AC Voltage in North America Calibration

For discussions related to emonPi, emonTx, emonBase and emonGLCD hardware and Arduino firmware

I have a new installation of emonPI with US AC to AC from shop and a emonTx with the same. I have a 15kw Solar Net meter here in the United States with Split phase 220/110 Vac at 200A total.
The voltage readings from the emonPi is reading 11 volts below my 4 different digital meters. Note that all are different from each other by small amounts.
The reading from the emonTx is 10 volts below.
Would this be a source of error in Power Readings?
Is there a way in software to correct the difference?
Do I need to correct the calculations in Emoncms?



Maybe not.

I assume you are looking at the results in emonCMS, but which emonCMS? Is it running on the emonPi, or do you have an emonBase as well, and you are using You’ve confused me by mentioning both emonBase and emonPi, and they’re not the same.

The cause of the discrepancy is component tolerances. What you have is a set of physical components, each of which is manufactured to be within a tolerance band - but we don’t know where the actual value will be within that band. The same applies to the current readings too. If you go to Resources > Building Blocks, you’ll find a full explanation of how the inaccuracies can build up. Fortunately, you rarely get a set of worst-case components, and statistics say that some of the errors will cancel each other out.

You can correct the errors at source in your emonTx and in the cut-down emonTx (the procedure is essentially the same). How is spelled out in Building Blocks.

If it’s an emonPi that you have (and not an emonBase and you’re not using, then it’s somewhat easier. You can adjust the values of the multipliers for each node in the config file emonhub.conf. They are in the lines “scales = …” and correspond 1 for 1 with the names in the line above. You will need to adjust both voltage and power independently, they do not interact at this stage as the multiplication was done a long way back in the signal chain.
If you correct the scaling there, you don’t need to add a scaling step in emonCMS. But if it’s, that’s the only place available to you without calibrating the emonTx itself.

Thank you Robert.

I have the emonPi and emonTx. Both are new installs this month (December 2016).

I saw the lines “scales” in the Pi software. I will play with that to adjust Voltage and Power readings.

The emonTx is in Building blocks and I have the programer to explore. Good luck to me!

Happy New Year!