Why readings of Klever differ significantly from the chemical analysis?

The Klever shows -С- or “Measurement error”. What is to be done?

The Klever shows the presence of water in the milk, although it should not be there. Why?

Can I fix the Klever myself? Please send me a circuit design for the power supply unit.

Where is it possible to get the Klever calibrated?

We have lost Operation Manual, the calibration method description, etc. Where can we get it?