Design a concentric cylindrical capacitive sensor to detect fertilizer solution concentration

which consists of voltage regulator

10 MHz oscillator

Schmitt trigger

capacitance sensor

and true root-mean-square（ RMS） detector. Test results is output in the form of DC voltage. In concentration range of 8 × 10

-1

1 ×10

-3

mol/L

take a series of experiments by using a series of fertilizer solution to test sensor: 1) calibration experiments show that output voltage of sensor have linear negative relationship with fertilizer solution concentration

the decision coefficient is R

2

> 0. 987; 2) r

esults of temperature variabilily test show that output voltage of sensor has linear positive relationship with temperature of fertilizer solution

the decision coefficient is R

2

= 0. 984 17; 3) results of stability test show that the biggest difference value in 10 h that under the same concentration of liquid fertilizer is 0. 000 3 mol/L

stability is good; 4) validation test show that the maximum detection error value is 9. 8 %. therefore

this sensor can meet application requirement of detection of fertilizer solution concentration in practical engineering.