Calibration Help

Hi, I am struggling to understand the calibration guide whilst trying to calibrate a feed.

I have two feeds that I was convinced was high. I turned off the RCBOs and the feeds read 13w-16w and 20w-24w.

So I tried to calibrate these in emonhub.conf however I got totally lost in the settings.

This is my (back to default) config

    nodename = emontx3
       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

I believe I need to change those scales down so that the two off RCBOs read zero watts. However I am not totally sure how I do that.

Or, do I need to calibrate the vrms so that both feeds read zero when off?

There’s nothing you can do in emonHub (which is where you are) to remove that standing error - which is probably noise picked up by your emonTx (possibly from the mains via its power supply, possibly from its own digital circuitry).

But you can subtract a fixed value in the input processing in emonCMS, so that might help.
(Were you to try to reduce those numbers to zero, the only way would be to put a zero in the scales = ... list - and then it would always read zero whatever the actual power, because it’s like it says, a scale factor. I don’t think that’s what you want. :thinking: )

The second point is you should calibrate Vrms (with scales =) so that the displayed voltage is correct. That won’t affect the power values because the power calculation is done in the emonTx, long before the numbers reach emonHub.

Equally, you calibrate the power values separately from Vrms. Unless you have a known good calibrated power meter, or multimeter and a stable mains voltage, you are probably better off leaving the default values.

If you do have accurate and reliable meter(s), then the proper place to calibrate is inside the emonTx’s sketch, following the procedure in the ‘Learn’ section. Those scales = ... factors, although they can be used for calibration, were never intended as such. Their intended purpose is to convert numbers like Vrms, which is sent as an integer - 23876 say - back into the correct value of 238.76

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Thank you for the detailed reply Robert.

But you can subtract a fixed value in the input processing in emonCMS, so that might help.

I considered this, but the “RCBO off” reading fluctuates a little. If I subtract a fixed value, I may end up with a negative feed value and/or incorrect readings (I think I can set a min value)

I also, couldn’t find a setting to subtract a value from a feed in EmonCMS


Unless you have a known good calibrated power meter, or multimeter
I do have a known good multimeter however I don’t think we have a stable mains voltage so that may be a non-starter.

I guess my take away from this is - there isn’t much I can do apart from set a fixed reduction?

It’s the same as adding a negative value.

By the look of that recording, you’re in the UK? That’s what I had in mind when I wrote that. :slightly_frowning_face:
The obvious difficulty is by the time you’ve swapped from measuring voltage to measuring current, everything has changed. The only real alternative is to put both c.t’s on your main incomer and check the long-term energy recorded against your meter - that’s probably better than nothing, but long-winded and somewhat hit and miss.