calibration at a distance from voltage sensor and CT clamp

right, so I have an EmonTX ( a second one)

my first one appears to be super accurate as the 9v AC transformer and CT clamp are right by the incoming mains.

the second setup has the ct clamp and a second ac transformer which is plugged in 20m away from the mains in (over a 20m piece of 6mm armoured) then the load is actually about another 40m away from the ct over another bit of 6mm

How can I calibrate this to be accurate, allow for the volatge drops etc.



Incoming mains--------20m 6mm cable ------consumer unit--------------40m 6mm cable------------load

(emontx1, ac trans, ct)                              (emontx2, ac trans, ct)

Robert Wall's picture

Re: calibration at a distance from voltage sensor and CT clamp

The first point to clear up, what power do you want to measure? The volt drop along your cable really is lost power - the cable gets warm - so if you need the true voltage at the load end, you need to know the cable resistance (loop - both conductors in series!) and do a simple sum on the voltage to correct it before using it for the power calculation:

 Vconsumer unit = Vload + I * R

of course using the instantaneous values for both V and I. You'll need to customise your emonLib to include this.

You can get the loop resistance from the published tables. If your cable is what I think it is, it's 7.3 mV/A/m. If the volt drop is greater than 4% 3% for lighting and 5% for other of the declared supply voltage, your installation doesn't comply with the regulations (BS 7671).

[Edited - Robin's bought an up-to date copy of BS7671 and corrected me.]

mattnj's picture

Re: calibration at a distance from voltage sensor and CT clamp

the total used power is already displayed at emontx1

I need to display the power at the "load"  on my dodgy diagram, which I have know currently is a tested 3kw, if that ends up being 3.1kw because of the cable loss then i need to show that.

With the standard calibration values in emonlib it was showing 4kw ish when measured with emontx2, I am assuming a 40m bit of 6mm isn't loosing a 1kw!

This is monitoring the swimming pool feed (under the garden on a 6mm armoured cable) the idea is to subract the two and show house/pool use. I did have the emontx at the pool end and it was accurate but I have had to more it for various reasons.

I applied the calibration calcs....but it is still wrong, then I noticed it says the 9v ac needs to be within 1m of the supply.

I have actually got it showing about right by trial and error, the calibration value is currently at 90 (from 111)

If you can provide a method to calibrate it exactly that would be great!

Robert Wall's picture

Re: calibration at a distance from voltage sensor and CT clamp

"it says the 9v ac needs to be within 1m of the supply."

That's because of the length of the low voltage cable on the adapter! If you want to measure the voltage at the pool and bring a pair of bell wire or similar back, that shouldn't be a problem. There is practically no volt drop in the voltage monitor because there's next to no current.

Obviously there are two sources of error in your power measurement, voltage and current. You need to get them both right, altering one too much to compensate for the other will lead to errors one way or the other.

If you're reading 3 kW at emonTx2, the current is nominally 12.5 A and you'll be losing 7.3 mV/A/m x 12.5 A x 40m =  3.65 V, or 45.635 W, so only 1.5% or so.

I think you need to move emonTx2 back up next to emonTx1 and calibrate it there - don't match the powers, adjust both voltage and current calibration to match voltage and current separately, then move it back and apply the volt drop correction and hope.

But to come back to the original question, if you subtract the two, you're in effect transferring the cable loss onto house consumption. Is that what you want?

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