Few loads are purely resistive or purely reactive. A water heater element is probably the closest thing to a purely resistive load that is easy to find. The strong cooling effect of the water prevents the temperature oscillating too much through the 50/60Hz cycles. By comparison, an incandescent light bulb can give you about 15-20% THD and some phase shift, due to the large oscillations of temperature, and therefore resistance, in the filament. If you have a purely resistive load there is a simple trick you can use, but it requires some alteration to the code... Buffer the voltage signal for enough samples to delay it by about 60 degrees. Use the exact mains frequency and sampling rate to work out the exact phase delay you have created. From this, work out the exact power factor this delay represents. It should be something like 0.5PF. In this area power measurements are very sensitive to small phase errors. Now make the software calculate power from the delayed voltage signal and the undelayed current signal, just as it calculates power from the undelayed voltage signal and the undelayed current signal. The power calculated from the undelayed signals should be a good estimate of the correct power into your resistive load, even if there is a small phase offset between the undelayed voltage and current signals. It takes a 2.5 degree phase error to cause just 0.1% of error in the measured power at unity power factor. Work out what power reading you would expect from the delayed voltage and undelayed current signals, using the effective power factor you worked out earlier. Now compare this with what you actually get. Even small phase errors cause a substantial difference between these numbers. If you adjust the phase until they match you should have a meter that is accurately calibrated for phase. Our metrology software used to incorporate this technique, so we could calibrate meters with a single measurement on a test set. However, we found that in practice none of the test sets we tried could be relied upon to consistently stabilise their phase to the required accuracy, so we took it out.
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