Vrms question

I have two emonTx’s each direct serially connected to RPi’s. They are both running the 3 phase firmware (modified by Robert Wall per an earlier forum exchange with him). Each has an ac/ac adapter with dc supplied from the RPi’s via the UART pins. The RPi’s are running the 7 Nov 2016 emonBase image.

Currently I am trying them out in my single phase home – two CT’s are clipped around my single phase live with each connected to CT1 on an emonTx. The emonTx’s have different Node numbers.

I’ve made the following changes to the 3 phase firmware …
Line 193 Max number of temp sensors = 1
Line 199 Amended to define the Node for each particular emonTx

I can see the inputs to each emonBase (RPi).

At the same instant (or within a second or two), Vrms as indicated was 259.4 on one ac adapter/emonTx combo and 260.5 on the other. That’s a less than 0.5% difference – probably explainable by component tolerances? And acceptable to me.

However, a Rapitest tester and EcoEye monitor were indicating 243v & 244v respectively at this same time. And these are more credible values.

What am I missing?

Should I be making more changes to the 3 phase firmware?

260 Vac does sound high, have you calibrated the sketch at all?

Looking at the default Vcal of 276.9 in the 3phase sketch it does appear to be a bit high to me, I thought about 260 to 265 was about right. (roughly calculated by 240Vin / 12Vout for the AC adapter ratio and x13 for the emonTx input voltage divider = 260) although the AC adapters do vary a bit.

As it happens, if you were to divide your emonTx readings by 276.9 and multiply by 260 you arrive at the values your “Rapitest” measured.

If you don’t want to calibrate in the firmware by uploading an edited sketch then you could apply a correction factor to the “scales” for voltage and each of the powers in emonhub.conf. It’s not an ideal way but it does do the job.

To determine the correction divide actual voltage by the reported voltage and then multiply that by the existing scales. eg 243 / 259.4 = 0.937 so the scales line would become something like

    scales = 0.937,0.937,0.937,0.937,0.00937,0.1,0.1,0.1,0.1,0.1,0.1

however the better route would be to use that correction and edit the Vcal in the sketch by multiplying the existing Vcal of 276.9 by the 0.937 = a new Vcal of 259.46. that way it gets applied to the voltage and the powers without playing with the scales in emonhub.conf.

There are many factors that affect calibration, there is a whole article about it in Learn. Unless you happen to have a full set of components that are exactly at their specified value, or a set where all the errors collectively cancel one another, you really should calibrate against a known good meter.

±3% to be exact, the nominal output voltage of the UK one is 11.6 V @ 240 V in, so its calibration constant should be 268.97, assuming it and everything else is spot on. 276.9 probably got left behind and not set back to the nominal value when the sketch got put on Github.
But that would still give you 252 V, which is very close to the 254 V maximum that the UK voltage is supposed not to exceed.