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Amperage - EmonPi How?

(Neo Web) #1

How/where can I see a feed, and log of amperage?

I have a split feed system:

(Robert Wall) #2

You can’t - easily.

You can, if you edit the sketch in the “emon” front end of the emonPi and send the currents - they are measured and calculated, just not sent. But if you do that (tortuous in itself), you must remember only ever to update your emonPi as an emonBase, never as an emonPi, else your carefully edited and uploaded sketch will be erased and returned to the default. Don’t ask me why it has to be like that, it must have seemed a good idea at the time.

This might help - if it is still accurate. If it doesn’t seem to make sense, ask again.

And when you’ve done that, you need to edit emonhub.conf to tell it about the new variables.

(Neo Web) #3

Can I just create a calculated variable?

Watts / Volts?

(Robert Wall) #4

Only if you promise never to use an appliance that is not a pure resistive load. :grin:

The reason is, the power calculated is real power. If you have a load that’s partially reactive, the true current will be higher than (power ÷ voltage), by a factor which is the reciprocal of the power factor.

Most of the time, and probably more so at higher loads, the “calculated” current should be less than 20% or so below the true current, but there’s really no knowing what the error will be, only that it’s likely to be bigger at low currents/powers.

(Neo Web) #5

So, thanks for helping me understand that. I do not want to log the data then, and will use it only for specific reasons that do not depend on the resistive load factor.

I edited the inputs to log, but I am thinking that I should not log to file, just memory…if I select mysql memory will it fill the ram?

I then setup a virtual feed that did the calcuations how I wanted. Will this virtual feed work with the mysql memory?

(Robert Wall) #6

Somebody else will have to answer the logging and virtual feeds questions, that’s not my area of expertise.

(Paul) #7

You could if you have no interest in real power, you could just run it without the AC adapter so it reports apparent power (not real power) and divide that by 120 to get current.

(Neo Web) #8

So, what is volts_vrms then?

I have this right now:

And the amps are currently:

(Robert Wall) #9

I would presume it has come from your a.c. adapter and it’s the true voltage - subject to calibration errors of course - of one leg of your split-phase supply. If the emonPi detects a voltage there at power-up, it uses it to calculate real power. If not, it assumes the voltage is the nominal (120 V or 230 V), and makes a best guess and gives you apparent power based on that voltage.

If you follow Paul’s suggestion and remove the a.c. adapter (and power down/power up the emonPi to force it to ignore the a.c. voltage input), then as he says, all your power measurements become the ‘best guess’ at apparent power, rather than real power. So you trade knowing the real power for being able to calculate the true rms current.

(Neo Web) #10

I understand then. For this scenario then, the measurement of power use is more important.

I know the amps will not be accurate.

Thanks for the all the help Robert and Paul!

I suppose the last thing I need to know about is the logging engine. I would like to log to mysql memory, but everything I am reading says it will max the memory out. I do not want to save the values, I just want to make simple calculations with them on the dashboard.

(Robert Wall) #11

I think the place to do the calculations is on the inputs page. Nothing there is saved until you ‘Log to feed’.

And I think you’re mistaken about SQL - my understanding is it’s only used for metadata, not the actual logging. The logging is done in ‘homebrew’ databases.