Traceable and accredited measurements
AC/DC transfer links quantum DC voltage traceability to a wide range of AC frequencies using the heating effect of electrical current. Decades of technology have refined this simple concept to give a measurement uncertainties at the ppm level for ac voltage and current beyond the audio frequency range.
Alternating currents and voltages are usually related to their direct counterparts using AC/DC transfer devices, that is devices which will respond to both AC and DC in a known way. The most common variety used in precision measurements is the thermal ac/dc transfer standard. The heart of such a standard is a thermal element – a short heater wire with a thermocouple attached and enclosed in an evacuated glass envelope. The AC and DC signals are applied in turn to the heater and the corresponding temperature rise is indicated by the electrical output from the thermocouple.
In a perfect device an equal temperature rise would indicate that the root mean square (rms) value of the AC waveform was equal to the value of the DC signal. For a more complicated device, the multi–junction thermal converter (MJTC), first developed at NPL in 1965. Unfortunately in the case of the more common single–junction thermal converter (SJTC) anomalous temperature distributions caused by Peltier and Thomson effects result in there being a difference in outputs even when the same dc and rms ac are applied to the input.
The AC/DC transfer difference is the difference between the AC and the DC signals that are required to give the same output. It is usually expressed as parts–per–million (ppm) of the total DC signal.
In use, the range of these simple SJTCs is extended from currents of typically 1 mA to currents of 20 A by the addition of resistive shunts. For voltages from 0.1 V to 1000 V series ranging resistors are added. For voltages between 1 mV and 0.1 V, resistive voltage dividers are used.
As MJTCs are only available over a limited current/voltage range, a build-up technique is performed on sets of working standards to relate their transfer differences to the known small errors of the MJTCs.
The major difficulty with such thermal transfer devices is that the output voltage generated by the thermocouple is very small (in the region of 7 mV) and is very prone to drifting. Measurements of AC/DC transfer difference at NPL have been made under computer control. In this way the switching between AC and DC and the reading of the devices can take place at regular, precise intervals. NPL uses a third order curve fitting techniques to remove the thermal drifts from the measurements.
By making a DC measurement using an electronic voltage standard, the ac/dc transfer system has been adapted for the direct calibration of ac digital voltmeters. These measurements are important for the highest grade instruments and using this method uncertainties of 7 ppm can be achieved at a 95% confidence level.
UKAS accredited service for AC/DC transfer
NPL is able to offer measurements over an extensive range of voltages/frequencies. Best measurement uncertainty is ± 5ppm.
- AC/DC voltage: range from 1 mV to 1000 V, 10 Hz to 1 MHz (100 kHz max above 20 V)
- AC/DC current: range from 1 mA to 20 A 10 Hz to 100 kHz
- AC voltage: range from 0.5 V to 1000 V, 10 Hz to 1 MHz (100 kHz max above 20 V).