Complete Newbie Help Calibrating AC input for 240v

Hi there this is my 1st post so I will introduce myself, I am an electrician in Australia, I want to make a unit that I can leave at a clients premises that will log power volts and current.

  1. I have purchased a 9v AC transformer we have a 240v nominal voltage here and my RMS voltage at my home is normally around 245v. How do I calibrate the volt reading to suit my transformer and local voltage to workout an accurate power value?

  2. I want to use the pulse counter to compare what the network operator’s meter is reading and what the open energy meter is reading (Some of my clients don’t trust the network operator and want a independent meter to test the consumer meter)

Dhueys Electrical

Welcome Toby,

You don’t say what you’ve got! And as my crystal ball is fogged up, I’ve got to point you at the ‘Learn’ section, where you’ll find the instructions for a full calibration of the emonTx. The front end of the emonPi (if that’s what you have) is identical, but it’s a pain to reload the sketch in the Pi. You can, however, adjust the voltage and power calibration in the config file for emonHub: emonhub.conf (and you do this in the web browser).
Scroll down to the section

     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

and adjust the “scales” multipliers by the reciprocal of the factor that the quantity is wrong by. The values in each line have a 1:1 relationship. (e.g., if the voltage is 5% high, the 4th value in ‘scales’ becomes 0.0095) Note that adjusting the voltage scale won’t change the power scale, because the multiplication has already been done.
Unfortunately, you can’t calibrate the phase/timing error this way, you must edit and reload the sketch in the Atmel 328 to correct that.

Thanks Robert

Your crystal ball was working okay, I do have the emonPi.
I cant seem to find the instructions to do a calibration. I want to be able to use voltage to calculate power accurately.

I did manage to get the voltage to display correctly by changing the scales value.


The voltage stays a constant 246v and doesn’t fluctuate at all like I think it should with the street voltage. I did buy my own Australian 9v ac power transformer. Even when I unplug the 9v supply I still have 246.1v showing in the input view.

Is there a guide on how to reload a sketch somewhere?

Thanks for your help and sorry about the newbie questions.

Don’t worry about the newbie questions - we were all one once.

The voltage should change - was the 9 V transformer plugged in when you powered your RPi? Everything needs to be connected then, else it is ignored by the sketch. Rebooting the Pi doesn’t affect the Atmel 328, you must cycle the power. This catches people regularly - it’s actually in the FAQ.

Calibration instructions: Learn | OpenEnergyMonitor

Current and voltage are multiplied per sample inside the sketch to give real power (watts). Do note as I said above, you must calibrate power independently of voltage. So when you’ve got the voltage reading accurately, you must transfer your meter to measuring current, find a unity p.f. load and calculate the power and tweak the calibration to suit.
With care, you should get as close as you meter(s) allow.

I can’t find the old instructions for reloading the sketch - I can’t look now, it may be a couple of days. The new ‘official’ method using platformio (somewhere in Resources I think) didn’t work for me.