Why does the SDR software show pH<5, although I know that my sample is basic?

The pH values are calculated using a sigmoidal calibration curve. If the measured raw values are not within the range of this calibration curve, the software cannot calculate a pH value. This is internally stored in the software as 0 value, which is smaller than 5. So the message pH < 5 is also shown if the real pH is e. g. 12.