National Physical Laboratory

AC power wattmeters and voltmeters

AC power measurement by samplingAC power measurement by sampling 

The measurement of power at NPL is based on a digital sampling technique. This works by using a pair of analogue to digital converters (ADC) to take instantaneous samples of the respective voltage and current waveforms involved in power measurement. By multiplying the sample pairs, the instantaneous power can be found, and by averaging instantaneous power over a defined period, the average power of that period can be calculated.

This technique relies on the performance of the ADC in-terms of its accuracy and its ability to take samples at well-defined instants in time. To meet these requirements NPL has developed an ADC system based on the fastest form of ADC, known as a flash converter. Unfortunately, these devices are of limited resolution and to overcome this limitation they are included in a configuration that uses a Digital to Analogue Converter (DAC). This system makes use of the repetitive properties of the waveforms of interest, where one cycle is very similar to the following cycle.

NPL ADC Schematic
NPL ADC Schematic 
The computer takes a 'guess' at the signal level at a given instant on the waveform. This digital guess is accurately converted to a voltage using the DAC. The guess can then be compared to the incoming signal at the required point in time and the “error” voltage is converted using the Flash converter and passed back to the computer. Because the waveforms are assumed to be repetitive, the computer can improve its guess for the next cycle of the waveform using the error information from the previous cycle. In this way, the computer’s guess converges on a digital representation of the input waveform. This representation can then be used for AC voltage, current or power measurement.

Two of these ADC systems are used for a single-phase power measurement. They are used in conjunction with transducers that convert different levels of voltage and current to the working signal levels of the ADCs. In order to give power traceability all of these components require individual calibration and characterisation.

Digital sampling gives the extra benefits of allowing the analysis of other parameters associated with power measurements. These include the measurement of phase angle, harmonic content and reactive power for sine wave signals.

Voltage 1 V to 1000 V
Current 2 mA to 130 A
Power Active and Reactive
Frequency 40 to 400 Hz
Phase Angle All angles
Typical Uncertainty 40 ppm of Applied VA

Table 1: Scope of measurements for UKAS accredited Power Measurements at NPL

NPL offers calibration of single phase Wattmeters over the ranges set out in the table. These measurements are UKAS accredited. Higher frequency power measurements are available up to 100 KHz using the NPL High Frequency Wattmeter.

The sampling equipment described also allows for the measurement of AC voltage and current. One application of this is the measurement of very low frequency voltage. This measurement is important in vibration work and NPL offers a calibration service for digital voltmeters working down to 0.001 Hz.


Customer Service tel: +44 20 8943 7166

Last Updated: 18 Aug 2015
Created: 28 Nov 2007


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