An impedance-measurement system for electrical characterization of rockwool substrates

An interface system for measuring rockwool water-content and water conductivity is presented. It is intended for measuring impedances that can be modeled as a parallel combination of a capacitance C_p and a resistance R_p. In the particular application, the capacitance varies from 1 pF to 100 pF and the resistance can be in the range from few tens of ohms up to 5 kΩ The measurements are conducted with a sinusoidal signal having a frequency of 20 MHz. At this frequency the full-scale error for C_p doesn´t exceed 3,5% for a 100 pF range. The full-scale error for the measured R_p remains below 3,7% for a full-scale value of 5 kΩ with the presented measurement principle resolution better than 0,2% can be achieved for both measured components.