What is the precision of the power measured by emonTx and emonPi?


I am wondering what is the actual precision of the power measured by emonTx or emonPi ?

If this depends on proper calibration, then what is the precision assuming that it is perfectly calibrated ?

Does the precision also depends on the total power ? E.g. Will the absolute error be bigger in case the power is bigger ?
What are the factors reducing the precision of the measurements ?


I’m not sure that I completely understand what you mean by ‘precision’, but if you look in the ‘Learn’ section, there is an article about the possible sources of error that affect calibration, there is also one about the errors that become more significant at low currents (and therefore by implication at low powers), but I’m not aware of anyone having done a complete analysis over the full range of measurements and waveforms. Those articles should also go some way to answering your final question about the factors that affect the accuracy of the measurements.

What I can say is that some users who have carefully calibrated their emonTx (in particular, the older and, we believe, slightly less ‘good’ V2) have reported that it agreed with their supplier’s meter to better than 1%. The present c.t. available from the shop has significantly better performance than those available when the emonTx V2 was being sold, and the emonTx V3 does have a better noise figure, so I would hope that the V3 would also be able to achieve that 1% accuracy.

You might also find the article about multimeters gives you some insight into the meaning of ‘precision’ and the amount of money you need to spend in relation to the accuracy band that you wish to achieve.

Hi Robert,

Thanks for the extensive response.
I have found the page about the source of errors you have mentioned.

So without calibration the power error for emonTx v3 is around 10% and with proper calibration this error can be reduced to 1%.
That is what I wanted to know.

many thanks