How to Calibrate?

I have built and deployed the current only power monitor using a SCT-013-000 and Arduino Uno.  The only thing I changed was the 33 ohm burden resister to 146 ohm to get better resolution at lower current since clamp is good for 100 A but I am monitoring only up to 15A.

I cannot seem to find the docs for the EmonLib - specifically what the values in lines like "emon1.current(1, 11.0); // Current: input pin, calibration" and "double Irms = emon1.calcIrms(1480); // Calculate Irms only" mean.

emon1.current(1, 11.0) was emon1.current(1, 111.1) in the example code - I know the second value it has to do with the burden resister size but how is it calculated/estimated?  I came up with 11.0 by trial and error but, being an engineer (not an electrical engineer, unfortunately), that approach does not make me happy.

What does the 1480 in the emon1.calcIrms(1480) mean?  Is it something that may need adjustment?  Current and apparent values now being reported are sensible.

Thanks in advance.


Robert Wall's picture

Re: How to Calibrate?

Welcome, Dennis.

The emonLib documentation is in the source files in the form of comments, assisted by the variables or parameters names.

"emon1.current(1, 11.0); // Current: input pin, calibration" 
The first parameter is the input pin - ADC1 or Channel 3 of the emonTx, the second parameter is the calibration constant as you discovered. How to calculate that is in the Building Blocks section, CT and AC power adaptor installation and calibration theory.

"double Irms = emon1.calcIrms(1480); // Calculate Irms only" 
The parameter is the number of samples over which the current is averaged. It will normally read between 100 and 120 per mains cycle - on a 50 Hz system. Obviously, it would be best to measure an exact number of cycles in order to get an accurate average, but there's no safe way to do this without a voltage reference (as the current waveform might be greatly distorted and may even have more than two zero crossings per cycle). So a large number was chosen so that the error caused by having only part of a cycle is diminished.

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