NMI is responsible for maintaining and disseminating Australia's standards of voltage (the volt) at dc and ac, resistance and current at ac, power (the watt) and energy (the joule).
A number of research and development activities are being undertaken to develop improved standards for comparing voltage at dc and ac, to develop novel current shunts for comparing resistance at dc and ac, to establish a new standard for power and energy at mains power frequency, and to upgrade Australia's standard of voltage.
NMI is undertaking an independent determination of ac-dc differences of its thermal voltage converters at frequencies up to 1 MHz. This is achieved in two stages. First, several primary TVCs rated at 1 to 4 V are studied to obtain their ac-dc transfer difference corrections at the lowest attainable level of uncertainty. Then these corrections are passed to three sets of working TVCs in the range from 0.5 to 1000 V through stepup and stepdown processes.
NMI primary TVCs are not of a calculable type and, therefore, their high-frequency ac-dc differences cannot be obtained theoretically with required accuracy. However, a simple and effective procedure has been devised allowing direct measurement of the significant error components of a particular TVC. As a result, 1 s uncertainties of 8 µV/V at 1 MHz have been claimed.
NMI has taken part in the following key comparisons of ac-dc transfer: CCEM-K6a, CCEM-K6c, CCEM-K9.
There are several instruments on the market capable of accurate power measurements at frequencies of 200 kHz and higher. The aim of this project is to provide traceability for these instruments and to establish a second power and energy standard at mains power frequencies that could be intercompared with the NMI Double Power Bridge. The new standard is based on a thermal principle. The thermal power comparator can relate the product of two 1 V ac signals that are proportional to voltage and current in the instrument under test to a product of two known dc voltages. The method used in the power comparator helps to reduce many errors that are associated with the comparison. Most of the errors of the standard are associated with the scaling devices, resistive voltage divider and shunts. These errors are presently under investigation.
NMI develops and manufactures unique measurement equipment to realise its electrical measurement standards and to provide high-accuracy calibration and testing services. This equipment is also available to the Australian industry and to national metrology institutes around the world.