Picture of a correct calibration setup

Hello everybody,

I’ve bougth one year ago an emonTx arduino shield and it is working fine even though the calibration is not perfect…I am planning to realize correctly the calibration and I’ve closely read the corresponding building blocks articles written by Robert Wall (very good metrological work)…

I found a picture on one of the open energy monitor github page, and I suppose it is the correct setup for the emonTX calibration :



Is the material with a screen on the left of the strip a multimeter ? I’ve never seen such one ? somebody would know the brand ?

The emonTx seems to be alimented by an extra ACDC 5V supply…I am surprised because I’ve read it could be alimented by the ACAC adapter used for the voltage sampling…is there a particular reason for that extra alimentation ? maybe the photo was made with an old version of the emonTX which could not be powered by the ACAC adapter…

best regards


That picture shows an emonTx V2, which does not have the ability to draw power from the a.c. adapter. It must have a separate 5 V d.c. power supply.

The device that you have labelled “Multi meter” will be an in-line energy monitor. I do not recognise the make or model. If you wish to buy one of those devices, I advise you to check the accuracy that is claimed, because many say “typical”, and that is not a guarantee that the one you receive will reach that standard.
It will be a near equivalent of this: Energy Monitoring Devices | Energy Monitors | Energenie
For this particular unit, the specification is Current range: 0.005 to 13A, ±2%. Power range: 0.1 to 3120W, ±2%. Power Factor: 0.00 to 1.00.

Or this one maplin | the electronics specialist | FREE delivery over £20! | Maplin claims voltage: 200V - 276V (± 1%), current: 0A - 13A ( ± 1.5%), power: 0.1 - 3120W ± 1.5% and frequency: 45 - 65Hz (±1) which if correct is approximately the same as a good (~€200) multimeter.

[Warning: Both these have UK plugs and sockets, so you would need to have two adapters or extension cables: French plug to UK socket, and UK plug to French socket.]

Thanks a lot for your response…

so the picture included in my initial post can be considered as the reference setup for the calibration process ? am I correct or mistaking something ?

as far as the in-line energy monitor is concerned, I’ve found a similar product on the internet with the euro plug :

Do these inline monitors work on the same principle as emonTX as far as the measurement process is concerned ? ie voltage sampling, current sampling, calculus of Irms and Vrms, phase correction of voltage, calculus of instant power, and then at the end of the loop, calculus of real power, apparent power and power factor…

I can imagine that energy distributors do have the same problem when they make the bills, aren’t they ? Their meters work on the same basis or the concept is different ?


The equipment that you use to calibrate will depend on what you have available. For example, instead of a lamp, I use a 500 W heater to calibrate phase errors, but my test equipment uses a resistor on a large heat sink. If you are thinking of buying something only to calibrate your emonTx or emonPi, then the Toutallantvert wattmeter at €22.00 seems a good buy. But I cannot see anywhere the accuracy, so beware - caveat emptor.

In general, yes. But they probably have inside an I.C. specially made for measuring power and energy, therefore they do it much better. If you look at the data sheets from the manufacturers of energy monitoring integrated circuits, you will find general descriptions of how they work. We would find it hard to use one of those i.c’s, because of European Law and the need for a CE mark. Your supplier’s energy meter could well have exactly the same measuring i.c. as the plug-in meter, but it will have been carefully calibrated in the factory and is guaranteed to be accurate for (I think) 20 years.