EmonTx readings

I’ve just bought an EmonTx with 4 CT sensors and UK AC PSU from the shop. The plan is to connect it to a mini PC and read the values over serial. Whilst testing out the unit to get some basic understanding on how it works I noticed one of the CT ports seems to read higher than the others.

As you can see from the readings below, power on CT1 is generally 4-5 higher than CT2 and CT3.

MSG:550,Vrms:244.38,P1:190,P2:186,P3:185,E1:304,E2:297,E3:297,pulse:1

MSG:551,Vrms:244.38,P1:185,P2:180,P3:180,E1:305,E2:298,E3:297,pulse:1

MSG:552,Vrms:244.22,P1:251,P2:246,P3:246,E1:305,E2:298,E3:298,pulse:1

MSG:553,Vrms:243.75,P1:210,P2:205,P3:205,E1:306,E2:299,E3:299,pulse:1

MSG:554,Vrms:243.77,P1:195,P2:191,P3:190,E1:307,E2:299,E3:299,pulse:1

MSG:555,Vrms:243.78,P1:191,P2:187,P3:186,E1:307,E2:300,E3:300,pulse:1

MSG:556,Vrms:243.78,P1:175,P2:171,P3:171,E1:308,E2:300,E3:300,pulse:1

MSG:557,Vrms:243.65,P1:172,P2:167,P3:167,E1:308,E2:301,E3:301,pulse:1

MSG:558,Vrms:243.51,P1:180,P2:176,P3:176,E1:309,E2:301,E3:301,pulse:1

MSG:559,Vrms:243.52,P1:185,P2:180,P3:179,E1:309,E2:302,E3:302,pulse:1

MSG:560,Vrms:243.41,P1:210,P2:206,P3:205,E1:310,E2:302,E3:302,pulse:1

I’ve swapped the CT sensors between ports and the higher reading stays with CT1.

With the CT sensors connected but not clamped round any cable I see the below:

MSG:1,Vrms:240.90,P1:1,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:2,Vrms:240.95,P1:0,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:3,Vrms:241.35,P1:0,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:4,Vrms:241.64,P1:0,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:5,Vrms:240.78,P1:1,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:6,Vrms:240.23,P1:1,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:7,Vrms:240.14,P1:1,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:8,Vrms:239.97,P1:1,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:9,Vrms:240.25,P1:2,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:10,Vrms:240.27,P1:2,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:11,Vrms:239.98,P1:3,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:12,Vrms:240.46,P1:4,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:13,Vrms:240.80,P1:4,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:14,Vrms:241.11,P1:4,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:15,Vrms:241.58,P1:5,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:16,Vrms:241.54,P1:5,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:17,Vrms:240.98,P1:5,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:18,Vrms:241.28,P1:5,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:19,Vrms:241.06,P1:5,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

MSG:20,Vrms:241.09,P1:5,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:1

…

MSG:163,Vrms:243.77,P1:7,P2:0,P3:0,E1:2,E2:0,E3:0,pulse:1

MSG:164,Vrms:243.86,P1:7,P2:0,P3:0,E1:2,E2:0,E3:0,pulse:1

MSG:165,Vrms:243.87,P1:6,P2:0,P3:0,E1:2,E2:0,E3:0,pulse:1

MSG:166,Vrms:243.91,P1:7,P2:0,P3:0,E1:2,E2:0,E3:0,pulse:1

MSG:167,Vrms:243.03,P1:6,P2:0,P3:0,E1:2,E2:0,E3:0,pulse:1

I also discovered that when running the EmonTx using a separate DC PSU (in addition to the AC for voltage measurement) the offset on CT1 increases.

MSG:11,Vrms:242.31,P1:11,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:0

MSG:12,Vrms:242.52,P1:11,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:0

MSG:13,Vrms:243.36,P1:10,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:0

MSG:14,Vrms:243.76,P1:10,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:0

MSG:15,Vrms:243.38,P1:11,P2:0,P3:0,E1:0,E2:0,E3:0,pulse:0

This is with stock firmware and no calibration performed as yet.

Is this to be expected due to manufacturing tolerances? It seems a bit odd that only 1 CT port is off.

Would calibration help here?

Appreciate any advice or experience other users have.

Thanks.

Paul