EmonTX vrms values too large


I recently bought an emonTX, then I flashed emonESP on a nodeMCU and yesterday I installed then with 3 CT-013-000 sensors, USB charger and the AC/DC charger included with emonTX.

Everything works fine but, vrms values are too high.
Values ranges from 24168.0 to 25207.0.

Is there anything wrong?

Any help will be appreciated.


Those values look a bit on the high side to me, assuming that your nominal voltage is 220 V. But without knowing the actual voltage at the same time, it’s hard to say how wrong it is. The different versions of a.c. adapter require slightly different calibration constants, but those values are too far out for it to be just that. (I’m not talking about the × 100 factor - that’s because the values are sent as integers. 241.68 to 252.07 are the measured voltages.)

To do it properly, you need to go through the calibration procedure in the Learn section. If you are using emonCMS, you could alternatively simply scale the voltage and power (because the voltage is used to calculate power but before the two values are sent - separately) somewhere in the processing chain, multiplying by a constant 0.9 or so. Or if it’s emonCMS on your own server/emonBase/emonPi, you can scale the values in emonhub.conf


Thanks a lot for your answer.

I didn’t´t know that values were sent as integers and I thought that they were wrong. :frowning:
Now I see that the values are correct but a bit high. Here, in Argentina voltage should be 220V but, in general, are a bit higher.

I am newbie in energy monitoring and I am learning how to configure all hardware and software.
I installed emonPI and I beginning to configure the feeds and graphs.
I have to learn how to do what your said about “processing chain” to get values around 220v.

Thank you very much.


I think there’s a bit of (understandable) confusion there.

“EmonPi” is a piece of hardware - it’s an aluminium box containing a Raspberry Pi with cut-down emonTx attached, with a small LCD screen.

What I think you mean is emonCMS. That is the software that is downloadable (or you can get if from our shop on an SD card) and runs on a Raspberry Pi, or any Linux machine, or on a Windows machine with a WAMP server installed (Windows-Apache-MySQL-PHP).

If you’ve got emonHub (which I think you’ll have, if you downloaded emonSD-07Nov16.zip), there should be a button to open emonhub.conf in your web browser window. In there, you’ll find a section for your emonTx (check the Node ID - probably 8) and you can edit the number in “scales = …”.
The section of file should look like this:

    nodename = emonTx_3
    firmware =V2_3_emonTxV3_4_DiscreteSampling
    hardware = emonTx_(NodeID_DIP_Switch1:OFF)
       names = power1, power2, power3, power4, Vrms, temp1, temp2, temp3, temp4, temp5, temp6, pulse
       datacodes = h,h,h,h,h,h,h,h,h,h,h,L
       scales = 1,1,1,1,0.01,0.1,0.1, 0.1,0.1,0.1,0.1,1
       units =W,W,W,W,V,C,C,C,C,C,C,p

so the scales line might end up reading something like this (assuming that 0.91 gives you the correct voltage - you might need to tweak the powers further and separately to make those correct too):
scales = 0.91,0.91,0.91,0.91,0.0091,0.1,0.1, 0.1,0.1,0.1,0.1,1

Sorry for my confusion. I have installed emonCMS (emonSD-07Nov16.zip) on a Raspberry Pi.

I found the menu option to edit emonhub.conf. I will modify it.

Thank you very very much! You save me a lot of reading and work.


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Hi Robert,

Me again :frowning:
I realize that emonhub is not scaling in any way. The data I receive as input or in the feeds is the same that emonESP module sent.
Here are the screenshots.

Could you please guide where I can found the information to debug it?

Thanks a lot!

I’m not sure where emonESP and nodeMCU come into the story. Your emonTx should be sending those values directly by radio to your emonPi, so if you’re routing the data some other way, you’re probably bypassing emonHub.
I misread you - Raspberry PI, not emonPi. So you are sending via Wi-fi and bypassing emonHub.

In that case, in emonCMS, on the Inputs page, you need to insert before the “log to feed” a “x input” step with the appropriate multiplier 0.01 (or a bit less to give the correct voltage) and similarly a multiplier of about 0.9 for each power (ct1 - ct4).
If you’re sending the number somewhere else as well, those will be wrong too. (That’s why calibrating the emonTx would be ‘better’ - but it may be impractical for you.)

It works!!
Now it’s look like it should be :slight_smile:
Thank you very much for your help.


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Adding to an earlier thread; I have an emonPi with an AC-AC adapter, currently showing an Input Vrms of 232.9 - which sounds “about right”.

On an emonTx fed from the same mains meter, with a second AC-AC adapter the Input Vrms value is showing 24433. Both are reporting through to emoncms.org

But I can’t work out why a) one is displaying a “normal” voltage, whilst the other is ~100 times greater, and b) why they are not the same since they are connected to the same mains feed.

Or is it that, with regards to “b)” above, the emonPi is measuring activity within the house (no high loads at the moment) and the emonTx is measuring activity within the garage (currently charging an EV at 32A)?


That bit is easy, the voltage is passed as an integer, not a float, to do that and provide accuracy to 2 decimal places the value is multiplied by 100 at source and then divided by 100 at the other end. This is done in emonHub on the emonPi for the emonPi sourced voltage, but your emonTx is sending direct to emoncms.org, so you will need to divide by 100 (or x 0.01 actually) there as it doesn’t go through emonhub.

As for why one is reading 232.9V whilst the other is reading (effectively) 244.33V could be down to calibration or put another way, the default calibration not being perfect for all devices due to component tolerances, the difference in those examples is only around 5%. It could partly also be voltage drop if one is measuring close to a load or source, like I believe you are measuring close to the inverter with the emonTx whilst at the other end of a 25mtr cable and a couple of CU’s with the emonPi. The way inverters work they do produce a higher voltage than the measured grid voltage deliberately to “tip” the power they produce towards the grid.

Do you have a multimeter with a good AC RMS voltage range ? Are you comfortable working on live wiring etc? You can confirm each device is reading accurately by checking the voltage close to where they are connected, that would either put your mind at rest they are fairly accurate or arm you with the info you need to make a small adjustment if needed.

As a sidenote, wither or both will be far more accurate than the fixed magnitude and PF Owl you are replacing.

. . . and just in case someone thinks I have crystal ball to conjour up all this info about your setup I’ll add a link here to your other thread for completeness :slight_smile: . Setting up an emonPi - some newbie questions

Many thanks. I have now therefore added another “step” to the Input “Garage_Vrms” within emoncms.org, so that rather than just a Log To Feed, there is also a “X 0.01” process that I have put above the log to feed. But … The little “x” to the left of the (blue) “log” icon is red, and won’t turn blue. I assume this means that there is an error with running this command, but can’t work out what. (Meanwhile the value of Garage_Vrms remains ~25200). I have even restarted the emonTx through emoncms on but that didn’t resolve anything.

How does one correct this process?

More than happy to work with wiring, even if live with due care; my multimeter doesn’t do Vrms, so perhaps I need something like this? https://www.amazon.co.uk/multimeter-Voltmeter-Capacitance-Frequency-Resistance/dp/B07PRTFFVC/ref=sr_1_4?keywords=rms+multimeter&qid=1565971895&s=gateway&sr=8-4

I assume you are refering to the “process list” display on the inputs page? The different process types have different colours and the x and + processes are red eg

That I don’t understand if you have made the change above.

Are we talking about the same emoncms.org page? (not confusing with the local emoncms?)

Are you looking at the inputs page or the feeds page?

If you look at the garage vrms line does it say 25000’ish at the end (and show the red “+” and blue log to feed symbols in the same line)? If you then click the spanner at the end so the processlist opens you should see the (now static) values down the right hand side show the result of each process so both the “x0.01” line and the log to feed line should be 250’ish?

The value on the input page is the raw input value not the processed value, the feeds page will show you the processed values (if they are saved to feeds). Each input process list could save multiple values to various feeds so it can never display one processed value, it is always the raw values on the input page.

Think of it as input and output, the inputs page is the data as collected and the feeds page is the results of your processing being saved to disk or displayed in your apps, dashboards and graphs ie “output”.

Oh dear - another please will the ground swallow me up moment. I was looking at / interacting with emoncms.org and not emonpi.local etc, but I was looking at the Input page; now I look at the Feeds page the Garage_Vrms is showing as 250.9, which is correct. I hadn’t twigged that the “x” process icon was always red, and now realise why the effect of the process wouldn’t have shown on the Input page anyway. Obvious in hindsight !

It’s easily done!

IMO, the emonESP should also have the ability to post to emonHub, so it can work as a true rfm replacement. The advantage there is that you would point your emonESP at emonHub, make any adjustments you need there just once and emonHub would send to as many emoncms instances as you wish. The same data would arrive as the same values at each emoncms instance. The current method is a bit messy when you use emonESP AND emonPi together.

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