Calibration VCAL ICAL PHASECAL

This guide covers calibration of AC factors VCAL ICAL PHASECAL used with RPICT series and Emonwrt3.

Overview

For AC related measures there are 3 coefficients. VCAL ICAL and PHASECAL.

These coefficients are set by default at manufacturing time. These are normally good enough to operate straight away and provide a decent reading.

All sensors although all produced identically have minor differences between them. This also applies for the ADC and passive components on the RPICT board. Therefore one may want to adjust the calibration coefficients to get more accurate values.

To achieve this a high quality multimeter or scope will be required. This will be the reference system that we will trust as being right.

Voltage VCAL

-1-

Take a note of the VCAL value currently setup in the configuration. Say this is VCAL = 545.0 in our example.

For this you might use the lcl-rpict-config.py function without options.

lcl-rpict-config.py

Reset the RPICT board. Then open the file located in /tmp/rpict.conf.

nano /tmp/rpict.conf

In this file you will find a line with KCAL. The KCAL parameter contains both VCAL and ICAL. See the section below to find which one is which.

Take a note of the VCAL value you are interested in and close nano with Ctrl X.

-2-

Connect the voltage sensor (ac/ac or zmpt) to the board to be tested and note the reading of the measured voltage given by the RPICT. Let's say in our case we get Vmes = 269.5V.

To achieve this you can just read the serial port with the cat command or any other program you might have.

stty -F /dev/ttyAMA0 raw speed 38400
cat /dev/ttyAMA0

-3-

Use the multimeter to read the real voltage value. In our case Vreal = 245.6

-4-

Calculate the new VCAL as such

newVCAL = VCAL*Vreal/Vmes

For us here this is newVCAL = 525*245.6/269.5 = 478.44

-5-

Enter the newVCAL value in the configuration.

Current ICAL

-1-

Setup in configuration an approximate value for ICAL. You might just use the default provided to start with. We will use ICAL = 83.33 in our example.

This can be anything but should be relatively close to the expected final value.

-2-

Irms must be selected as output in the RPICT config (obviously). Connect the CT sensor to the board to be tested. Use a significant load of approx 2kW (this can be a kettle for example). Apply the load and note the reading of the measured voltage given by the RPICT.

Let's say in our case we get Imes = 2048.5

-3-

At the same time the above load is being measured use the multimeter to read the real current value. In our case Ireal = 2105.2

-4-

Calculate the new ICAL as such

newICAL = ICAL*Ireal/Imes

For us here this is newVCAL = 83.33*2105.2/2048.5 = 85.64

-5-

Enter the newICAL value in the configuration.

PHASECAL

In general Phasecal does not need to be any different from 0. Giving any other value to PhaseCal will shift the voltage signal against the current signal to compute power.

Only modify this parameter if you have strong belief there are needs to shift signal once against the other. e.g. you are using a sensor inducing phase shift. We anticipate this to probably never happen. Nevertheless the option is available.

-1-

Setup in configuration an approximate value for PHASECAL. .

-2-

Configure the board to output Power Factor.

If Power Factor is not configurable take records of RealPower, Vrms, Irms. Calculate Power Factor using RealPower/Irms/Vrms.

-3-

Use a purely resistive load like a radiator or a kettle.

-4-

Adjust Phasecal in the configuration until PowerFactor equal to 1 or slightly smaller. We recommend iterating in steps of 1. If PhaseCal = 0 then try -1 +1 -2 +2 etc until PowerFactor=1 is reached.