Energy metering has never been done for fun. There is money involved, so the meter accuracy has to be verified.
In the past without any electronics it was really challenging. You needed a voltage/current source with a high stability, calibrated wattmeters and a calibrated stop-watch.
The source was transformer operated with regulators for current, voltage and phase shift.
First you adjust voltage and current. Then you read the power from the wattmeters.
The meters connected to the test bench had to be tested one by one.
Each meter has a meter constant, indicating the no. of revolutions per kWh.
Imagine a single-phase meter with a constant of 75 r/kWh. We have adjusted the voltage to 230 V and the current to 10 A. Power factor is 1.
We see on the wattmeter a power of 2300 W.
Now we can calculate the theoretical time for one rotation based on the meter constant.
The theoretical time for one rotation under a power of 2300 W is 20,87 s. (Note that we had no calculator, in best case we calculated with a slide rule.)
Next step: We stop the time for one rotation. The rotating disk of the electromechanical meter has a coloured mark. Once the mark is in front of the meter you start the watch and you stop it once the mark reaches the same position after one revolution.
Let’s say, you stop a time of 21.1 seconds.
Now we use the common formula for percentage error:
So we come to an error of +1.1 % . The meter is faster than it should be. At that time the meters had typically an accuracy class 2, so it’s PASS.
As you can see there are a lot of influence factors which can spoil the measurement result like
- inaccurate reading from the wattmeter
- source instability (wattmeter needle is moving during the test)
- inaccurate reading from the stop-watch
- too late or too early stopping of the time
- rounding errors in the calculations
Especially the stopping time made the life for the meter testers very hard. So the trick was to stop the time for several revolutions to minimise the human error for start/stop.
If you have portable equipment like our calibrator RS350 with a start/stop switch you can compare the error results vs. using a scanning head.
In the animation below we see the rotating disk of a single-phase meter (top view without meter case ). This meter has a constant of 100 r/kWh. It’s running with a load of 3,450 W.
You have already calculated the theoretical run time per revolution (10.43 s).
Use your satellite synchronised smart phone stop watch to check the meter error.
Leave me a comment with your result :)
Thank you for reading.
Editor's note: This article was originally published in December 2019 and has been updated for comprehensiveness.