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Soil Moisture and Temperature Sensor Hardware and Reading Conversions

See ICT at http://www.ictinternational.com.au/soils.htm for more information about the soil moisture and temperature sensors

Soil Moisture Decagon Echo20 Sensor

Soil moisture on the Mica2s and Flecks is read by an ADC (analogue to digital converter) returning a value proportional to the voltage.

For flecks with Echo20 sensor board

Reading_in_milliVolts = ADC * 3300 / 65535

Note: we are currently getting values that are too high from this – SW needs testing Power seems too high too

For Mica2s with MDA300 sensor board

Reading_in_milliVolts = ADC * 2500 / 4096

Echo20 range should be from 260mV (probe in air) up to 1000 mV max

The EC-20 Volumetric Water Content (VWC) calculation is VWC = 1.738 * (mV/mVe) - 0.29

when the excitation voltage (mVe) is 2500mV (as for Flecks)

Temperature Sensor ECT Air and Soil Temperature Probe

Soil (or air) temperature in degrees C is calculated by the cubic polynomial

T = -0.1087 x3 + 1.6066 x2 – 22.801 x + 25.0

where x = ln( (V0/V)-1)

and V0 is 2500 (excitation voltage) and V is the milliV reading from the probe.

For example, for temperature probe reading 1206mV, the calculated temperature is 23.409 degrees C

Battery and Solar Voltage from the Fleck is

milliV = (ADC/1023) * 4.3 * 3.02 * 1000 = ADC * 12986L / 1023L

Solar Current is

milliAmp = ADC/1023 * 3.02 * 303 = ADC / 1023L * 915.06

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