Overview

The WATERMARK Soil Moisture Sensor measures electrical resistance inside of a granular matrix to determine soil water tension. Originally developed in the 1980's, many different methods and devices have been used to read the sensor both by IRROMETER as well as others.

To measure resistance inside the sensor, typically a voltage divider circuit is used. The circuit uses a known input voltage, output voltage, and series resistor value to calculate the value of another resistor (the sensor). Figure 1 is a simple example using a digital pin for power and an analog input to read and measure the resistance of the sensor.

Once the resistance is known, the value is calibrated to soil water tension via a series of equations or referencing look-up tables. The WATERMARK sensor can be read by devices such as Arduino/ Raspberry Pi etc., provided some guidelines are maintained with the reading circuitry and program.

For adapters which will read the sensor and output the values in a more standard format such as an analog 0-3V or SDI-12, click here.

Sensor Isolation

Devices reading WATERMARK sensors must isolate the sensor reading circuit from any earth ground. Mains powered devices will need transformer isolation to prevent a loop to ground through nearby equipment, and any communication line intended to be permanently connected to another grounded device will require optical isolation. If a ground loop exists, the readings will be wrong and current leakage may quickly destroy the sensor electrodes. Battery powered devices are more isolated by nature, but grounds for lightning protection etc. can cause the same problem.

In addition to general device isolation from ground, when reading multiple sensors the circuit must be designed to accommodate isolating sensors from each other. The wet soil in which sensors are installed creates a common conductive path between sensors. In effect, without isolation a device can be reading partially or fully between electrodes in different sensors rather than between electrodes inside each sensor.

Sensors must be powered individually, and the ground must be isolated from sensor to sensor. Even if powered individually, an open ground path to another un-powered sensor can't be allowed. This is best accomplished using multiplexers to open and close the appropriate channels.

Caution: Since WATERMARK sensors provide a direct path from earth to any device, protection via Zener or TVS diodes should be included on all sensor lines. In the interest of simplicity these are not included in any of examples to follow, but are highly recommended.

Arduino Example

The example below reads one soil temperature and three WATERMARK sensors using one 74HC4052 dual channel multiplexer. Two outputs are used to control the multiplexers (PWM3 and PWM4) while PWM5 and PWM11 are alternated as excitation power and GND. A1 is used for the analog measurement.

Initial Steps:

1. PWM 6 set LOW to enable multiplexer
2. PWM 3 and 4 are set to the appropriate channel


Reading the Sensors:

PWM 3 and 4 are set to the next channel and the reading is repeated for the next sensor


Shutdown:

PWM 6 set HIGH to disable multiplexer

Calculate Resistance:

Where SenVWM1 is the analog voltage measured in the first direction, SenVWM2 is the same in the second direction, and Rx is the value of R1:

					
double WM_ResistanceA = (Rx * (SupplyV - SenVWM1) / SenVWM1); //voltage divider
double WM_ResistanceB = Rx * SenVWM2 / (SupplyV - SenVWM2);  // reverse
double WM_Resistance = ((WM_ResistanceA + WM_ResistanceB) / 2); //average the two directions
					
					

Convert to Centibars (kPa):

Where res is the measured resistance, TC is the soil temperature in C, and cF is an optional calibration factor:

					
int myCBvalue(int res, float TC, float cF) {   //conversion of ohms to CB

	int WM_CB;
	float resK = res / 1000.0;
	float tempD = 1.00 + 0.018 * (TC - 24.00);

	if (res > 550.00) { //if in the normal calibration range
		if (res > 8000.00) { //above 8k
			WM_CB = (-2.246 - 5.239 * resK * (1 + .018 * (TC - 24.00)) - .06756 * resK * resK * (tempD * tempD)) * cF;
		} else if (res > 1000.00) { //between 1k and 8k
			WM_CB = (-3.213 * resK - 4.093) / (1 - 0.009733 * resK - 0.01205 * (TC)) * cF ;
		} else { //below 1k
			WM_CB = (resK * 23.156 - 12.736) * tempD;
		}
	} else { //below normal range but above short (new, unconditioned sensors)
		if (res > 300.00)  {
			WM_CB = 0.00;
		}
		if (res < 300.00 && res >= short_resistance) { //wire short
			WM_CB = short_CB; //240 is a fault code for sensor terminal short
			Serial.print("Sensor Short WM \n");
		}
	}
	if (res >= open_resistance || res==0) {
		WM_CB = open_CB; //255 is a fault code for open circuit or sensor not present
	}
	
	return WM_CB;
}
					
					

Download the code for this sample as well as a simplified, single sensor example HERE

Download the calibration look-up table HERE