Vrms value

I assume that Vrms in emontx should roughly be the same as I read on an analogue multimeter. However the meter reads approx 230V, whereas Vrms fluctuates around 250V. All my power readings seem too high. Lloyd

Robert Wall's picture

Re: Vrms value

Yes, they should read the same. Provided both are reading the same thing and both are perfectly accurate and calibrated properly. In practice they won't, but should be within a few percent of one another. You don't give us much of a clue where to start advising you.

Have you looked here: http://openenergymonitor.org/emon/buildingblocks/measuring-voltage-with-an-acac-power-adapter

and here: http://openenergymonitor.org/emon/buildingblocks/ct-and-ac-power-adaptor-installation-and-calibration-theory

What is your nominal system voltage? (I ask because if you are in the UK, although the nominal voltage is now 230 V, in practice it is centred on 240 V, so that means your meter might be reading 4% low, and the emonTx 4% high).

Is your analogue meter accurate? (Unless you paid a lot of money, it is unlikely to be much better than 2.5% accuracy).

Which voltage transformer are you using?

What calibration values are you using in the sketch?

Have you changed any component values? (The voltage divider resistors on the emonTx board might be 5% components - gold band on the end? - so if you have a pair that are both at the wrong end of the band, they alone could give an error of 10%).


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