Appendix 1: Watermark sensors' interpretation reference
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The next table shows the resistance value of the sensor at different soil water tension conditions (at a 75 Fahrenheit degree, equivalent to 23.8 Celsius degree):
Soil Water Tension (cbar)
Sensor Resistance (Ohms)
0
550
9
1000
10
1100
15
2000
35
6000
55
9200
75
12200
100
15575
200
28075
This series may be approached by this equation:
Where TA is the soil water tension expressed in centibars.
From the real values and the approximation we obtain the next graph of the sensor resistance versus the soil water tension.
In the next figure we can see the frequency of the output of the adaptation circuit for the sensor, for the real resistance values and for the linearly approximated resistance values.
The formula used to draw this graph, from the sensor resistance, is shown bellow:
Where F is the output frequency in Hz and Rs the sensor resistance in ohms.
If we substitute Equation 1 in Equation 2, we get the output frequency in function of the soil water tension:
Equation 4. $F = \frac{\left( 137.5 \times \text{TA} + 150940 \right)}{\left( 2.8875 \times \text{TA} + 19.74 \right)}$
We can see that the frequency output for the working range is between 300 Hz (corresponding to the 200 cbar of maximum soil water tension) and 7600 Hz approximately for 0 cbar measurement. It has been empirically checked that for very wet soils, bellow 10 cbar, the behavior of different sensors is very variable, so calibration is highly recommended if accuracy under these conditions is needed.
To obtain the response of the sensor beyond this range, over the 200 cbar, we must extrapolate those soil water tension values from the linear approximation obtained in equation 1. These sensors are not prepared for working under those conditions, so these graph must be only taken as a reference.
