Scale adjustment for 200A Clip-on CT when using scaling for graphs in EmonCMS?

Hey folks,

I purchased the 200A Clip-on CTs a while back and just the other day got around to using them on fat wire feeds to a sub-panel.
Somewhere, at some point, I remember seeing an article saying that when using these CTs, the configuration file (emon hub) on the EmonPi receiver that the emonTX4 is sending to needs to be modified to reflect a scale of 3.

I forgot to do this when I was setting this system up at a client’s home (I’m an energy auditor and move my equipment frequently).
I assumed that I could simply input a scale of 3 down the line when creating graphs in from that original data and it would be accurate, but I suspect I’m totally wrong with this assumption and that I’ll need to change the config file on the receiving emonPi unit.

Can someone confirm if this scaling will be accurate or not when performed in graphing in EmonCMS? If not, can you explain the mechanism to me?

You need to know what the “CT Type” is set to in the EmonTx4 Serial Config Tool in the emonCMS running on your client’s emonPi. It could be 20, 25, 50, 100 or 200 A. You then need to multiply the values you’re seeing in by the appropriate ratio (10, 8, 4 or 2).

I think the “3” value was probably for the emonTx V3. This uses the same range of c.t’s (current output) as the emonTx V2 but those are totally different to the voltage output ones required by the emonTx4.

Hey Robert, thanks again for your wisdom!

“EmonTx4 Serial Config Tool” is not something I’m familiar with. I only know the emon hub config file. Is this the same thing?

I also use one TX3 and one TX4, that, depending on the situation, I send their data to either an older or newer EmonPi unit. So if I’m following you correctly, every time I wanted to use those 200 amp clip-on CTs, I would need to change the config on each of those (TX3 and TX4) units in addition to config on each EmonPi? Seems really convoluted…

And still, this doesn’t answer my question as to whether or not I can simply adjust for this down the line when viewing the logged data in EmonCMS graphing by using the “scale” entry for these feeds. If this is possible, what would that scale value be? Depends on the TX and EmonPi unit I was sending to?

No. The config tool configures the emonTx4 itself when plugged into the emonPi2 using the USB connection. But if you look in emonCMS, you’ll see the standard options available. My bet is the emonTx4 will be set to one of those, so if you can’t move the emonTx4 to the emonPi to connect it (it can’t be done by ISM band radio and LPL), and if you can guess which if you have a good idea of the load and can see what emonCMS says - then divide by your guess and multiply by 200 in emonub.conf :smile:

This probably hints at a possible solution for the wider question, if I now understand your problem correctly: You have a number of emonTx V3 and emonTx4, and a selection of c.t’s. The important bit is, you’re only interested in being able to set up the emonCMS each is working into, to give the correct calibration. You’re not per se interested in the actual number coming out of the emonTx.

What you need to do is set up each channel on each sensor node to give a common number in relation to some standard (which you decide - bearing in mind the biggest signed integer is 32767) So let’s say that every input is calibrated in the emonTx so that the c.t. at rated current and your nominal system voltage reports 32 kW. Then, in emonCMS, on the Inputs page you multiply by a constant that’s the result of multiplying the rated c.t. current by the nominal voltage and dividing by 32000. You can indeed do the same instead (not both together) in the graph “Scale” box of the emonCMS graph, or in the dashboard dials.

[I only chose 32 kW so you won’t be troubled by loss of accuracy due to integer truncation.]