikely that conventional thermal converters will be replaced as primary standards in the short term. However, computable ac generators and alternative thermal devices may be expected to make a significant contribution in the field of ac-dc transfer in the near future.