Dummy's guide to emonTx calibration

Is there a dummy's guide to emonTx calibration?

To be clear, I'm the dummy :)

The step-by-step build guides for emonTx & emonGLCD were massively helpful (this is my first electronics project since school so the first time I've had success with a soldering iron for >20 years.....) but I'm struggling to understand what I need to do to calibrate the emonTx.

I feel this might be something to do with it ... but that sounds like I need a whole new circuit and I think I got one of these AC/AC with the kit I ordered but apart from knowing which bit of the emonTx to plug it into I don't know what to do next.  The calibration page gives me some hints regarding what code to change but it's not clear where I get the readings I need.  This page about accuracy implies there's some experimentation I could do but for that I need another piece of kit ... a calibrated meter ... anyone got a recommendation?

Robert Wall's picture

Re: Dummy's guide to emonTx calibration

Try reading this (if you haven't already): http://openenergymonitor.org/emon/buildingblocks/ct-and-ac-power-adaptor...

Yes, if you want accuracy better than the limits of the tolerances on the components, then you do need a reasonably accurate multimeter. You need to measure two out of three from current, voltage and power. You already have one (hopefully) accurate Watt-hour meter (courtesy of your regional electricity company) but as the emonTx measures current and voltage independently, you need to measure one other quantity out of the three. Without that, your best bet is to assume the current readings are accurate (the error could be a little more, but is likely to be less than 1%), and adjust the voltage calibration so that you record the correct power (which you can establish with a stop watch and your supply meter - you must have a constant load while you take the measurements).
The voltage reading depends on two resistors and the adapter, so is likely to have a greater error than the current, which depends on one resistor and the c.t.

"it's not clear where I get the readings I need"

You modify your sketch to print voltage and current, and real power to the serial port, and look at the terminal display. If you tell me which sketch you're using, I can give you a better steer.

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