EmonTX4 measurements are off


I finished my new energy monitoring setup.
Everything is working despite the values returned by the EmonTX4.

For example for the total consumed power,I have a difference about 200w with the amount displayed on my grid counter.

The VRMS displayed in emonpi is almost accurate ( maybe 3 or 4v off): 236v in place of 240v at this very moment.

In my setup I’m not using the exact same CT as the one offered in the shop.
Would that be the cause of the problem? Do I need to calibrate them?


Over what size of power (what are both actual values)

What are you using for your ‘accurate’ figure?

Just checking that you have set the right values for the CT in use on the TX4 firmware?

Yes :slight_smile:

@Robert.Wall is better on the more technical aspects.

For the vmrs, I compared with the value given by the grid counter. I have also checked it by measuring it directly with a DMM.

For comparison:
EmonTX4 : 4521w
Grid computer: 4774w

This is the Elektrizitätszähler which your electricity supplier uses to calculate your energy cost?

This is an error of 1.66% (low).

A total error of a little less than 5.3% low means the CT is also reading nearly 4% low.

This is more than we would expect, therefore can you give me details of the current transformer you are using?

[If you are not sure of the electrical words to use, look at https://www.electropedia.org/]

Sorry not grid computer but grid counter, autocorrect got me again :frowning:

I’m using the following CT

On the AC voltage sensing board I’m using the one listed on the BOM.

The c.t. you are using should be within 0.5% at the current you are measuring, I cannot think of a reason why you are seeing an error this large.

This appears to be a single phase installation with the c.t. expansion board giving you 12 channels in total. Therefore are you using the demonstration sketch and emonLibDB? If you are, you can adjust the calibration by editing the sketch. You can adjust the calibration of voltage from the emonVs and current from the c.t. separately if you wish, calibrating the voltage will make this correct and affect the power reading from all the c.t’s, or you can change only the calibration for each c.t. if the voltage error is acceptable.

For the voltage, change the value “100” in the line
EmonLibDB_set_vInput(1, 100.0, 0.16); // emonVS Input channel 1, voltage calibration 100, phase error 0.16°
to 1.017 (but adjust to give the correct reading).

Then for each c.t (and when you have done this, you must not move the c.t to a different input channel), change the value (100 here, 20.0 for your 20 A c.t) in lines like
EmonLibDB_set_cInput(1, 100.0, 0.3); // emonTx4 Current/power Input channel 1, 0.3° phase error for 100 A CT
to give the correct power - 20.8 using the numbers you gave above.

The value for phase error should not have a major effect on the power with most loads, but you can if you wish adjust for a power factor (Leistungsfaktor) closest to 1.0. To see this, in your sketch print the value with Serial.print(EmonLibDB_getPF(ch),4); - “ch” is the channel number 1 - 12.

thanks for the detailed answer.

I have applied the changes yesterday evening.
I did another check this morning and the numbers looks better but only for the 50A CTs

For the 20A CTs, I have used the following setup based on the number used for the 50A CTs:
EmonLibDB_set_cInput(7, 20.32, 0.3);

Unfortunately the measures are completely off with the 20A CTs.
For example with the hot water tank which is 3200W, I got the following:
EmonTX4 : 865W
EmonTX2 : 3220W

Something is very seriously wrong if a 20 A c.t. with a calibration factor anywhere close to 20 reads a quarter of what it should. Are you sure you have the c.t. you think you have, connected to the input you think it is?

Are you able to measure the voltage directly from the c.t? Do you see 233 mV at 14 A with your water heater?

Ok I checked the CTs directly with a DMM.

Current consumed by the water tank : 12.5A
50A CT : 98.592mV
20A CT : 61.62mV

These CT are completely off and the 20A are the worth.
All the CT should have a 0.5% accuracy.

I have not tested all the CT but only 2 of each type anc the results are the same.

No, you are completely wrong. Look at your image:

You have voltage output c.t’s. These have their burden inside the body of the c.t. and the output is a voltage. The current is meaningless. Le courant n’a aucun sens.

I wrote:

Êtes-vous capable de mesurer la tension directement à partir du c.t ? Voyez-vous 233 mV à 14 A avec votre chauffe-eau ?

Sorry I did a mistake in my reports.
It was not mA but mV.

OK, but the voltages are completely wrong.

For the 50 A c.t., you should have 12.5 / 50 × 333 mV = 83.25 mV
For the 20 A c.t., you should have 12.5 / 20 × 333 mV = 208.125 mV

I can only think that you have faulty c.t’s, the 50 A c.t. is really a 42.2 A c.t. :roll_eyes:
and the 20 A c.t. is really a 67.5 A c.t. :exploding_head:

I only look at the numbers and know these c.t’s are not 0.5% accuracy. :rage:

One final check: if you restore the calibration of your emonTx4 to 50.0 and 20.0, does the emonTx4 read,
for the 50 A c.t. 50 A × 98.592 / 333 × 240 V = 3552 W approximately, and
for the 20 A c.t. 20 A × 61.62 / 333 × 240 V = 888 W approximately.
If it does, your emonTx4 is correct, or nearly so.

Indeed these CTs are bad, I already contacted the vendor to get a replacement or refound :confused:

I will do the test with the default settings tomorrow.