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Don't touch the pot for this.
How long are the cables between the Solar / UV interface and the TX unit? If possible use twisted par cables, and keep them as short as possible.

Those wires are about 6 inches long. They are 2 pairs of UTP from a CAT5 jumper I sacrificed to get 2 pairs of wires. I used the Blue pair and the Green pair since they are twisted more than the other color pairs. Don't know if it makes a difference but I paired the Vcc with ADC0 and Gnd with ADC1. Should I have paired ADC1 with ADC0 and Vcc with Gnd?

Usually readings in the dark are bellow 4, however for some reason they are above that value.
Search for this code on the RX software:

Code:

      if (RX_Data[2] < 4) RX_Data[2] = 0;        SolarRad = map(RX_Data[2], 0, 1023, 0, 1400);    // Outputs an integer, between 0 and 1400 W/m2

change to this

Code:

      if (RX_Data[2] <= 6) RX_Data[2] = 0;        SolarRad = map(RX_Data[2], 0, 1023, 0, 1400);    // Outputs an integer, between 0 and 1400 W/m2

For a final calibration, wait for a clear sky day, and try to compare your readings with a nearby trustable station.
If they don't differ too much, try to increase or decrease the 1400 value on the code above. You really need to do this in a clear sky day.

OK, I'll try that.

I was watching two other semi-local weather stations on wunderground that have pyranometers and my readings were comparable to theirs for the couple of hours of sun I had after installing the solar/uv board.