Calibration emontx shield with arduino

I’m sorry but i really don’t understand the calibration for the emontx shield with arduino
(i’am french, i understand all the words…but not the meaning of the procedure)

this is in my sketch “emonTX Shield CT123 Voltage”
if (CT1) ct1.voltage(0, 260, 1.7);
i am in france in the euro aera and i have the ideal power adaptator for the 9v AC. so 260 is the good value
the emon tx send me a mesure of 255 v
in real i have nearly between 239.5 and 240.5 v

so what do i have to change ?
the 260 value ?
the 1.7 value (what it is ?)

other question not in this theme, is it normal that i mesure some what if i only have the SCt plugged ? like 12w ?

thanks for your answer


It corrects for a property of the transformers - phase error. If you wish to adjust it, get a resistive load (e.g. an electric kettle) and adjust the value so that apparent power and real power are the same (or as close as possible).

Yes. It is electrical noise. It might come from the power supply that you are using to supply your Arduino.

ok thank you rober.wall

is the tension calibration linear ?
240*260/255= 244.7
so i put
if (CT1) ct1.voltage(0, 244.7, 1.7);

ok for electrical noise, i use a temporary 5v power supply , i am waiting for a 9v power supply for my arduino, maybe it will be better quality.

for the phase error, how to monitor it ? because it have incidence to the mesure of current and it may have be calibrate too ?

what is the order between calibrate the phase error and the current ?




You should set the current calibration first. Monitor with

You must temporarily monitor power factor
and adjust for maximum, with a pure resistance load (e.g. a water heater).

Note: all this is written in great detail: Learn→Electricity Monitoring→Current & Voltage→1. Calibration Procedure→Recommended Calibration Method