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Setting custom "calculated" channels

(Rory) #1

I’m doing research before purchase right now so apologies if some of my nomenclature is wrong.
I’m planning on buying an emonTX with the wifi adapter to upload directly to emoncms.org.

I’m planning how I might use the power sensing channels.

#1 Total consumption
#2 Consumption through circuit A
#3 Consumption through circuit C
#4 Consumption through circuit F

Will this be counted and shown correctly in emoncms as parts of the whole? Is it possible to easily display (for example) consumption on the remainder of the system (total from circuits B, D and E calcutated as [total - A - C - F]?

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(Robert Wall) #2

Welcome aboard, Rory.

I don’t have the WiFi adapter, so I’ve no experience of it, but in any case that makes little difference.
First, a caution. The standard c.t. from our on-line shop is rated at 100 A. That’s fine for measuring your total consumption, but it’s going to struggle from the accuracy standpoint on individual circuits running less than about 5 A (as the c.t. is only specified down to 10 A). It will measure with falling accuracy well below that, so all is not lost and it depends wholly on your needs. There are lower rated c.t’s available, but the choice is limited.

Back to the original question: emoncms.org can do the maths you’re asking for. All four inputs will arrive on the inputs page separately, you then do the maths and send the result to be stored and displayed as a feed.

I’ve thrown together a quick example for you:

and here’s the maths that does it - in the first (total) Input:

image
(Missing from what you see here are the 3 other inputs pages that simply do “Log to feed” - though they could do more, as indeed could that page.)

Back to the accuracy question again: If “total” ≈ (A+C+F), then there could be a big error in “remainder”. It’s something to be cautious about.

EmonLibCM (not used yet by the default emonTx sketch) appears to make a better job of reading low currents, but it’s still at the mercy of the c.t’s inherent shortcomings at low currents.

One more point to add:
You can run emoncms on a Raspberry Pi (or any other server) on your own LAN, should you wish. That version tends to be more capable than the emoncms.org version.

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(Rory) #3

Thank you very much for taking the time to reply in so much detail. I take your point about accuracy - the individual circuits have things like heaters, a hot tub and an electric car on so fairly large power draws.
Take your point also about emoncms. I’ve messed around extensively with things on raspis in the past but I really want a simple solution that I don’t have to worry too much about maintaining / looking after myself.

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