EmonTX CT sensor for immersion divert showing incorrect value

Hi I have recently purchased an Emontx to go with my Emonpi.

The idea behind this is to be able to capture the amount that I am diverting to my immersion tank.

I have a SCT-013-000 CT sensor connected to the CT4 socket.

The issue I have is that the value I get is higher than the amount of solar being generated, and I know that I am not importing at this point.

the input is set up like this

I am using an Optimmersion unit, I have the CT on the feed to the control box, not on the output to the immersion.

Does anyone have any ideas why this is?



My first guess is you’re not measuring what you think you’re measuring. The second is you’re not doing the additions and subtractions that you think you’re doing.

So what is driving each of those dials? Do you have a.c. adapters so you are measuring real power, or are you assuming a voltage and using apparent power? I think the difference is too great to be a calibration issue, but have you checked that?

Hi Robert,

Thanks for reply.

Dials, Import, Solar and Use are all fed from the emonpi, which has an ac adpator to measure the mains voltage.

Immersion Divert is on the emontx, which does not have an ac adaptor to measure mains voltage
it is powered from a usb at 5v

These are the Feeds

Does this help?

OK, you have an immediate problem with the emonTx “power”. It won’t be accurate. Without an a.c. adapter, it assumes 230 V, even thought the UK ‘centre’ voltage is 240 V. Your error there, if the current is correct, can be ±10%. I’d expect it to be reading 4-5% low, which makes your problem worse. You can improve its accuracy by, in emoncms, dividing the power by 230 (multiply by 0.004348) and multiplying by the Pi’s Vrms.

I still don’t know how you get those feeds. Personally, I would have put a Pi c.t. on the grid infeed and used a centre-zero dial to show nett import/export, then the second Pi c.t. on the solar infeed. Generally, because there will be a variation in the sensitivity between all your inputs (unless you’ve calibrated very carefully), it’s a bad idea to derive a feed by subtracting two nearly equal quantities. So I wouldn’t put a c.t. on the house side of where the solar infeed joins, to measure use directly, and then derive the nett grid power by subtraction, because by design your immersion controller aims to make that near-zero when it can (and Robin’s diverter would make it exactly zero).

I’m guessing the controller uses phase angle control, the emonTx will measure the rms current of a load like that reasonably accurately, so I don’t anticipate that being a problem.

So sorry, but I can’t add much more.

Many Thanks Robert, I will look at this further and try to sort it out using your comments to assist me.

PSU added to emontx, inputs tweaked.
now will wait till we get some sun tomorrow to see the results.
Will post results that I hope will be much better.
thanks again for assistance