VRMS different between emonPi and emonTx3


I have an emonPi and an emonTx together monitoring the mains, solarPv, diversion and EV through CT clamps.
Both of them have AC-AC adapters. The adapters are plugged into an extension lead, itself plugged into the single socket I have available in the utility (the other is being used by the washing machine)

2 questions:

  1. Is going through an extension lead acceptable to get correct AC measurements?
  2. I’m getting very different VRMS measurements from the emonPi and the emonTx. The Pi is telling me 230V, the Tx is saying 247V. Is that normal?



  1. Yes, provided that you do not have a high-current load also using the extension lead.
  2. Yes, it is caused by component tolerances, and that is why we always suggest you calibrate for best results. At over 7% difference, you appears to be rather unlucky. The full calibration instructions for your emonTx are in the ‘Learn’ section, but although exactly the same applies to your emonPi, it’s hard to actually carry out the proper procedure. If you have an accurate voltmeter, you can adjust the scale constants in emonHub so that both read the same, correct, value. Adjusting the voltage in emonHub will not also calibrate the power, you must also calibrate each power separately.

For your emonPi, vrms is item no.4, therefore you change the 4th value (0.01) in “scales =” to adjust the voltage reading sent to emonCMS. Changing it to 0.0104 would increase the voltage by 4%.

    nodename = emonpi
        names = power1,power2,power1pluspower2,vrms,t1,t2,t3,t4,t5,t6,pulsecount
        datacodes = h, h, h, h, h, h, h, h, h, h, L
        scales = 1,1,1,0.01,0.1,0.1,0.1,0.1,0.1,0.1,1
        units = W,W,W,V,C,C,C,C,C,C,p