Calibration of soil moisture sensor

mlnz · June 30, 2019, 9:01am

Hello,
I am using new soil moisture sensor and when is whole sensor in the water, it shows 80% moisture… Probably I can calibrate the sensor, but I am not sure if I am doing something wrong… I have two sensors and both works exactly same.

Thank you for any idea!
Milan

Esquive · August 3, 2021, 7:09am

Hi Minz,

were you able to find a solution to that?

Best,
Eric

hub.martin · August 4, 2021, 7:51am

SoilSensor has support for calibration, but it is still experimental. Please see note in the SoilSensor project.

github.com

hardwario/bcf-radio-soil-sensor/blob/master/app/application.c#L152




    if (bc_soil_sensor_get_cap_raw(self, device_address, &raw_cap_u16))
    {
        snprintf(topic, sizeof(topic), "soil-sensor/%llx/raw", device_address);


        // Publish raw capacitance value message on radio
        int raw_cap = (int)raw_cap_u16;
        bc_radio_pub_int(topic, &raw_cap);


        /*
        // Experimental - send percent moisture value based on sensor calibration
        int moisture;
        bc_soil_sensor_get_moisture(self, device_address, &moisture);
        snprintf(topic, sizeof(topic), "soil-sensor/%llx/moisture", device_address);
        bc_radio_pub_int(topic, &moisture);
        */
    }
}
else if (event == BC_SOIL_SENSOR_EVENT_ERROR)
{
    int error = bc_soil_sensor_get_error(self);

We added small EEPROM inside and added some calibration support, but after tests in the field we discovered that 0-100% output is not practical. So we suggest basic RAW reading which has bigger range (it was somewhere 6000-11000) and users can “calibrate” that in for example in Node-RED.

Can you try to submerge your sensor not fully, but just to the begining of the heat shrink tube? Sometimes if you submerge the part with electronics or cable then the capacitive coupling is decreasing which could explain 80% instead of 100%.

If you would like to re-calibrate your Soil, you can try our code here, but as I said, calibration is experimental and we suggest to use only RAW values from the sensor.

github.com

blavka/bcf-soil-sensor-calibration/blob/master/app/application.c

#include <application.h>

// LED instance
bc_led_t led;

// Button instance
bc_button_t button;
bc_button_t lcd_left;
bc_button_t lcd_right;

bc_gfx_t *gfx;

bc_soil_sensor_t soil;

typedef enum
{
    STATE_SCAN = 0,
    STATE_READ_0,
    STATE_READ_100,
    STATE_CONFIRM,

This file has been truncated. show original

This calibration program needs LCD Module, but could be easily changed to output intructions over uart. You basically connect sensor, keep it dry and confirm that by pressing button. Then you submerge sensor from the tip to the start of heat shrink tube and set the 100% calibration. Then you confirm that and it is saved to the internal EEPROM of Soil.