# SCT013 value consistently too low **URL:** \ **Category:** Getting Started\ **Tags:** emonlib\ **Created:** [25 July 2024 22:00 UTC](https://community.openenergymonitor.org/t/sct013-value-consistently-too-low/26480 "2024-07-25T22:00:13Z")\ **Posts on this page:** 1\ **Showing post:** 6
**Author:** ![Robert.Wall](https://community.openenergymonitor.org/user_avatar/community.openenergymonitor.org/robert.wall/32/38246_2.png) [@Robert.Wall](https://community.openenergymonitor.org/u/Robert.Wall)\ **Post date:** [5 August 2024 19:21 UTC](https://community.openenergymonitor.org/t/sct013-value-consistently-too-low/26480/6 "2024-08-05T19:21:26Z")
> [@tge](#): > > voltage is “U”, is that maybe a German thing? It is indeed. I’m happy for you to use U, if this is what you learned. > [@tge](#): > > we don’t need to worry about the vCal nor iCal But you **DO**. VCal & ICal are applied to the voltage, the current and the power calculations separately. You should not need to adjust ICal, but the European a.c. adapter requires a different VCal to the UK one (which is the default value in the sketch), in addition to it’s output being subject to a tolerance of ± 3%, so you will very likely need to get the correct value for this by measurement and comparison with a known good meter. > [@tge](#): > > How would I determine the phase error of the V and I transformers? You either get it from the manufacturer, or you measure it - and this is the hard part. You can read how I tested the UK a.c. adapter here: [Ideal Power 9V AC-AC Adaptor (Voltage Transformer) — OpenEnergyMonitor 0.0.1 documentation](https://docs.openenergymonitor.org/electricity-monitoring/voltage-sensing/report-idealpower-9v-acac-adaptor.html) and the SCT-013-000 current transformer here: [YHDC SCT-013-000 Current Transformer — OpenEnergyMonitor 0.0.1 documentation](https://docs.openenergymonitor.org/electricity-monitoring/ct-sensors/yhdc-sct-013-000-ct-sensor-report.html) > [@tge](#): > > which software did you use to take those pictures?) I didn’t – those are Megni pictures from their Digital Storage Oscilloscope. Since I did those tests on the Ideal v.t and YHDC c.t’s, I’ve got a two channel DSO too (not the same make), and I now use it to record the two waves (primary side and secondary side) and I use a FFT in a spreadsheet to determine the phase error. It is much more precise than trying to read the time difference at the zero crossings: [ZMPT101B, powerfactor and current shown - #52 by Robert.Wall](https://community.openenergymonitor.org/t/zmpt101b-powerfactor-and-current-shown/11939/52) and the rest of this thread is well worth reading too. > [@tge](#): > > we have 208ms/6000 = ~34.7 microsecs per one calcVI() cycle, and therefore also between two readings of V (and I). Yes to the first part, but it reads both V & I in this 34.7 µs. It’s true that there is 34.7 µs between one V sample and the next, it will not be true that there is 17.35 µs between V & I samples. > [@tge](#): > > Can that be correct? If so, is it really possible, that the deltaT of 5 microsecs = 0.09° between sampling V and I has that much of a influence? The faster the controller is, the less the phaseCal problem should be, at least the deltaT between sampling I and V (?) This does look reasonable to me. But the next part is wrong - with 0.63° between samples of the same V or I, and maybe 0.09° (though I suspect it might be a little longer) between V&I) you still have something like an angle of 2° _difference_ between the phase errors of the two transformers, and it is this error that is the principle part of the total phase error. (2° comes from my test results in the links above.) You cannot have a phasecal of 2 / 0.63 = 3.17 and not have a huge error in the synthetic voltage sample that’s created in the interpolation maths (it is now **EXTRAPOLATING)**. As I suspected, you will need to store some samples, or slow the rate at which your version of emonLib reads the sample pair. --- _[View the full topic](https://community.openenergymonitor.org/t/sct013-value-consistently-too-low/26480)._