Do you mean to calibrate your emonTx after you’ve changed the value of the burden? Do you have a programmer?
If you do have a programmer, you need to download the default sketch (comprising 3 files, the main one is “EmonTxV3CM.ino” from Github, change the value of
i4Cal to 90.91 (line 98 or so), compile and upload to the emonTx. Then ignore the change to P4 & E4 in emonhub.conf that follows. All those calibration constants assume our YHDC SCT-013-000 c.t., if you have a different one, the relevant constant(s) will need changing.
If not, the best place is in emonHub.conf - when you get your emonPi working. Find the section for your emonTx, and there is the specification that allows it to decode the data that the emonTx sends. It should be
nodename = emontx3cm15
names = MSG, Vrms, P1, P2, P3, P4, E1, E2, E3, E4, T1, T2, T3, pulse
datacodes = L,h,h,h,h,h,L,L,L,L,h,h,h,L
scales = 1,0.01,1,1,1,1,1,1,1,1,0.01,0.01,0.01,1
units = n,V,W,W,W,W,Wh,Wh,Wh,Wh,C,C,C,p
whitening = 1
If you get silly numbers for the energy, you need to change the ‘datacodes’ line to
datacodes = L,h,h,h,h,h,l,l,l,l,h,h,h,L
Notwithstanding that, the names, datacodes, scales and units all have a 1:1 relationship left to right, so to change the scale of the 4th input, you must change the scale of “P4” (for the power) and “E4” (for the accumulated energy). These will be the 6th & 10th items in the “scales=” line. Change the “1” to 5.44 ( = 120 / 22 where 22 is the 120 Ω & 27 Ω in parallel).
That is my new not-yet-released CM version for the emonPi. Like I said, you must substitute the paths and filenames for the one you compiled from this step:
Your ‘sourcefile’ will have the path where you downloaded the source, and the name of the file you downloaded - suffixed with “.hex”; your ‘targetfile’ should have the same path as mine and the same filename as the sourcefile.